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1995-07-25
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PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
NNNNAAAAMMMMEEEE
perlfunc - Perl builtin functions
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
The functions in this section can serve as terms in an
expression. They fall into two major categories: list
operators and named unary operators. These differ in their
precedence relationship with a following comma. (See the
precedence table in the _p_e_r_l_o_p manpage.) List operators
take more than one argument, while unary operators can never
take more than one argument. Thus, a comma terminates the
argument of a unary operator, but merely separates the
arguments of a list operator. A unary operator generally
provides a scalar context to its argument, while a list
operator may provide either scalar and list contexts for its
arguments. If it does both, the scalar arguments will be
first, and the list argument will follow. (Note that there
can only ever be one list argument.) For instance, _s_p_l_i_c_e()
has three scalar arguments followed by a list.
In the syntax descriptions that follow, list operators that
expect a list (and provide list context for the elements of
the list) are shown with LIST as an argument. Such a list
may consist of any combination of scalar arguments or list
values; the list values will be included in the list as if
each individual element were interpolated at that point in
the list, forming a longer single-dimensional list value.
Elements of the LIST should be separated by commas.
Any function in the list below may be used either with or
without parentheses around its arguments. (The syntax
descriptions omit the parens.) If you use the parens, the
simple (but occasionally surprising) rule is this: It _L_O_O_K_S
like a function, therefore it _I_S a function, and precedence
doesn't matter. Otherwise it's a list operator or unary
operator, and precedence does matter. And whitespace
between the function and left parenthesis doesn't count--so
you need to be careful sometimes:
print 1+2+3; # Prints 6.
print(1+2) + 3; # Prints 3.
print (1+2)+3; # Also prints 3!
print +(1+2)+3; # Prints 6.
print ((1+2)+3); # Prints 6.
If you run Perl with the ----wwww switch it can warn you about
this. For example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
For functions that can be used in either a scalar or list
Page 1 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
context, non-abortive failure is generally indicated in a
scalar context by returning the undefined value, and in a
list context by returning the null list.
Remember the following rule:
o+ _T_H_E_R_E _I_S _N_O _G_E_N_E_R_A_L _R_U_L_E _F_O_R _C_O_N_V_E_R_T_I_N_G _A _L_I_S_T _I_N_T_O _A
_S_C_A_L_A_R!
Each operator and function decides which sort of value it
would be most appropriate to return in a scalar context.
Some operators return the length of the list that would have
been returned in a list context. Some operators return the
first value in the list. Some operators return the last
value in the list. Some operators return a count of
successful operations. In general, they do what you want,
unless you want consistency.
-X FILEHANDLE
-X EXPR
-X A file test, where X is one of the letters listed
below. This unary operator takes one argument,
either a filename or a filehandle, and tests the
associated file to see if something is true about
it. If the argument is omitted, tests $_, except
for -t, which tests STDIN. Unless otherwise
documented, it returns 1 for TRUE and '' for FALSE,
or the undefined value if the file doesn't exist.
Despite the funny names, precedence is the same as
any other named unary operator, and the argument may
be parenthesized like any other unary operator. The
operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size.
-s File has non-zero size (returns size).
Page 2 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO).
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is a text file.
-B File is a binary file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
The interpretation of the file permission operators
-r, -R, -w, -W, -x and -X is based solely on the
mode of the file and the uids and gids of the user.
There may be other reasons you can't actually read,
write or execute the file. Also note that, for the
superuser, -r, -R, -w and -W always return 1, and -x
and -X return 1 if any execute bit is set in the
mode. Scripts run by the superuser may thus need to
do a _s_t_a_t() in order to determine the actual mode of
the file, or temporarily set the uid to something
else.
Example:
while (<>) {
chop;
next unless -f $_; # ignore specials
...
}
Note that -s/a/b/ does not do a negated
substitution. Saying -exp($foo) still works as
expected, however--only single letters following a
minus are interpreted as file tests.
The -T and -B switches work as follows. The first
block or so of the file is examined for odd
characters such as strange control codes or
characters with the high bit set. If too many odd
characters (>30%) are found, it's a -B file,
otherwise it's a -T file. Also, any file containing
null in the first block is considered a binary file.
Page 3 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
If -T or -B is used on a filehandle, the current
stdio buffer is examined rather than the first
block. Both -T and -B return TRUE on a null file,
or a file at EOF when testing a filehandle.
If any of the file tests (or either the _s_t_a_t() or
_l_s_t_a_t() operators) are given the special filehandle
consisting of a solitary underline, then the stat
structure of the previous file test (or stat
operator) is used, saving a system call. (This
doesn't work with -t, and you need to remember that
_l_s_t_a_t() and -l will leave values in the stat
structure for the symbolic link, not the real file.)
Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
abs VALUE
Returns the absolute value of its argument.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the
_a_c_c_e_p_t(2) system call does. Returns the packed
address if it succeeded, FALSE otherwise. See
example in the _p_e_r_l_i_p_c manpage.
alarm SECONDS
Arranges to have a SIGALRM delivered to this process
after the specified number of seconds have elapsed.
(On some machines, unfortunately, the elapsed time
may be up to one second less than you specified
because of how seconds are counted.) Only one timer
may be counting at once. Each call disables the
previous timer, and an argument of 0 may be supplied
to cancel the previous timer without starting a new
one. The returned value is the amount of time
remaining on the previous timer.
For sleeps of finer granularity than one second, you
may use Perl's _s_y_s_c_a_l_l() interface to access
_s_e_t_i_t_i_m_e_r(2) if your system supports it, or else see
Page 4 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
the select() entry elsewhere in this documentbelow.
atan2 Y,X
Returns the arctangent of Y/X in the range - to .
bind SOCKET,NAME
Binds a network address to a socket, just as the
bind system call does. Returns TRUE if it
succeeded, FALSE otherwise. NAME should be a packed
address of the appropriate type for the socket. See
example in the _p_e_r_l_i_p_c manpage.
binmode FILEHANDLE
Arranges for the file to be read or written in
"binary" mode in operating systems that distinguish
between binary and text files. Files that are not
in binary mode have CR LF sequences translated to LF
on input and LF translated to CR LF on output.
Binmode has no effect under Unix; in DOS, it may be
imperative. If FILEHANDLE is an expression, the
value is taken as the name of the filehandle.
bless REF,PACKAGE
bless REF
This function tells the referenced object (passed as
REF) that it is now an object in PACKAGE--or the
current package if no PACKAGE is specified, which is
the usual case. It returns the reference for
convenience, since a _b_l_e_s_s() is often the last thing
in a constructor. See the _p_e_r_l_o_b_j manpage for more
about the blessing (and blessings) of objects.
caller EXPR
caller Returns the context of the current subroutine call.
In a scalar context, returns TRUE if there is a
caller, that is, if we're in a subroutine or _e_v_a_l()
or _r_e_q_u_i_r_e(), and FALSE otherwise. In a list
context, returns
($package,$filename,$line) = caller;
With EXPR, it returns some extra information that
the debugger uses to print a stack trace. The value
of EXPR indicates how many call frames to go back
before the current one.
chdir EXPR
Changes the working directory to EXPR, if possible.
If EXPR is omitted, changes to home directory.
Returns TRUE upon success, FALSE otherwise. See
Page 5 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
example under _d_i_e().
chmod LIST
Changes the permissions of a list of files. The
first element of the list must be the numerical
mode. Returns the number of files successfully
changed.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
chomp VARIABLE
chomp LIST
chomp This is a slightly safer version of chop (see
below). It removes any line ending that corresponds
to the current value of $/ (also known as
$INPUT_RECORD_SEPARATOR in the English module). It
returns the number of characters removed. It's
often used to remove the newline from the end of an
input record when you're worried that the final
record may be missing its newline. When in
paragraph mode ($/ = ""), it removes all trailing
newlines from the string. If VARIABLE is omitted,
it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
...
}
You can actually chomp anything that's an lvalue,
including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and
the total number of characters removed is returned.
chop VARIABLE
chop LIST
chop Chops off the last character of a string and returns
the character chopped. It's used primarily to
remove the newline from the end of an input record,
but is much more efficient than s/\n// because it
neither scans nor copies the string. If VARIABLE is
Page 6 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
omitted, chops $_. Example:
while (<>) {
chop; # avoid \n on last field
@array = split(/:/);
...
}
You can actually chop anything that's an lvalue,
including an assignment:
chop($cwd = `pwd`);
chop($answer = <STDIN>);
If you chop a list, each element is chopped. Only
the value of the last chop is returned.
chown LIST
Changes the owner (and group) of a list of files.
The first two elements of the list must be the
_N_U_M_E_R_I_C_A_L uid and gid, in that order. Returns the
number of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up non-numeric uids in
the passwd file:
print "User: ";
chop($user = <STDIN>);
print "Files: "
chop($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = <${pattern}>; # expand filenames
chown $uid, $gid, @ary;
chr NUMBER
Returns the character represented by that NUMBER in
the character set. For example, chr(65) is "A" in
ASCII.
chroot FILENAME
Does the same as the system call of that name. If
you don't know what it does, don't worry about it.
If FILENAME is omitted, does chroot to $_.
Page 7 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
close FILEHANDLE
Closes the file or pipe associated with the file
handle, returning TRUE only if stdio successfully
flushes buffers and closes the system file
descriptor. You don't have to close FILEHANDLE if
you are immediately going to do another open on it,
since open will close it for you. (See _o_p_e_n().)
