InfoMagic Source Code 1993 July/THE_SOURCE_CODE_CD_ROM.iso/bsd_srcs/share/doc/ps1/04.pascal/puman2.n

Text File | 1991-04-17 | 22.2 KB | 859 lines

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.\"    @(#)puman2.n    6.3 (Berkeley) 4/17/91
.\"
.if !\n(xx \{\
.so tmac.p \}
'if n 'ND
.nr H1 1
.NH
Basic UNIX Pascal
.PP
The following sections
explain the basics of using
.UP .
In examples here we use the text editor
.I ex
(1).
Users of the text editor
.I ed
should have little trouble following these examples,
as
.I ex
is similar to
.I ed .
We use
.I ex
because it
allows us to make clearer examples.\(dg
.FS
\(dg Users with \s-2CRT\s0 terminals should find the editor
.I vi
more pleasant to use;
we do not show its use here because its display oriented nature
makes it difficult to illustrate.
.FE
The new
.UX
user will find it helpful to read one of the text editor documents
described in section 1.4 before continuing with this section.
.NH 2
A first program
.PP
To prepare a program for
.UP
we first need to have an account on
.UX
and to `login'
to the system on this account.
These procedures are described in the documents
.I "Communicating with UNIX"
and
.I "UNIX for Beginners".
.PP
Once we are logged in we need to choose a name for our program;
let us call it `first' as this is the first example.
We must also choose a name for the file in which the program will be stored.
The
.UP
system requires that programs reside in files which have names ending with
the sequence `.p' so we will call our file `first.p'.
.PP
A sample editing session to create this file would begin:
.LS
% \*bex first.p\fR
"first.p" [New file]
:
.LE
We didn't expect the file to exist, so the error diagnostic doesn't
bother us.
The editor now knows the name of the file we are creating.
The `:' prompt indicates that it is ready for command input.
We can add the text for our program using the `append'
command as follows.
.LS
:\*bappend\fR
.B
program first(output)
begin
    writeln('Hello, world!')
end.
\&.
.R
:
.LE
The line containing the single `\*b.\fR' character here indicated
the end of the appended text.
The `:' prompt indicates that
.I ex
is ready for another command.
As the editor operates in a temporary work space we must now store the contents
of this work space in the file `first.p'
so we can use the Pascal
translator and executor
.IX
on it.
.LS
:\*bwrite\fR
"first.p" [New file] 4 lines, 59 characters
:\*bquit\fR
%
.LE
We wrote out the file from the edit buffer here with the
`write'
command, and
.I ex
indicated the number of lines and characters written.
We then quit the editor, and now have a prompt from the shell.\(dd
.FS
\(dd Our examples here assume you are using
.I csh.
.FE
.KS
.PP
We are ready to try
to translate and execute our program.
.DS
.tr '\(aa^\(ua
% \*bpix first.p\fR
.so firstout
.tr ''^^
%
.DE
.KE
.PP
The translator first printed a syntax error diagnostic.
The number 2 here indicates that the rest of the line is an image
of the second line of our program.
The translator is saying that it expected to find a `;' before the
keyword
.B begin
on this line.
If we look at the Pascal syntax charts in the Jensen-Wirth
.I "User Manual" ,
or at some of the sample programs therein, we will see that
we have omitted the terminating `;' of the
.B program
statement on the first
line of our program.
.PP
One other thing to notice about the error diagnostic is the letter `e'
at the beginning.
It stands for `error',
indicating that our input was not legal Pascal.
The fact that it is an `e' rather than an `E'
indicates that the translator managed to recover from this error well
enough that generation of code and execution could take place.
Execution is possible whenever no fatal `E' errors
occur during translation.
The other classes of diagnostics are `w' warnings,
which do not necessarily indicate errors in the program,
but point out inconsistencies which are likely to be due to program bugs,
and `s' standard-Pascal violations.\*(dg
.FS
\*(dgThe standard Pascal warnings occur only when the associated
.B s
translator option is enabled.
The
.B s
option is discussed in sections 5.1 and A.6 below.
Warning diagnostics are discussed at the end of section 3.2,
the associated
.B w
option is described in section 5.2.
.FE
.PP
After completing the translation of the program to interpretive code,
the Pascal system indicates that execution of the translated program began.
The output from the execution of the program then appeared.
At program termination, the Pascal runtime system indicated the
number of statements executed, and the amount of cpu time
used, with the resolution of the latter being 1/60'th of a second.
.PP
Let us now fix the error in the program and translate it to a permanent
object code file
.I obj
using
.PI .
The program
.PI
translates Pascal programs but stores the object code instead of executing it\*(dd.
.FS
\*(ddThis script indicates some other useful approaches to debugging
Pascal programs.
As in
.I ed
we can shorten commands in
.I ex
to an initial prefix of the command name as we did
with the
.I substitute
command here.
We have also used the `!' shell escape command here to execute other
commands with a shell without leaving the editor.
.FE
.LS
% \*bex first.p\fR
"first.p" 4 lines, 59 characters
:\*b1 print\fR
program first(output)
:\*bs/$/;\fR
program first(output);
:\*bwrite\fR
"first.p" 4 lines, 60 characters
:\*bquit\fR
% \*bpi first.p\fR
%
.LE
If we now use the
.UX
.I ls
list files command we can see what files we have:
.LS
% \*bls\fR
first.p
obj
%
.LE
The file `obj' here contains the Pascal interpreter code.
We can execute this by typing:
.LS
% \*bpx obj\fR
.so firstobjout
%
.LE
Alternatively, the command:
.LS
% \*bobj\fR
.LE
will have the same effect.
Some examples of different ways to execute the program follow.
.LS
% \*bpx\fR
.so firstobjout
% \*bpi -p first.p\fR
% \*bpx obj\fR
.so firstobjout2
% \*bpix -p first.p\fR
.so firstobjout2
%
.LE
.PP
Note that
.I px
will assume that `obj' is the file we wish to execute
if we don't tell it otherwise.
The last two translations use the
.B \-p
no-post-mortem option to eliminate
execution statistics and
`Execution begins'
and
`Execution terminated'
messages.
See section 5.2 for more details.
If we now look at the files in our directory we will see:
.LS
% \*bls\fR
first.p
obj
%
.LE
We can give our object program a name other than `obj' by using the move
command
.I mv
(1).
Thus to name our program `hello':
.LS
% \*bmv obj hello\fR
% \*bhello\fR
Hello, world!
% \*bls\fR
first.p
hello
%
.LE
Finally we can get rid of the Pascal object code by using the
.I rm
(1) remove file command, e.g.:
.LS
% \*brm hello\fR
% \*bls\fR
first.p
%
.LE
.PP
For small programs which are being developed
.IX
tends to be more convenient to use than
.PI
and
.X .
Except for absence of the
.I obj
file after a
.IX
run,
a
.IX
command is equivalent to a
.PI
command followed by a
.X
command.
For larger programs,
where a number of runs testing different parts of the program are
to be made,
.PI
is useful as this
.I obj
file can be executed any desired number of times.
.. >>> INSERT SECTION FOR PC <<<
.NH 2
A larger program
.PP
Suppose that we have used the editor to put a larger program
in the file `bigger.p'.
We can list this program with line numbers by using the program
.I cat -n
i.e.:
.LS
% \*bcat -n bigger.p\fR
.so bigger3.p
%
.LE
This program is similar to program 4.9 on page 30 of the
Jensen-Wirth
.I "User Manual" .
A number of problems have been introduced into this example for
pedagogical reasons.
.br
.PP
If we attempt to translate and execute the program using
.IX
we get the following response:
.LS
% \*bpix bigger.p\fR
.so bigout1
%
.LE
.PP
Since there were fatal `E' errors in our program,
no code was generated and execution was necessarily suppressed.
One thing which would be useful at this point is a listing of the
program with the error messages.
We can get this by using the command:
.LS
% \*bpi -l bigger.p\fR
.LE
There is no point in using
.IX
here, since we know there are fatal errors in the program.