However, an explicit close on an input file resets
the line counter ($.), while the implicit close done
by _o_p_e_n() does not. Also, closing a pipe will wait
for the process executing on the pipe to complete,
in case you want to look at the output of the pipe
afterwards. Closing a pipe explicitly also puts the
status value of the command into $?. Example:
open(OUTPUT, '|sort >foo'); # pipe to sort
... # print stuff to output
close OUTPUT; # wait for sort to finish
open(INPUT, 'foo'); # get sort's results
FILEHANDLE may be an expression whose value gives
the real filehandle name.
closedir DIRHANDLE
Closes a directory opened by _o_p_e_n_d_i_r().
connect SOCKET,NAME
Attempts to connect to a remote socket, just as the
connect system call does. Returns TRUE if it
succeeded, FALSE otherwise. NAME should be a
package address of the appropriate type for the
socket. See example in the _p_e_r_l_i_p_c manpage.
cos EXPR
Returns the cosine of EXPR (expressed in radians).
If EXPR is omitted takes cosine of $_.
crypt PLAINTEXT,SALT
Encrypts a string exactly like the _c_r_y_p_t(3) function
in the C library. Useful for checking the password
file for lousy passwords, amongst other things.
Only the guys wearing white hats should do this.
Here's an example that makes sure that whoever runs
this program knows their own password:
$pwd = (getpwuid($<))[1];
$salt = substr($pwd, 0, 2);
Page 8 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
system "stty -echo";
print "Password: ";
chop($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $salt) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever
asks you for it is unwise at best.
dbmclose ASSOC_ARRAY
[This function has been superseded by the _u_n_t_i_e()
function.]
Breaks the binding between a DBM file and an
associative array.
dbmopen ASSOC,DBNAME,MODE
[This function has been superseded by the _t_i_e()
function.]
This binds a _d_b_m(3) or _n_d_b_m(3) file to an
associative array. ASSOC is the name of the
associative array. (Unlike normal open, the first
argument is _N_O_T a filehandle, even though it looks
like one). DBNAME is the name of the database
(without the ._d_i_r or ._p_a_g extension). If the
database does not exist, it is created with
protection specified by MODE (as modified by the
_u_m_a_s_k()). If your system only supports the older
DBM functions, you may perform only one _d_b_m_o_p_e_n() in
your program. If your system has neither DBM nor
ndbm, calling _d_b_m_o_p_e_n() produces a fatal error.
If you don't have write access to the DBM file, you
can only read associative array variables, not set
them. If you want to test whether you can write,
either use file tests or try setting a dummy array
entry inside an _e_v_a_l(), which will trap the error.
Note that functions such as _k_e_y_s() and _v_a_l_u_e_s() may
return huge array values when used on large DBM
files. You may prefer to use the _e_a_c_h() function to
iterate over large DBM files. Example:
Page 9 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
defined EXPR
Returns a boolean value saying whether the lvalue
EXPR has a real value or not. Many operations
return the undefined value under exceptional
conditions, such as end of file, uninitialized
variable, system error and such. This function
allows you to distinguish between an undefined null
scalar and a defined null scalar with operations
that might return a real null string, such as
referencing elements of an array. You may also
check to see if arrays or subroutines exist. Use of
defined on predefined variables is not guaranteed to
produce intuitive results.
When used on a hash array element, it tells you
whether the value is defined, not whether the key
exists in the hash. Use _e_x_i_s_t_s() for that.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
eval '@foo = ()' if defined(@foo);
die "No XYZ package defined" unless defined %_XYZ;
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
See also _u_n_d_e_f().
delete EXPR
Deletes the specified value from its hash array.
Returns the deleted value, or the undefined value if
nothing was deleted. Deleting from $ENV{} modifies
the environment. Deleting from an array tied to a
DBM file deletes the entry from the DBM file. (But
deleting from a _t_i_e()d hash doesn't necessarily
return anything.)
The following deletes all the values of an
associative array:
Page 10 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
foreach $key (keys %ARRAY) {
delete $ARRAY{$key};
}
(But it would be faster to use the _u_n_d_e_f() command.)
Note that the EXPR can be arbitrarily complicated as
long as the final operation is a hash key lookup:
delete $ref->[$x][$y]{$key};
die LIST
Outside of an _e_v_a_l(), prints the value of LIST to
STDERR and exits with the current value of $!
(errno). If $! is 0, exits with the value of ($? >>
8) (`command` status). If ($? >> 8) is 0, exits
with 255. Inside an _e_v_a_l(), the error message is
stuffed into $@. and the _e_v_a_l() is terminated with
the undefined value.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the
current script line number and input line number (if
any) are also printed, and a newline is supplied.
Hint: sometimes appending ", stopped" to your
message will cause it to make better sense when the
string "at foo line 123" is appended. Suppose you
are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also _e_x_i_t() and _w_a_r_n().
do BLOCK
Not really a function. Returns the value of the
last command in the sequence of commands indicated
by BLOCK. When modified by a loop modifier,
executes the BLOCK once before testing the loop
condition. (On other statements the loop modifiers
test the conditional first.)
Page 11 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
do SUBROUTINE(LIST)
A deprecated form of subroutine call. See the
_p_e_r_l_s_u_b manpage.
do EXPR Uses the value of EXPR as a filename and executes
the contents of the file as a Perl script. Its
primary use is to include subroutines from a Perl
subroutine library.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient, more concise, keeps
track of the current filename for error messages,
and searches all the ----IIII libraries if the file isn't
in the current directory (see also the @INC array in
the section on _P_r_e_d_e_f_i_n_e_d _N_a_m_e_s in the _p_e_r_l_v_a_r
manpage). It's the same, however, in that it does
reparse the file every time you call it, so you
probably don't want to do this inside a loop.
Note that inclusion of library modules is better
done with the _u_s_e() and _r_e_q_u_i_r_e() operators.
dump LABEL
This causes an immediate core dump. Primarily this
is so that you can use the uuuunnnndddduuuummmmpppp program to turn
your core dump into an executable binary after
having initialized all your variables at the
beginning of the program. When the new binary is
executed it will begin by executing a goto LABEL
(with all the restrictions that goto suffers).
Think of it as a goto with an intervening core dump
and reincarnation. If LABEL is omitted, restarts
the program from the top. WARNING: any files opened
at the time of the dump will NOT be open any more
when the program is reincarnated, with possible
resulting confusion on the part of Perl. See also
----uuuu option in the _p_e_r_l_r_u_n manpage.
Example:
Page 12 (printed 6/30/95)
PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLFFFFUUUUNNNNCCCC((((1111))))
#!/usr/bin/perl
require 'getopt.pl';
require 'stat.pl';
%days = (
'Sun' => 1,
'Mon' => 2,
'Tue' => 3,
'Wed' => 4,
'Thu' => 5,
'Fri' => 6,
'Sat' => 7,
);
dump QUICKSTART if $ARGV[0] eq '-d';
QUICKSTART:
Getopt('f');
each ASSOC_ARRAY
Returns a 2 element array consisting of the key and
value for the next value of an associative array, so
that you can iterate over it. Entries are returned
in an apparently random order. When the array is
entirely read, a null array is returned (which when
assigned produces a FALSE (0) value). The next call
to _e_a_c_h() after that will start iterating again.
The iterator can be reset only by reading all the
elements from the array. You should not add
elements to an array while you're iterating over it.
There is a single iterator for each associative
array, shared by all _e_a_c_h(), _k_e_y_s() and _v_a_l_u_e_s()
function calls in the program. The following prints
out your environment like the _p_r_i_n_t_e_n_v(1) program,
only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also _k_e_y_s() and _v_a_l_u_e_s().
eof FILEHANDLE
eof Returns 1 if the next read on FILEHANDLE will return
end of file, or if FILEHANDLE is not open.
FILEHANDLE may be an expression whose value gives
the real filehandle name. (Note that this function
actually reads a character and then _u_n_g_e_t_c()s it, so
it is not very useful in an interactive context.)
An eof without an argument returns the eof status
for the last file read. Empty parentheses () may be
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used to indicate the pseudo file formed of the files
listed on the command line, i.e. eof() is
reasonable to use inside a while (<>) loop to detect
the end of only the last file. Use eof(ARGV) or eof
without the parentheses to test _E_A_C_H file in a while
(<>) loop. Examples:
# insert dashes just before last line of last file
while (<>) {
if (eof()) {
print "--------------\n";
}
print;
}
# reset line numbering on each input file
while (<>) {
print "$.\t$_";
if (eof) { # Not eof().
close(ARGV);
}
}
Practical hint: you almost never need to use eof in
Perl, because the input operators return undef when
they run out of data.
eval EXPR
eval BLOCK
EXPR is parsed and executed as if it were a little
Perl program. It is executed in the context of the
current Perl program, so that any variable settings,
subroutine or format definitions remain afterwards.
The value returned is the value of the last
expression evaluated, or a return statement may be
used, just as with subroutines.
If there is a syntax error or runtime error, or a
_d_i_e() statement is executed, an undefined value is
returned by _e_v_a_l(), and $@ is set to the error
message. If there was no error, $@ is guaranteed to
be a null string. If EXPR is omitted, evaluates $_.
The final semicolon, if any, may be omitted from the
expression.
Note that, since _e_v_a_l() traps otherwise-fatal
errors, it is useful for determining whether a
particular feature (such as _d_b_m_o_p_e_n() or _s_y_m_l_i_n_k())
is implemented. It is also Perl's exception
trapping mechanism, where the die operator is used
to raise exceptions.
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If the code to be executed doesn't vary, you may use
the eval-BLOCK form to trap run-time errors without
incurring the penalty of recompiling each time. The
error, if any, is still returned in $@. Examples:
# make divide-by-zero non-fatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = };
# a run-time error
eval '$answer ='; # sets $@
With an _e_v_a_l(), you should be especially careful to
remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++" # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the
code contained in the variable $x. (Although case 2
has misleading double quotes making the reader
wonder what else might be happening (nothing is).)
Cases 3 and 4 likewise behave in the same way: they
run the code <$x>, which does nothing at all. (Case
4 is preferred for purely visual reasons.) Case 5 is
a place where normally you _W_O_U_L_D like to use double
quotes, except that in particular situation, you can
just use symbolic references instead, as in case 6.
exec LIST
The _e_x_e_c() function executes a system command _A_N_D
_N_E_V_E_R _R_E_T_U_R_N_S. Use the _s_y_s_t_e_m() function if you
want it to return.
If there is more than one argument in LIST, or if
LIST is an array with more than one value, calls
_e_x_e_c_v_p(3) with the arguments in LIST. If there is
only one scalar argument, the argument is checked
for shell metacharacters. If there are any, the
entire argument is passed to /bin/sh -c for parsing.
If there are none, the argument is split into words
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and passed directly to _e_x_e_c_v_p(), which is more
efficient. Note: _e_x_e_c() (and _s_y_s_t_e_m(0) do not flush
your output buffer, so you may need to set $| to
avoid lost output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first
argument, but want to lie to the program you are
executing about its own name, you can specify the
program you actually want to run as an "indirect
object" (without a comma) in front of the LIST.
(This always forces interpretation of the LIST as a
multi-valued list, even if there is only a single
scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
exists EXPR
Returns TRUE if the specified hash key exists in its
hash array, even if the corresponding value is
undefined.
print "Exists\n" if exists $array{$key};
print "Defined\n" if defined $array{$key};
print "True\n" if $array{$key};
A hash element can only be TRUE if it's defined, and
defined if it exists, but the reverse doesn't
necessarily hold true.
Note that the EXPR can be arbitrarily complicated as
long as the final operation is a hash key lookup:
if (exists $ref->[$x][$y]{$key}) { ... }
exit EXPR
Evaluates EXPR and exits immediately with that
value. (Actually, it calls any defined END routines
first, but the END routines may not abort the exit.
Likewise any object destructors that need to be
called are called before exit.) Example:
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$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also _d_i_e(). If EXPR is omitted, exits with 0
status.
exp EXPR
Returns _e (the natural logarithm base) to the power
of EXPR. If EXPR is omitted, gives exp($_).
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the _f_c_n_t_l(2) function. You'll probably
have to say
use Fcntl;
first to get the correct function definitions.
Argument processing and value return works just like
_i_o_c_t_l() below. Note that _f_c_n_t_l() will produce a
fatal error if used on a machine that doesn't
implement _f_c_n_t_l(2). For example:
use Fcntl;
fcntl($filehandle, F_GETLK, $packed_return_buffer);
fileno FILEHANDLE
Returns the file descriptor for a filehandle. This
is useful for constructing bitmaps for _s_e_l_e_c_t(). If
FILEHANDLE is an expression, the value is taken as
the name of the filehandle.
flock FILEHANDLE,OPERATION
Calls _f_l_o_c_k(2) on FILEHANDLE. See the _f_l_o_c_k(_2)
manpage for definition of OPERATION. Returns TRUE
for success, FALSE on failure. Will produce a fatal
error if used on a machine that doesn't implement
_f_l_o_c_k(2). Here's a mailbox appender for BSD
systems.
$LOCK_SH = 1;
$LOCK_EX = 2;
$LOCK_NB = 4;
$LOCK_UN = 8;
sub lock {
flock(MBOX,$LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
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sub unlock {
flock(MBOX,$LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock();
print MBOX $msg,"\n\n";
unlock();
Note that _f_l_o_c_k() can't lock things over the
network. You need to do locking with _f_c_n_t_l() for
that.
fork Does a _f_o_r_k(2) system call. Returns the child pid
to the parent process and 0 to the child process, or
undef if the fork is unsuccessful. Note: unflushed
buffers remain unflushed in both processes, which
means you may need to set $| ($AUTOFLUSH in English)
or call the _a_u_t_o_f_l_u_s_h() FileHandle method to avoid
duplicate output.
If you _f_o_r_k() without ever waiting on your children,
you will accumulate zombies:
$SIG{'CHLD'} = sub { wait };
There's also the double-fork trick (error checking
on _f_o_r_k() returns omitted);
unless ($pid = fork) {
unless (fork) {
exec "what you really wanna do";
die "no exec";
# ... or ...
some_perl_code_here;
exit 0;
}
exit 0;
}
waitpid($pid,0);
formline PICTURE, LIST
This is an internal function used by formats, though
you may call it too. It formats (see the _p_e_r_l_f_o_r_m
manpage) a list of values according to the contents
of PICTURE, placing the output into the format
output accumulator, $^A. Eventually, when a _w_r_i_t_e()
is done, the contents of $^A are written to some
filehandle, but you could also read $^A yourself and
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then set $^A back to "". Note that a format
typically does one _f_o_r_m_l_i_n_e() per line of form, but
the _f_o_r_m_l_i_n_e() function itself doesn't care how many
newlines are embedded in the PICTURE. Be careful if
you put double quotes around the picture, since an
"@" character may be taken to mean the beginning of
an array name. _f_o_r_m_l_i_n_e() always returns TRUE.
getc FILEHANDLE
getc Returns the next character from the input file
attached to FILEHANDLE, or a null string at end of
file. If FILEHANDLE is omitted, reads from STDIN.
getlogin
Returns the current login from /_e_t_c/_u_t_m_p, if any.
If null, use _g_e_t_p_w_u_i_d().
$login = getlogin || (getpwuid($<))[0] || "Kilroy";
getpeername SOCKET
Returns the packed sockaddr address of other end of
the SOCKET connection.
# An internet sockaddr
$sockaddr = 'S n a4 x8';
$hersockaddr = getpeername(S);
($family, $port, $heraddr) = unpack($sockaddr,$hersockaddr);
getpgrp PID
Returns the current process group for the specified
PID, 0 for the current process. Will produce a
fatal error if used on a machine that doesn't
implement _g_e_t_p_g_r_p(2). If PID is omitted, returns
process group of current process.
getppid Returns the process id of the parent process.
getpriority WHICH,WHO
Returns the current priority for a process, a
process group, or a user. (See the _g_e_t_p_r_i_o_r_i_t_y(_2)
manpage.) Will produce a fatal error if used on a
machine that doesn't implement _g_e_t_p_r_i_o_r_i_t_y(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
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getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
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endservent
These routines perform the same functions as their
counterparts in the system library. Within a list
context, the return values from the various get
routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
Within a scalar context, you get the name, unless
the function was a lookup by name, in which case you
get the other thing, whatever it is. (If the entry
doesn't exist you get the undefined value.) For
example:
$uid = getpwnam
$name = getpwuid
$name = getpwent
$gid = getgrnam
$name = getgrgid
$name = getgrent
etc.
The $members value returned by _g_e_t_g_r*() is a space
separated list of the login names of the members of
the group.
For the _g_e_t_h_o_s_t*() functions, if the h_errno
variable is supported in C, it will be returned to
you via $? if the function call fails. The @addrs
value returned by a successful call is a list of the
raw addresses returned by the corresponding system
library call. In the Internet domain, each address
is four bytes long and you can unpack it by saying
something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
getsockname SOCKET
Returns the packed sockaddr address of this end of
the SOCKET connection.
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# An internet sockaddr
$sockaddr = 'S n a4 x8';
$mysockaddr = getsockname(S);
($family, $port, $myaddr) =
unpack($sockaddr,$mysockaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Returns the socket option requested, or undefined if
there is an error.
glob EXPR
Returns the value of EXPR with filename expansions
such as a shell would do. This is the internal
function implementing the <*.*> operator.
gmtime EXPR
Converts a time as returned by the time function to
a 9-element array with the time analyzed for the
Greenwich timezone. Typically used as follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All array elements are numeric, and come straight
out of a struct tm. In particular this means that
$mon has the range 0..11 and $wday has the range
0..6. If EXPR is omitted, does gmtime(time()).
goto LABEL
goto &NAME
The goto-LABEL form finds the statement labeled with
LABEL and resumes execution there. It may not be
used to go into any construct that requires
initialization, such as a subroutine or a foreach
loop. It also can't be used to go into a construct
that is optimized away. It can be used to go almost
anywhere else within the dynamic scope, including
out of subroutines, but it's usually better to use
some other construct such as last or die. The
author of Perl has never felt the need to use this
form of goto (in Perl, that is--C is another
matter).
The goto-&NAME form is highly magical, and
substitutes a call to the named subroutine for the
currently running subroutine. This is used by
AUTOLOAD subroutines that wish to load another
subroutine and then pretend that the other
subroutine had been called in the first place
(except that any modifications to @_ in the current
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subroutine are propagated to the other subroutine.)