This command will produce the output at our terminal.
If we are at a terminal which does not produce a hard copy
we may wish to print this
listing off-line on a line printer.
We can do this with the command:
.LS
% \*bpi -l bigger.p | lpr\fR
.LE
.PP
In the next few sections we will illustrate various aspects of the
Berkeley
Pascal system by correcting this program.
.NH 2
Correcting the first errors
.PP
Most of the errors which occurred in this program were
.I syntactic
errors, those in the format and structure of the program rather than
its content.
Syntax errors are flagged by printing the offending line, and then a line
which flags the location at which an error was detected.
The flag line also gives an explanation
stating either a possible cause of the error,
a simple action which can be taken to recover from the error so
as to be able to continue the analysis,
a symbol which was expected at the point of error,
or an indication that the input was `malformed'.
In the last case, the recovery may skip ahead in the input
to a point where analysis of the program can continue.
.PP
In this example,
the first error diagnostic indicates that the translator detected
a comment within a comment.
While this is not considered an error in `standard'
Pascal, it usually corresponds to an error in the program which
is being translated.
In this case, we have accidentally omitted the trailing `*)' of the comment
on line 8.
We can begin an editor session to correct this problem by doing:
.LS
% \*bex bigger.p\fR
"bigger.p" 24 lines, 512 characters
:\*b8s/$/ *)\fR
        s = 32;       (* 32 character width for interval [x, x+1] *)
:
.LE
.PP
The second diagnostic, given after line 16,
indicates that the keyword
.B do
was expected before the keyword
.B begin
in the
.B for
statement.
If we examine the
.I statement
syntax chart on page 118 of the
Jensen-Wirth
.I "User Manual"
we will discover that
.B do
is a necessary part of the
.B for
statement.
Similarly, we could have referred to section C.3 of the
Jensen-Wirth
.I "User Manual"
to learn about the
.B for
statement and gotten the same information there.
It is often useful to refer to these syntax charts and to the
relevant sections of this book.
.PP
We can correct this problem by first scanning for the keyword
.B for
in the file and then substituting the keyword
.B do
to appear in front of the keyword
.B begin
there.
Thus:
.LS
:\*b/for\fR
    for i := 0 to lim begin
:\*bs/begin/do &\fR
    for i := 0 to lim do begin
:
.LE
The next error in the program is easy to pinpoint.
On line 18, we didn't hit the shift key and got a `9'
instead of a `)'.
The translator diagnosed that `x9'
was an undefined variable and, later,
that a `)' was missing in the statement.
It should be stressed that
.PI
is not suggesting that you should insert a `)' before the `;'.
It is only indicating that making this change will help it to be able to
continue analyzing the program so as to be able to diagnose further
errors.
You must then determine the true cause of the error and make the
appropriate correction to the source text.
.PP
This error also illustrates the fact that one error in the input may lead
to multiple error diagnostics.
.I Pi
attempts
to give only one diagnostic for each error,
but single errors in the input sometimes appear to be more than
one error.
It is also the case that
.PI
may not detect an error when it occurs, but may detect it later in
the input.
This would have happened
in this example if we had typed `x' instead of `x9'.
.PP
The translator next detected, on line 19, that the function
.I Round
and the variable
.I h
were undefined.
It does not know about
.I Round
because
.UP
normally distinguishes between upper and lower case.\*(dg
.FS
\*(dgIn ``standard'' Pascal no distinction is made based on case.
.FE
On
.UX
lower-case is preferred\*(dd,
.FS
\*(ddOne good reason for using lower-case is that it is easier to type.
.FE
and all keywords and built-in
.B procedure
and
.B function
names are composed of lower-case letters,
just as they are in the Jensen-Wirth
.I "Pascal Report" .
Thus we need to use the function
.I round
here.
As far as
.I h
is concerned,
we can see why it is undefined if we look back to line 9
and note that its definition was lost in the non-terminated
comment.
This diagnostic need not, therefore, concern us.