After the goto, not even _c_a_l_l_e_r() will be able to
tell that this routine was called first.
grep BLOCK LIST
grep EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST
(locally setting $_ to each element) and returns the
list value consisting of those elements for which
the expression evaluated to TRUE. In a scalar
context, returns the number of times the expression
was TRUE.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that, since $_ is a reference into the list
value, it can be used to modify the elements of the
array. While this is useful and supported, it can
cause bizarre results if the LIST is not a named
array.
hex EXPR
Returns the decimal value of EXPR interpreted as an
hex string. (To interpret strings that might start
with 0 or 0x see _o_c_t().) If EXPR is omitted, uses
$_.
import There is no built-in _i_m_p_o_r_t() function. It is
merely an ordinary method subroutine defined (or
inherited) by modules that wish to export names to
another module. The _u_s_e() function calls the
_i_m_p_o_r_t() method for the package used. See also the
use entry elsewhere in this documentbelow and the
_p_e_r_l_m_o_d manpage.
index STR,SUBSTR,POSITION
index STR,SUBSTR
Returns the position of the first occurrence of
SUBSTR in STR at or after POSITION. If POSITION is
omitted, starts searching from the beginning of the
string. The return value is based at 0, or whatever
you've set the $[ variable to. If the substring is
not found, returns one less than the base,
ordinarily -1.
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int EXPR
Returns the integer portion of EXPR. If EXPR is
omitted, uses $_.
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the _i_o_c_t_l(2) function. You'll probably
have to say
require "ioctl.ph"; # probably /usr/local/lib/perl/ioctl.ph
first to get the correct function definitions. If
ioctl.ph doesn't exist or doesn't have the correct
definitions you'll have to roll your own, based on
your C header files such as <sys/ioctl.h>. (There
is a Perl script called hhhh2222pppphhhh that comes with the
Perl kit which may help you in this.) SCALAR will
be read and/or written depending on the FUNCTION--a
pointer to the string value of SCALAR will be passed
as the third argument of the actual ioctl call. (If
SCALAR has no string value but does have a numeric
value, that value will be passed rather than a
pointer to the string value. To guarantee this to
be TRUE, add a 0 to the scalar before using it.)
The _p_a_c_k() and _u_n_p_a_c_k() functions are useful for
manipulating the values of structures used by
_i_o_c_t_l(). The following example sets the erase
character to DEL.
require 'ioctl.ph';
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$TIOCGETP,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,$TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
The return value of ioctl (and fcntl) is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns TRUE on success and FALSE on
failure, yet you can still easily determine the
actual value returned by the operating system:
($retval = ioctl(...)) || ($retval = -1);
printf "System returned %d\n", $retval;
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join EXPR,LIST
Joins the separate strings of LIST or ARRAY into a
single string with fields separated by the value of
EXPR, and returns the string. Example:
$_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
See the split entry in the _p_e_r_l_f_u_n_c manpage.
keys ASSOC_ARRAY
Returns a normal array consisting of all the keys of
the named associative array. (In a scalar context,
returns the number of keys.) The keys are returned
in an apparently random order, but it is the same
order as either the _v_a_l_u_e_s() or _e_a_c_h() function
produces (given that the associative array has not
been modified). Here is yet another way to print
your environment:
@keys = keys %ENV;
@values = values %ENV;
while ($#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
kill LIST
Sends a signal to a list of processes. The first
element of the list must be the signal to send.
Returns the number of processes successfully
signaled.
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
Unlike in the shell, in Perl if the _S_I_G_N_A_L is
negative, it kills process groups instead of
processes. (On System V, a negative _P_R_O_C_E_S_S number
will also kill process groups, but that's not
portable.) That means you usually want to use
positive not negative signals. You may also use a
signal name in quotes.
last LABEL
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last The last command is like the break statement in C
(as used in loops); it immediately exits the loop in
question. If the LABEL is omitted, the command
refers to the innermost enclosing loop. The
continue block, if any, is not executed:
line: while (<STDIN>) {
last line if /^$/; # exit when done with header
...
}
lc EXPR Returns an lowercased version of EXPR. This is the
internal function implementing the \L escape in
double-quoted strings.
lcfirst EXPR
Returns the value of EXPR with the first character
lowercased. This is the internal function
implementing the \l escape in double-quoted strings.
length EXPR
Returns the length in characters of the value of
EXPR. If EXPR is omitted, returns length of $_.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename.
Returns 1 for success, 0 otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen system call
does. Returns TRUE if it succeeded, FALSE
otherwise. See example in the _p_e_r_l_i_p_c manpage.
local EXPR
In general, you should be using "my" instead of
"local", because it's faster and safer. Format
variables have to use "local" though, as do any
other variables whose local value must be visible to
called subroutines. This is known as dynamic
scoping. Lexical scoping is done with "my", which
works more like C's auto declarations.
A local modifies the listed variables to be local to
the enclosing block, subroutine, eval or "do". If
more than one value is listed, the list must be
placed in parens. All the listed elements must be
legal lvalues. This operator works by saving the
current values of those variables in LIST on a
hidden stack and restoring them upon exiting the
block, subroutine or eval. This means that called
subroutines can also reference the local variable,
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but not the global one. The LIST may be assigned to
if desired, which allows you to initialize your
local variables. (If no initializer is given for a
particular variable, it is created with an undefined
value.) Commonly this is used to name the
parameters to a subroutine. Examples:
sub RANGEVAL {
local($min, $max, $thunk) = @_;
local $result = '';
local $i;
# Presumably $thunk makes reference to $i
for ($i = $min; $i < $max; $i++) {
$result .= eval $thunk;
}
$result;
}
if ($sw eq '-v') {
# init local array with global array
local @ARGV = @ARGV;
unshift(@ARGV,'echo');
system @ARGV;
}
# @ARGV restored
# temporarily add to digits associative array
if ($base12) {
# (NOTE: not claiming this is efficient!)
local(%digits) = (%digits,'t',10,'e',11);
parse_num();
}
Note that _l_o_c_a_l() is a run-time command, and so gets
executed every time through a loop. In Perl 4 it
used up more stack storage each time until the loop
was exited. Perl 5 reclaims the space each time
through, but it's still more efficient to declare
your variables outside the loop.
When you assign to a localized EXPR, the local
doesn't change whether EXPR is viewed as a scalar or
an array. So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
both supply a list context to the righthand side,
while
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local $foo = <STDIN>;
supplies a scalar context.
localtime EXPR
Converts a time as returned by the time function to
a 9-element array with the time analyzed for the
local timezone. Typically used as follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All array elements are numeric, and come straight
out of a struct tm. In particular this means that
$mon has the range 0..11 and $wday has the range
0..6. If EXPR is omitted, does _l_o_c_a_l_t_i_m_e(time).
In a scalar context, prints out the _c_t_i_m_e(3) value:
$now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
See also the timelocal entry in the _p_e_r_l_m_o_d manpage
and the _s_t_r_f_t_i_m_e(3) function available via the POSIX
modulie.
log EXPR
Returns logarithm (base _e) of EXPR. If EXPR is
omitted, returns log of $_.
lstat FILEHANDLE
lstat EXPR
Does the same thing as the _s_t_a_t() function, but
stats a symbolic link instead of the file the
symbolic link points to. If symbolic links are
unimplemented on your system, a normal _s_t_a_t() is
done.
m// The match operator. See the _p_e_r_l_o_p manpage.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST
(locally setting $_ to each element) and returns the
list value composed of the results of each such
evaluation. Evaluates BLOCK or EXPR in a list
context, so each element of LIST may produce zero,
one, or more elements in the returned value.
@chars = map(chr, @nums);
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translates a list of numbers to the corresponding
characters. And
%hash = map {&key($_), $_} @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
$hash{&key($_)} = $_;
}
mkdir FILENAME,MODE
Creates the directory specified by FILENAME, with
permissions specified by MODE (as modified by
umask). If it succeeds it returns 1, otherwise it
returns 0 and sets $! (errno).
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl. If CMD is
&IPC_STAT, then ARG must be a variable which will
hold the returned msqid_ds structure. Returns like
ioctl: the undefined value for error, "0 but true"
for zero, or the actual return value otherwise.
msgget KEY,FLAGS
Calls the System V IPC function msgget. Returns the
message queue id, or the undefined value if there is
an error.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the
message MSG to the message queue ID. MSG must begin
with the long integer message type, which may be
created with pack("L", $type). Returns TRUE if
successful, or FALSE if there is an error.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a
message from message queue ID into variable VAR with
a maximum message size of SIZE. Note that if a
message is received, the message type will be the
first thing in VAR, and the maximum length of VAR is
SIZE plus the size of the message type. Returns
TRUE if successful, or FALSE if there is an error.
my EXPR A "my" declares the listed variables to be local
(lexically) to the enclosing block, subroutine, eval
or "do". If more than one value is listed, the list
must be placed in parens. All the listed elements
must be legal lvalues. Only alphanumeric
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identifiers may be lexically scoped--magical
builtins like $/ must be localized with "local"
instead. In particular, you're not allowed to say
my $_; # Illegal.
Unlike the "local" declaration, variables declared
with "my" are totally hidden from the outside world,
including any called subroutines (even if it's the
same subroutine--every call gets its own copy).
(An _e_v_a_l(), however, can see the lexical variables
of the scope it is being evaluated in so long as the
names aren't hidden by declarations within the
_e_v_a_l() itself. See the _p_e_r_l_r_e_f manpage.)
The EXPR may be assigned to if desired, which allows
you to initialize your variables. (If no
initializer is given for a particular variable, it
is created with an undefined value.) Commonly this
is used to name the parameters to a subroutine.
Examples:
sub RANGEVAL {
my($min, $max, $thunk) = @_;
my $result = '';
my $i;
# Presumably $thunk makes reference to $i
for ($i = $min; $i < $max; $i++) {
$result .= eval $thunk;
}
$result;
}
if ($sw eq '-v') {
# init my array with global array
my @ARGV = @ARGV;
unshift(@ARGV,'echo');
system @ARGV;
}
# Outer @ARGV again visible
When you assign to the EXPR, the "my" doesn't change
whether EXPR is viewed as a scalar or an array. So
my($foo) = <STDIN>;
my @FOO = <STDIN>;
both supply a list context to the righthand side,
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while
my $foo = <STDIN>;
supplies a scalar context.
Some users may wish to encourage the use of
lexically scoped variables. As an aid to catching
implicit references to package variables, if you say
use strict 'vars';
then any variable reference from there to the end of
the enclosing block must either refer to a lexical
variable, or must be fully qualified with the
package name. A compilation error results
otherwise. An inner block may countermand this with
"no strict 'vars'".
next LABEL
next The next command is like the continue statement in
C; it starts the next iteration of the loop:
line: while (<STDIN>) {
next line if /^#/; # discard comments
...
}
Note that if there were a continue block on the
above, it would get executed even on discarded
lines. If the LABEL is omitted, the command refers
to the innermost enclosing loop.
no Module LIST
See the "use" function, which "no" is the opposite
of.
oct EXPR
Returns the decimal value of EXPR interpreted as an
octal string. (If EXPR happens to start off with
0x, interprets it as a hex string instead.) The
following will handle decimal, octal, and hex in the
standard Perl or C notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_.
open FILEHANDLE,EXPR
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open FILEHANDLE
Opens the file whose filename is given by EXPR, and
associates it with FILEHANDLE. If FILEHANDLE is an
expression, its value is used as the name of the
real filehandle wanted. If EXPR is omitted, the
scalar variable of the same name as the FILEHANDLE
contains the filename. If the filename begins with
"<" or nothing, the file is opened for input. If
the filename begins with ">", the file is opened for
output. If the filename begins with ">>", the file
is opened for appending. (You can put a '+' in
front of the '>' or '<' to indicate that you want
both read and write access to the file.) If the
filename begins with "|", the filename is
interpreted as a command to which output is to be
piped, and if the filename ends with a "|", the
filename is interpreted as command which pipes input
to us. (You may not have a command that pipes both
in and out.) Opening '-' opens STDIN and opening
'>-' opens STDOUT. Open returns non-zero upon
success, the undefined value otherwise. If the open
involved a pipe, the return value happens to be the
pid of the subprocess. Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
open(article, "caesar <$article |"); # decrypt article
open(extract, "|sort >/tmp/Tmp$$"); # $$ is our process id
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
local($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
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while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
... # whatever
}
}
You may also, in the Bourne shell tradition, specify
an EXPR beginning with ">&", in which case the rest
of the string is interpreted as the name of a
filehandle (or file descriptor, if numeric) which is
to be duped and opened. You may use & after >, >>,
<, +>, +>> and +<. The mode you specify should
match the mode of the original filehandle. Here is
a script that saves, redirects, and restores STDOUT
and STDERR:
#!/usr/bin/perl
open(SAVEOUT, ">&STDOUT");
open(SAVEERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&SAVEOUT");
open(STDERR, ">&SAVEERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify "<&=N", where N is a number, then
Perl will do an equivalent of C's _f_d_o_p_e_n() of that
file descriptor. For example:
open(FILEHANDLE, "<&=$fd")
If you open a pipe on the command "-", i.e. either
"|-" or "-|", then there is an implicit fork done,
and the return value of open is the pid of the child
within the parent process, and 0 within the child
process. (Use _d_e_f_i_n_e_d($pid) to determine whether
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the open was successful.) The filehandle behaves
normally for the parent, but i/o to that filehandle
is piped from/to the STDOUT/STDIN of the child
process. In the child process the filehandle isn't
opened--i/o happens from/to the new STDOUT or STDIN.
Typically this is used like the normal piped open
when you want to exercise more control over just how
the pipe command gets executed, such as when you are
running setuid, and don't want to have to scan shell
commands for metacharacters. The following pairs
are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, "-|") || exec 'cat', '-n', $file;
Explicitly closing any piped filehandle causes the
parent process to wait for the child to finish, and
returns the status value in $?. Note: on any
operation which may do a fork, unflushed buffers
remain unflushed in both processes, which means you
may need to set $| to avoid duplicate output.
The filename that is passed to open will have
leading and trailing whitespace deleted. In order
to open a file with arbitrary weird characters in
it, it's necessary to protect any leading and
trailing whitespace thusly:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by
_r_e_a_d_d_i_r(), _t_e_l_l_d_i_r(), _s_e_e_k_d_i_r(), _r_e_w_i_n_d_d_i_r() and
_c_l_o_s_e_d_i_r(). Returns TRUE if successful. DIRHANDLEs
have their own namespace separate from FILEHANDLEs.
ord EXPR
Returns the numeric ascii value of the first
character of EXPR. If EXPR is omitted, uses $_.
pack TEMPLATE,LIST
Takes an array or list of values and packs it into a
binary structure, returning the string containing
the structure. The TEMPLATE is a sequence of
characters that give the order and type of values,
as follows:
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A An ascii string, will be space padded.
a An ascii string, will be null padded.
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char value.
C An unsigned char value.
s A signed short value.
S An unsigned short value.
i A signed integer value.
I An unsigned integer value.
l A signed long value.
L An unsigned long value.
n A short in "network" order.
N A long in "network" order.
v A short in "VAX" (little-endian) order.
V A long in "VAX" (little-endian) order.
f A single-precision float in the native format.
d A double-precision float in the native format.
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
Each letter may optionally be followed by a number
which gives a repeat count. With all types except
"a", "A", "b", "B", "h" and "H", and "P" the pack
function will gobble up that many values from the
LIST. A * for the repeat count means to use however
many items are left. The "a" and "A" types gobble
just one value, but pack it as a string of length
count, padding with nulls or spaces as necessary.
(When unpacking, "A" strips trailing spaces and
nulls, but "a" does not.) Likewise, the "b" and "B"
fields pack a string that many bits long. The "h"
and "H" fields pack a string that many nybbles long.
The "P" packs a pointer to a structure of the size
indicated by the length. Real numbers (floats and
doubles) are in the native machine format only; due
to the multiplicity of floating formats around, and
the lack of a standard "network" representation, no
facility for interchange has been made. This means
that packed floating point data written on one
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machine may not be readable on another - even if
both use IEEE floating point arithmetic (as the
endian-ness of the memory representation is not part
of the IEEE spec). Note that Perl uses doubles
internally for all numeric calculation, and
converting from double into float and thence back to
double again will lose precision (i.e. unpack("f",
pack("f", $foo)) will not in general equal $foo).
Examples:
$foo = pack("cccc",65,66,67,68);
# foo eq "ABCD"
$foo = pack("c4",65,66,67,68);
# same thing
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
The same template may generally also be used in the
unpack function.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the
corresponding system call. Note that if you set up
a loop of piped processes, deadlock can occur unless
you are very careful. In addition, note that Perl's
pipes use stdio buffering, so you may need to set $|
to flush your WRITEHANDLE after each command,
depending on the application.
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pop ARRAY
Pops and returns the last value of the array,
shortening the array by 1. Has a similar effect to
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the
undefined value.
pos SCALAR
Returns the offset of where the last m//g search
left off for the variable in question. May be
modified to change that offset.
print FILEHANDLE LIST
print LIST
print Prints a string or a comma-separated list of
strings. Returns non-zero if successful.
FILEHANDLE may be a scalar variable name, in which
case the variable contains the name of the
filehandle, thus introducing one level of
indirection. (NOTE: If FILEHANDLE is a variable and
the next token is a term, it may be misinterpreted
as an operator unless you interpose a + or put
parens around the arguments.) If FILEHANDLE is
omitted, prints by default to standard output (or to
the last selected output channel--see _s_e_l_e_c_t()). If
LIST is also omitted, prints $_ to STDOUT. To set
the default output channel to something other than
STDOUT use the select operation. Note that, because
print takes a LIST, anything in the LIST is
evaluated in a list context, and any subroutine that
you call will have one or more of its expressions
evaluated in a list context. Also be careful not to
follow the print keyword with a left parenthesis
unless you want the corresponding right parenthesis
to terminate the arguments to the print--interpose a
+ or put parens around all the arguments.
printf FILEHANDLE LIST
printf LIST
Equivalent to a "print FILEHANDLE _s_p_r_i_n_t_f(LIST)".
The first argument of the list will be interpreted
as the printf format.
push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of
LIST onto the end of ARRAY. The length of ARRAY
increases by the length of LIST. Has the same
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effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the new number of
elements in the array.
q/STRING/
qq/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See the _p_e_r_l_o_p manpage.
quotemeta EXPR
Returns the value of EXPR with with all regular
expression metacharacters backslashed. This is the
internal function implementing the \Q escape in
double-quoted strings.
rand EXPR
rand Returns a random fractional number between 0 and the
value of EXPR. (EXPR should be positive.) If EXPR
is omitted, returns a value between 0 and 1. This
function produces repeatable sequences unless
_s_r_a_n_d() is invoked. See also _s_r_a_n_d().