.PP
The next error which occurred in the program caused the translator
to insert a `;' before the statement calling
.I writeln
on line 23.
If we examine the program around the point of error we will see
that the actual error is that the keyword
.B until
and an associated expression have been omitted here.
Note that the diagnostic from the translator does not indicate the actual
error, and is somewhat misleading.
The translator made the correction which seemed to be most plausible.
As the omission of a `;' character is a common mistake,
the translator chose to indicate this as a possible fix here.
It later detected that the keyword
.B until
was missing, but not until it saw the keyword
.B end
on line 24.
The combination of these diagnostics indicate to us the true problem.
.PP
The final syntactic error message indicates that the translator needed an
.B end
keyword to match the
.B begin
at line 15.
Since the
.B end
at line 24 is supposed to match this
.B begin ,
we can infer that another
.B begin
must have been mismatched, and have matched this
.B end .
Thus we see that we need an
.B end
to match the
.B begin
at line 16,
and to appear before the final
.B end .
We can make these corrections:
.LS
:\*b/x9/s//x)\fR
                y := exp(-x) * sin(i * x);
:\*b+s/Round/round\fR
                n := round(s * y) + h;
:\*b/write\fR
                        write(' ');
:\*b/\fR
                writeln('*')
:\*binsert\fR
                \*buntil n = 0;\fR
\&\*b.\fR
:\*b$\fR
end.
:\*binsert\fR
        \*bend\fR
\&\*b.\fR
:
.LE
.PP
At the end of each
.B procedure
or
.B function
and the end of the
.B program
the translator summarizes references to undefined variables
and improper usages of variables.
It also gives
warnings about potential errors.
In our program, the summary errors do not indicate any further problems
but the warning that
.I c
is unused is somewhat suspicious.
Examining the program we see that the constant was intended
to be used in the expression which is an argument to
.I sin ,
so we can correct this expression, and translate the program.
We have now made a correction for each diagnosed error
in our program.
.LS
:\*b?i ?s//c /\fR
        y := exp(-x) * sin(c * x);
:\*bwrite\fR
"bigger.p" 26 lines, 538 characters
:\*bquit\fR
% \*bpi bigger.p\fR
%
.LE
It should be noted that the translator suppresses warning
diagnostics for a particular
.B procedure ,
.B function
or the main
.B program
when it finds severe syntax errors in that part of the source
text.
This is to prevent possibly confusing and
incorrect warning diagnostics from being produced.
Thus these warning diagnostics may not appear in a program with
bad syntax errors until these errors are corrected.
.KS
.PP
We are now ready to execute our program for the first
time.
We will do so in the next section after giving a listing
of the corrected program for reference purposes.
.LS
% \*bcat -n bigger.p\fR
.so bigger6.p
%
.LE
.NH 2
Executing the second example
.PP
We are now ready to execute the second example.
The following output was produced by our first run.
.LS
% \*bpx\fR
.so bigout2
%
.LE
Here the interpreter is presenting us with a runtime error diagnostic.
It detected a `division by zero' at line 17.
Examining line 17, we see that we have written
the statement `x := d / i' instead of `x := d * i'.
We can correct this and rerun the program:
.LS
% \*bex bigger.p\fR
"bigger.p" 26 lines, 538 characters
:\*b17\fR
        x := d / i
:\*bs'/'*\fR
        x := d * i
:\*bwrite\fR
"bigger.p" 26 lines, 538 characters
:\*bq\fR
% \*bpix bigger.p\fR
.so bigout3
%
.LE
.KS
.PP
This appears to be the output we wanted.
We could now save the output in a file if we wished by using the shell
to redirect the output:
.LS
% \*bpx > graph\fR
.LE
.KE
We can use
.I cat
(1) to see the contents of the file graph.
We can also make a listing of the graph on the line printer without
putting it into a file, e.g.
.LS
% \*bpx | lpr\fR
.so bigout4
%
.LE
Note here that the statistics lines came out on our terminal.
The statistics line comes out on the diagnostic output (unit 2.)