(Note: if your rand function consistently returns
numbers that are too large or too small, then your
version of Perl was probably compiled with the wrong
number of RANDBITS. As a workaround, you can
usually multiply EXPR by the correct power of 2 to
get the range you want. This will make your script
unportable, however. It's better to recompile if
you can.)
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable
SCALAR from the specified FILEHANDLE. Returns the
number of bytes actually read, or undef if there was
an error. SCALAR will be grown or shrunk to the
length actually read. An OFFSET may be specified to
place the read data at some other place than the
beginning of the string. This call is actually
implemented in terms of stdio's fread call. To get
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a true read system call, see _s_y_s_r_e_a_d().
readdir DIRHANDLE
Returns the next directory entry for a directory
opened by _o_p_e_n_d_i_r(). If used in a list context,
returns all the rest of the entries in the
directory. If there are no more entries, returns an
undefined value in a scalar context or a null list
in a list context.
readlink EXPR
Returns the value of a symbolic link, if symbolic
links are implemented. If not, gives a fatal error.
If there is some system error, returns the undefined
value and sets $! (errno). If EXPR is omitted, uses
$_.
recv SOCKET,SCALAR,LEN,FLAGS
Receives a message on a socket. Attempts to receive
LENGTH bytes of data into variable SCALAR from the
specified SOCKET filehandle. Actually does a C
_r_e_c_v_f_r_o_m(), so that it can returns the address of
the sender. Returns the undefined value if there's
an error. SCALAR will be grown or shrunk to the
length actually read. Takes the same flags as the
system call of the same name.
redo LABEL
redo The redo command restarts the loop block without
evaluating the conditional again. The continue
block, if any, is not executed. If the LABEL is
omitted, the command refers to the innermost
enclosing loop. This command is normally used by
programs that want to lie to themselves about what
was just input:
Page 39 (printed 6/30/95)
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# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
line: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front{|;
redo line;
}
}
}
print;
}
ref EXPR
Returns a TRUE value if EXPR is a reference, FALSE
otherwise. The value returned depends on the type
of thing the reference is a reference to. Builtin
types include:
REF
SCALAR
ARRAY
HASH
CODE
GLOB
If the referenced object has been blessed into a
package, then that package name is returned instead.
You can think of _r_e_f() as a _t_y_p_e_o_f() operator.
if (ref($r) eq "HASH") {
print "r is a reference to an associative array.\n";
}
if (!ref ($r) {
print "r is not a reference at all.\n";
}
See also the _p_e_r_l_r_e_f manpage.
rename OLDNAME,NEWNAME
Changes the name of a file. Returns 1 for success,
0 otherwise. Will not work across filesystem
boundaries.
require EXPR
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require Demands some semantics specified by EXPR, or by $_
if EXPR is not supplied. If EXPR is numeric,
demands that the current version of Perl ($] or
$PERL_VERSION) be equal or greater than EXPR.
Otherwise, demands that a library file be included
if it hasn't already been included. The file is
included via the do-FILE mechanism, which is
essentially just a variety of _e_v_a_l(). Has semantics
similar to the following subroutine:
sub require {
local($filename) = @_;
return 1 if $INC{$filename};
local($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
die $@ if $@;
die "$filename did not return true value" unless $result;
$INC{$filename} = $realfilename;
$result;
}
Note that the file will not be included twice under
the same specified name. The file must return TRUE
as the last statement to indicate successful
execution of any initialization code, so it's
customary to end such a file with "1;" unless you're
sure it'll return TRUE otherwise. But it's better
just to put the "1;", in case you add more
statements.
If EXPR is a bare word, the require assumes a "._p_m"
extension for you, to make it easy to load standard
modules. This form of loading of modules does not
risk altering your namespace.
For a yet more powerful import facility, see the the
use() entry elsewhere in this documentbelow, and
also the _p_e_r_l_m_o_d manpage.
reset EXPR
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reset Generally used in a continue block at the end of a
loop to clear variables and reset ?? searches so
that they work again. The expression is interpreted
as a list of single characters (hyphens allowed for
ranges). All variables and arrays beginning with
one of those letters are reset to their pristine
state. If the expression is omitted, one-match
searches (?pattern?) are reset to match again. Only
resets variables or searches in the current package.
Always returns 1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?? searches
Resetting "A-Z" is not recommended since you'll wipe
out your ARGV and ENV arrays. Only resets package
variables--lexical variables are unaffected, but
they clean themselves up on scope exit anyway, so
anymore you probably want to use them instead. See
the my entry elsewhere in this document.
return LIST
Returns from a subroutine or eval with the value
specified. (Note that in the absence of a return a
subroutine or eval will automatically return the
value of the last expression evaluated.)
reverse LIST
In a list context, returns a list value consisting
of the elements of LIST in the opposite order. In a
scalar context, returns a string value consisting of
the bytes of the first element of LIST in the
opposite order.
rewinddir DIRHANDLE
Sets the current position to the beginning of the
directory for the _r_e_a_d_d_i_r() routine on DIRHANDLE.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like index except that it returns the
position of the LAST occurrence of SUBSTR in STR.
If POSITION is specified, returns the last
occurrence at or before that position.
rmdir FILENAME
Deletes the directory specified by FILENAME if it is
empty. If it succeeds it returns 1, otherwise it
returns 0 and sets $! (errno). If FILENAME is
omitted, uses $_.
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s/// The substitution operator. See the _p_e_r_l_o_p manpage.
scalar EXPR
Forces EXPR to be interpreted in a scalar context
and returns the value of EXPR.
seek FILEHANDLE,POSITION,WHENCE
Randomly positions the file pointer for FILEHANDLE,
just like the _f_s_e_e_k() call of stdio. FILEHANDLE may
be an expression whose value gives the name of the
filehandle. The values for WHENCE are 0 to set the
file pointer to POSITION, 1 to set the it to current
plus POSITION, and 2 to set it to EOF plus offset.
You may use the values SEEK_SET, SEEK_CUR, and
SEEK_END for this is usin the POSIX module. Returns
1 upon success, 0 otherwise.
seekdir DIRHANDLE,POS
Sets the current position for the _r_e_a_d_d_i_r() routine
on DIRHANDLE. POS must be a value returned by
_t_e_l_l_d_i_r(). Has the same caveats about possible
directory compaction as the corresponding system
library routine.
select FILEHANDLE
select Returns the currently selected filehandle. Sets the
current default filehandle for output, if FILEHANDLE
is supplied. This has two effects: first, a write
or a print without a filehandle will default to this
FILEHANDLE. Second, references to variables related
to output will refer to this output channel. For
example, if you have to set the top of form format
for more than one output channel, you might do the
following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives
the name of the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
With Perl 5, filehandles are objects with methods,
and the last example is preferably written
use FileHandle;
STDERR->autoflush(1);
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select RBITS,WBITS,EBITS,TIMEOUT
This calls the select _s_y_s_t_e_m(2) call with the
bitmasks specified, which can be constructed using
_f_i_l_e_n_o() and _v_e_c(), along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might
wish to write a subroutine:
sub fhbits {
local(@fhlist) = split(' ',$_[0]);
local($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = &fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready:
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Any of the bitmasks can also be undef. The timeout,
if specified, is in seconds, which may be
fractional. Note: not all implementations are
capable of returning the $timeleft. If not, they
always return $timeleft equal to the supplied
$timeout.
You can effect a 250 microsecond sleep this way:
select(undef, undef, undef, 0.25);
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function semctl. If CMD is
&IPC_STAT or &GETALL, then ARG must be a variable
which will hold the returned semid_ds structure or
semaphore value array. Returns like ioctl: the
undefined value for error, "0 but true" for zero, or
the actual return value otherwise.
Page 44 (printed 6/30/95)
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semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns the
semaphore id, or the undefined value if there is an
error.
semop KEY,OPSTRING
Calls the System V IPC function semop to perform
semaphore operations such as signaling and waiting.
OPSTRING must be a packed array of semop structures.
Each semop structure can be generated with
pack("sss", $semnum, $semop, $semflag). The number
of semaphore operations is implied by the length of
OPSTRING. Returns TRUE if successful, or FALSE if
there is an error. As an example, the following
code waits on semaphore $semnum of semaphore id
$semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with "1".
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Takes the same flags
as the system call of the same name. On unconnected
sockets you must specify a destination to send TO,
in which case it does a C _s_e_n_d_t_o(). Returns the
number of characters sent, or the undefined value if
there is an error.
setpgrp PID,PGRP
Sets the current process group for the specified
PID, 0 for the current process. Will produce a
fatal error if used on a machine that doesn't
implement _s_e_t_p_g_r_p(2).
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process
group, or a user. (See _L_s_e_t_p_r_i_o_r_i_t_y(2)>.) Will
produce a fatal error if used on a machine that
doesn't implement _s_e_t_p_r_i_o_r_i_t_y(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined
if there is an error. OPTVAL may be specified as
undef if you don't want to pass an argument.
shift ARRAY
Page 45 (printed 6/30/95)
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shift Shifts the first value of the array off and returns
it, shortening the array by 1 and moving everything
down. If there are no elements in the array,
returns the undefined value. If ARRAY is omitted,
shifts the @ARGV array in the main program, and the
@_ array in subroutines. (This is determined
lexically.) See also _u_n_s_h_i_f_t(), _p_u_s_h(), and _p_o_p().
_S_h_i_f_t() and _u_n_s_h_i_f_t() do the same thing to the left
end of an array that _p_u_s_h() and _p_o_p() do to the
right end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. If CMD is
&IPC_STAT, then ARG must be a variable which will
hold the returned shmid_ds structure. Returns like
ioctl: the undefined value for error, "0 but true"
for zero, or the actual return value otherwise.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the
shared memory segment id, or the undefined value if
there is an error.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment
ID starting at position POS for size SIZE by
attaching to it, copying in/out, and detaching from
it. When reading, VAR must be a variable which will
hold the data read. When writing, if STRING is too
long, only SIZE bytes are used; if STRING is too
short, nulls are written to fill out SIZE bytes.
Return TRUE if successful, or FALSE if there is an
error.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner
indicated by HOW, which has the same interpretation
as in the system call of the same name.
sin EXPR
Returns the sine of EXPR (expressed in radians). If
EXPR is omitted, returns sine of $_.
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or
forever if no EXPR. May be interrupted by sending
the process a SIGALRM. Returns the number of
seconds actually slept. You probably cannot mix
_a_l_a_r_m() and _s_l_e_e_p() calls, since _s_l_e_e_p() is often
Page 46 (printed 6/30/95)
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implemented using _a_l_a_r_m().
On some older systems, it may sleep up to a full
second less than what you requested, depending on
how it counts seconds. Most modern systems always
sleep the full amount.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it
to filehandle SOCKET. DOMAIN, TYPE and PROTOCOL are
specified the same as for the system call of the
same name. You should "use Socket;" first to get
the proper definitions imported. See the example in
the _p_e_r_l_i_p_c manpage.
socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified
domain, of the specified type. DOMAIN, TYPE and
PROTOCOL are specified the same as for the system
call of the same name. If unimplemented, yields a
fatal error. Returns TRUE if successful.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
Sorts the LIST and returns the sorted list value.
Nonexistent values of arrays are stripped out. If
SUBNAME or BLOCK is omitted, sorts in standard
string comparison order. If SUBNAME is specified,
it gives the name of a subroutine that returns an
integer less than, equal to, or greater than 0,
depending on how the elements of the array are to be
ordered. (The <=> and cmp operators are extremely
useful in such routines.) SUBNAME may be a scalar
variable name, in which case the value provides the
name of the subroutine to use. In place of a
SUBNAME, you can provide a BLOCK as an anonymous,
in-line sort subroutine.
In the interests of efficiency the normal calling
code for subroutines is bypassed, with the following
effects: the subroutine may not be a recursive
subroutine, and the two elements to be compared are
passed into the subroutine not via @_ but as $a and
$b (see example below). They are passed by
reference, so don't modify $a and $b.
Examples:
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# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming integers
}
@sortedclass = sort byage @class;
sub backwards { $b cmp $a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
Removes the elements designated by OFFSET and LENGTH
from an array, and replaces them with the elements
of LIST, if any. Returns the elements removed from
the array. The array grows or shrinks as necessary.
If LENGTH is omitted, removes everything from OFFSET
onward. The following equivalencies hold (assuming
$[ == 0):
push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$x] = $y splice(@a,$x,1,$y);
Example, assuming array lengths are passed before
Page 48 (printed 6/30/95)
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arrays:
sub aeq { # compare two list values
local(@a) = splice(@_,0,shift);
local(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split Splits a string into an array of strings, and
returns it.
If not in a list context, returns the number of
fields found and splits into the @_ array. (In a
list context, you can force the split into @_ by
using ?? as the pattern delimiters, but it still
returns the array value.) The use of implicit split
to @_ is deprecated, however.
If EXPR is omitted, splits the $_ string. If
PATTERN is also omitted, splits on whitespace (/[
\t\n]+/). Anything matching PATTERN is taken to be
a delimiter separating the fields. (Note that the
delimiter may be longer than one character.) If
LIMIT is specified and is not negative, splits into
no more than that many fields (though it may split
into fewer). If LIMIT is unspecified, trailing null
fields are stripped (which potential users of _p_o_p()
would do well to remember). If LIMIT is negative,
it is treated as if an arbitrarily large LIMIT had
been specified.
A pattern matching the null string (not to be
confused with a null pattern C<//., which is just
one member of the set of patterns matching a null
string) will split the value of EXPR into separate
characters at each point it matches that way. For
example:
print join(':', split(/ */, 'hi there'));
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produces the output 'h:i:t:h:e:r:e'.
The LIMIT parameter can be used to partially split a
line
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, Perl
supplies a LIMIT one larger than the number of
variables in the list, to avoid unnecessary work.
For the list above LIMIT would have been 4 by
default. In time critical applications it behooves
you not to split into more fields than you really
need.
If the PATTERN contains parentheses, additional
array elements are created from each matching
substring in the delimiter.
split(/([,-])/, "1-10,20");
produces the list value
(1, '-', 10, ',', 20)
The pattern /PATTERN/ may be replaced with an
expression to specify patterns that vary at runtime.
(To do runtime compilation only once, use
/$variable/o.) As a special case, specifying a
space (' ') will split on white space just as split
with no arguments does, but leading white space does
_N_O_T produce a null first field. Thus, split(' ')
can be used to emulate aaaawwwwkkkk's default behavior,
whereas split(/ /) will give you as many null
initial fields as there are leading spaces.
Example:
open(passwd, '/etc/passwd');
while (<passwd>) {
($login, $passwd, $uid, $gid, $gcos, $home, $shell) = split(/:/);
...
}
(Note that $shell above will still have a newline on
it. See the chop, chomp, and join entries
elsewhere in this document.)
sprintf FORMAT,LIST
Returns a string formatted by the usual printf
conventions of the C language. (The * character for
an indirectly specified length is not supported, but
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you can get the same effect by interpolating a
variable into the pattern.)
sqrt EXPR
Return the square root of EXPR. If EXPR is omitted,
returns square root of $_.
srand EXPR
Sets the random number seed for the rand operator.
If EXPR is omitted, does srand(time). Of course,
you'd need something much more random than that for
cryptographic purposes, since it's easy to guess the
current time. Checksumming the compressed output of
rapidly changing operating system status programs is
the usual method. Examples are posted regularly to
comp.security.unix.
stat FILEHANDLE
stat EXPR
Returns a 13-element array giving the status info
for a file, either the file opened via FILEHANDLE,
or named by EXPR. Returns a null list if the stat
fails. Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
If stat is passed the special filehandle consisting
of an underline, no stat is done, but the current
contents of the stat structure from the last stat or
filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This only works on machines for which the device
number is negative under NFS.)
study SCALAR
study Takes extra time to study SCALAR ($_ if unspecified)
in anticipation of doing many pattern matches on the
string before it is next modified. This may or may
not save time, depending on the nature and number of
patterns you are searching on, and on the
distribution of character frequencies in the string
to be searched--you probably want to compare
runtimes with and without it to see which runs
faster. Those loops which scan for many short
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constant strings (including the constant parts of
more complex patterns) will benefit most. You may
have only one study active at a time--if you study a
different scalar the first is "unstudied". (The way
study works is this: a linked list of every
character in the string to be searched is made, so
we know, for example, where all the 'k' characters
are. From each search string, the rarest character
is selected, based on some static frequency tables
constructed from some C programs and English text.
Only those places that contain this "rarest"
character are examined.)
For example, here is a loop which inserts index
producing entries before any line containing a
certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
...
print;
}
In searching for /\bfoo\b/, only those locations in
$_ that contain "f" will be looked at, because "f"
is rarer than "o". In general, this is a big win
except in pathological cases. The only question is
whether it saves you more time than it took to build
the linked list in the first place.
Note that if you have to look for strings that you
don't know till runtime, you can build an entire
loop as a string and eval that to avoid recompiling
all your patterns all the time. Together with
undefining $/ to input entire files as one record,
this can be very fast, often faster than specialized
programs like _f_g_r_e_p(1). The following scans a list
of files (@files) for a list of words (@words), and
prints out the names of those files that contain a
match:
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$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delim
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
substr EXPR,OFFSET,LEN
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it.
First character is at offset 0, or whatever you've
set $[ to. If OFFSET is negative, starts that far
from the end of the string. If LEN is omitted,
returns everything to the end of the string. You
can use the _s_u_b_s_t_r() function as an lvalue, in which
case EXPR must be an lvalue. If you assign
something shorter than LEN, the string will shrink,
and if you assign something longer than LEN, the
string will grow to accommodate it. To keep the
string the same length you may need to pad or chop
your value using _s_p_r_i_n_t_f().
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the
old filename. Returns 1 for success, 0 otherwise.
On systems that don't support symbolic links,
produces a fatal error at run time. To check for
that, use eval:
$symlink_exists = (eval 'symlink("","");', $@ eq '');
syscall LIST
Calls the system call specified as the first element
of the list, passing the remaining elements as
arguments to the system call. If unimplemented,
produces a fatal error. The arguments are
interpreted as follows: if a given argument is
numeric, the argument is passed as an int. If not,
the pointer to the string value is passed. You are
responsible to make sure a string is pre-extended
long enough to receive any result that might be
written into a string. If your integer arguments
are not literals and have never been interpreted in
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a numeric context, you may need to add 0 to them to
force them to look like numbers.