There are two ways to get rid of the statistics line.
We can redirect the statistics message to the printer using the
syntax `|\|&' to the shell rather than `|', i.e.:
.LS
% \*bpx |\|& lpr\fR
%
.LE
or we can translate the program with the
.B p
option disabled on the command line as we did above.
This will disable all post-mortem dumping including the statistics line,
thus:
.LS
% \*bpi -p bigger.p\fR
% \*bpx | lpr\fR
%
.LE
This option also disables the statement limit which normally guards
against infinite looping.
You should not use it until your program is debugged.
Also if
.B p
is specified and an error occurs, you will
not get run time diagnostic information to help you
determine what the problem is.
.NH 2
Formatting the program listing
.PP
It is possible to use special lines within the source text of a program
to format the program listing.
An empty line (one with no characters on it) corresponds to a
`space' macro in an assembler, leaving a completely blank line
without a line number.
A line containing only a control-l (form-feed) character
will cause a page eject in the listing with the corresponding line number
suppressed.
This corresponds to an `eject' pseudo-instruction.
See also section 5.2 for details on the
.B n
and
.B i
options of
.PI .
.NH 2
Execution profiling
.PP
An execution profile consists of a structured listing of (all or part of)
a program with information about the number of times each statement in
the program was executed for a particular run of the program.
These profiles can be used for several purposes.
In a program which was abnormally terminated due to excessive looping
or recursion or by a program fault, the counts can facilitate location
of the error.
Zero counts mark portions of the program which were not executed;
during the early debugging stages they should prompt new test data or
a re-examination of the program logic.
The profile is perhaps most valuable, however, in drawing
attention to the (typically small)
portions of the program that dominate execution time.
This information can be used for source level optimization.
.SH
An example
.PP
A prime number is a number which is divisible only by itself and the
number one.
The program
.I primes ,
written by Niklaus Wirth,
determines the first few prime numbers.
In translating the program we have specified the
.B z
option to
.IX .
This option causes the translator to generate counters and count instructions
sufficient in number to determine the number of times each statement in the
program was executed.\*(dg
.FS
\*(dgThe counts
are completely accurate only in the absence of runtime errors and nonlocal
.B goto
statements.
This is not generally a problem, however, as in structured programs
nonlocal
.B goto
statements occur infrequently,
and counts are incorrect after abnormal termination only when the
.I "upward look"
described below to get a count passes a suspended call point.
.FE
When execution of the program completes, either normally or abnormally,
this count data is written to the file
.I pmon.out
in the current directory.\*(dd
.FS
\*(dd\c
.I Pmon.out
has a name similar to
.I mon.out
the monitor file produced by the profiling facility of the C compiler
.I cc
(1).
See
.I prof
(1) for a discussion of the C compiler profiling facilities.
.FE
It is then possible to prepare an execution profile by giving
.XP
the name of the file associated with this data, as was done in the following
example.
.LS
% \*bpix -l -z primes.p\fR
.so primeout1
%
.LE
.SH
Discussion
.PP
The header lines of the outputs of
.IX
and
.XP
in this example indicate the version of the translator and execution
profiler in use at the time this example was prepared.
The time given with the file name (also on the header line)
indicates the time of last modification of the program source file.
This time serves to
.I "version stamp"
the input program.
.I Pxp
also indicates the time at which the profile data was gathered.
.LS
% \*bpxp -z primes.p\fR
.so primeout2
%
.LE
.KE
.PP
To determine the number of times a statement was executed,
one looks to the left of the statement and finds the corresponding
vertical bar `|'.
If this vertical bar is labelled with a count then that count gives the
number of times the statement was executed.
If the bar is not labelled, we look up in the listing to find the first
`|' which directly above the original one which has a count and that
is the answer.
Thus, in our example,
.I k
was incremented 157 times on line 18,
while the
.I write
procedure call on line 24 was executed 48 times as given by the count
on the
.B repeat .
.PP
More information on
.I pxp
can be found in its manual section
.XP
(1)
and in sections 5.4, 5.5 and 5.10.