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
Note that Perl only supports passing of up to 14
arguments to your system call, which in practice
should usually suffice.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable
SCALAR from the specified FILEHANDLE, using the
system call _r_e_a_d(2). It bypasses stdio, so mixing
this with other kinds of reads may cause confusion.
Returns the number of bytes actually read, or undef
if there was an error. SCALAR will be grown or
shrunk to the length actually read. An OFFSET may
be specified to place the read data at some other
place than the beginning of the string.
system LIST
Does exactly the same thing as "exec LIST" except
that a fork is done first, and the parent process
waits for the child process to complete. Note that
argument processing varies depending on the number
of arguments. The return value is the exit status
of the program as returned by the _w_a_i_t() call. To
get the actual exit value divide by 256. See also
the exec entry elsewhere in this document.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
Attempts to write LENGTH bytes of data from variable
SCALAR to the specified FILEHANDLE, using the system
call _w_r_i_t_e(2). It bypasses stdio, so mixing this
with prints may cause confusion. Returns the number
of bytes actually written, or undef if there was an
error. An OFFSET may be specified to place the read
data at some other place than the beginning of the
string.
tell FILEHANDLE
tell Returns the current file position for FILEHANDLE.
FILEHANDLE may be an expression whose value gives
the name of the actual filehandle. If FILEHANDLE is
omitted, assumes the file last read.
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telldir DIRHANDLE
Returns the current position of the _r_e_a_d_d_i_r()
routines on DIRHANDLE. Value may be given to
_s_e_e_k_d_i_r() to access a particular location in a
directory. Has the same caveats about possible
directory compaction as the corresponding system
library routine.
tie VARIABLE,PACKAGENAME,LIST
This function binds a variable to a package that
will provide the implementation for the variable.
VARIABLE is the name of the variable to be
enchanted. PACKAGENAME is the name of a package
implementing objects of correct type. Any
additional arguments are passed to the "new" method
of the package. Typically these are arguments such
as might be passed to the _d_b_m__o_p_e_n() function of C.
Note that functions such as _k_e_y_s() and _v_a_l_u_e_s() may
return huge array values when used on large DBM
files. You may prefer to use the _e_a_c_h() function to
iterate over large DBM files. Example:
# print out history file offsets
tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A package implementing an associative array should
have the following methods:
TIEHASH objectname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
DELETE this, key
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
A package implementing an ordinary array should have
the following methods:
TIEARRAY objectname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
[others TBD]
A package implementing a scalar should have the
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following methods:
TIESCALAR objectname, LIST
DESTROY this
FETCH this,
STORE this, value
time Returns the number of non-leap seconds since
00:00:00 UTC, January 1, 1970. Suitable for feeding
to _g_m_t_i_m_e() and _l_o_c_a_l_t_i_m_e().
times Returns a four-element array giving the user and
system times, in seconds, for this process and the
children of this process.
($user,$system,$cuser,$csystem) = times;
tr/// The translation operator. See the _p_e_r_l_o_p manpage.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by
EXPR, to the specified length. Produces a fatal
error if truncate isn't implemented on your system.
uc EXPR Returns an uppercased version of EXPR. This is the
internal function implementing the \U escape in
double-quoted strings.
ucfirst EXPR
Returns the value of EXPR with the first character
uppercased. This is the internal function
implementing the \u escape in double-quoted strings.
umask EXPR
umask Sets the umask for the process and returns the old
one. If EXPR is omitted, merely returns current
umask.
undef EXPR
undef Undefines the value of EXPR, which must be an
lvalue. Use only on a scalar value, an entire
array, or a subroutine name (using "&"). (Using
_u_n_d_e_f() will probably not do what you expect on most
predefined variables or DBM list values, so don't do
that.) Always returns the undefined value. You can
omit the EXPR, in which case nothing is undefined,
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but you still get an undefined value that you could,
for instance, return from a subroutine. Examples:
undef $foo;
undef $bar{'blurfl'};
undef @ary;
undef %assoc;
undef &mysub;
return (wantarray ? () : undef) if $they_blew_it;
unlink LIST
Deletes a list of files. Returns the number of
files successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: unlink will not delete directories unless you
are superuser and the ----UUUU flag is supplied to Perl.
Even if these conditions are met, be warned that
unlinking a directory can inflict damage on your
filesystem. Use rmdir instead.
unpack TEMPLATE,EXPR
Unpack does the reverse of pack: it takes a string
representing a structure and expands it out into a
list value, returning the array value. (In a scalar
context, it merely returns the first value
produced.) The TEMPLATE has the same format as in
the pack function. Here's a subroutine that does
substring:
sub substr {
local($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()
In addition, you may prefix a field with a %<number>
to indicate that you want a <number>-bit checksum of
the items instead of the items themselves. Default
is a 16-bit checksum. For example, the following
computes the same number as the System V sum
program:
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while (<>) {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;
The following efficiently counts the number of set
bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
untie VARIABLE
Breaks the binding between a variable and a package.
(See _t_i_e().)
unshift ARRAY,LIST
Does the opposite of a shift. Or the opposite of a
push, depending on how you look at it. Prepends
list to the front of the array, and returns the new
number of elements in the array.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at
a time, so the prepended elements stay in the same
order. Use reverse to do the reverse.
use Module LIST
use Module
Imports some semantics into the current package from
the named module, generally by aliasing certain
subroutine or variable names into your package. It
is exactly equivalent to
BEGIN { require Module; import Module LIST; }
If you don't want your namespace altered, use
require instead.
The BEGIN forces the require and import to happen at
compile time. The require makes sure the module is
loaded into memory if it hasn't been yet. The
import is not a builtin--it's just an ordinary
static method call into the "Module" package to tell
the module to import the list of features back into
the current package. The module can implement its
import method any way it likes, though most modules
just choose to derive their import method via
inheritance from the Exporter class that is defined
in the Exporter module.
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Because this is a wide-open interface, pragmas
(compiler directives) are also implemented this way.
Currently implemented pragmas are:
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
These pseudomodules import semantics into the
current block scope, unlike ordinary modules, which
import symbols into the current package (which are
effective through the end of the file).
There's a corresponding "no" command that unimports
meanings imported by use.
no integer;
no strict 'refs';
See the _p_e_r_l_m_o_d manpage for a list of standard
modules and pragmas.
utime LIST
Changes the access and modification times on each
file of a list of files. The first two elements of
the list must be the NUMERICAL access and
modification times, in that order. Returns the
number of files successfully changed. The inode
modification time of each file is set to the current
time. Example of a "touch" command:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
values ASSOC_ARRAY
Returns a normal array consisting of all the values
of the named associative array. (In a scalar
context, returns the number of values.) The values
are returned in an apparently random order, but it
is the same order as either the _k_e_y_s() or _e_a_c_h()
function would produce on the same array. See also
_k_e_y_s() and _e_a_c_h().
vec EXPR,OFFSET,BITS
Treats a string as a vector of unsigned integers,
and returns the value of the bitfield specified.
May also be assigned to. BITS must be a power of
two from 1 to 32.
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Vectors created with _v_e_c() can also be manipulated
with the logical operators |, & and ^, which will
assume a bit vector operation is desired when both
operands are strings.
To transform a bit vector into a string or array of
0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used
in place of the *.
wait Waits for a child process to terminate and returns
the pid of the deceased process, or -1 if there are
no child processes. The status is returned in $?.
waitpid PID,FLAGS
Waits for a particular child process to terminate
and returns the pid of the deceased process, or -1
if there is no such child process. The status is
returned in $?. If you say
use POSIX "wait_h";
...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process.
Non-blocking wait is only available on machines
supporting either the _w_a_i_t_p_i_d(2) or _w_a_i_t_4(2) system
calls. However, waiting for a particular pid with
FLAGS of 0 is implemented everywhere. (Perl
emulates the system call by remembering the status
values of processes that have exited but have not
been harvested by the Perl script yet.)
wantarray
Returns TRUE if the context of the currently
executing subroutine is looking for a list value.
Returns FALSE if the context is looking for a
scalar.
return wantarray ? () : undef;
warn LIST
Produces a message on STDERR just like _d_i_e(), but
doesn't exit or throw an exception.
write FILEHANDLE
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write EXPR
write Writes a formatted record (possibly multi-line) to
the specified file, using the format associated with
that file. By default the format for a file is the
one having the same name is the filehandle, but the
format for the current output channel (see the
_s_e_l_e_c_t() function) may be set explicitly by
assigning the name of the format to the $~ variable.
Top of form processing is handled automatically: if
there is insufficient room on the current page for
the formatted record, the page is advanced by
writing a form feed, a special top-of-page format is
used to format the new page header, and then the
record is written. By default the top-of-page
format is the name of the filehandle with "_TOP"
appended, but it may be dynamically set to the
format of your choice by assigning the name to the
$^ variable while the filehandle is selected. The
number of lines remaining on the current page is in
variable $-, which can be set to 0 to force a new
page.
If FILEHANDLE is unspecified, output goes to the
current default output channel, which starts out as
STDOUT but may be changed by the select operator.
If the FILEHANDLE is an EXPR, then the expression is
evaluated and the resulting string is used to look
up the name of the FILEHANDLE at run time. For more
on formats, see the _p_e_r_l_f_o_r_m manpage.
Note that write is _N_O_T the opposite of read.
Unfortunately.
y/// The translation operator. See the section on _t_r///
in the _p_e_r_l_o_p manpage.
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