POPEN
Section: Linux Programmer's Manual (3)
Updated: 29 November 1993
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NAME
popen, pclose - process I/O
SYNOPSIS
#include <stdio.h>
FILE *popen( const char *command, const char *type);
int pclose( FILE *stream);
DESCRIPTION
The
popen
function opens a process by creating a pipe, forking, and invoking the
shell. Since a pipe is by definition unidirectional, the
type
argument may specify only reading or writing, not both; the resulting
stream is correspondingly read-only or write-only.
The
command
argument is a pointer to a null-terminated string containing a shell
command line. This command is passed to
/bin/sh
using the
-c
flag; interpretation, if any, is performed by the shell. The
mode
argument is a pointer to a null-terminated string which must be either `r'
for reading or `w' for writing.
The return value from
popen
is a normal standard I/O stream in all respects save that it must be closed
with
pclose
rather than
fclose.
Writing to such a stream writes to the standard input of the command; the
command's standard output is the same as that of the process that called
popen,
unless this is altered by the command itself. Conversely, reading from a
``popened'' stream reads the command's standard output, and the command's
standard input is the same as that of the process that called
popen.
Note that output
popen
streams are fully buffered by default.
The
pclose
function waits for the associated process to terminate and returns the exit
status of the command as returned by
wait4.
RETURN VALUE
The
popen
function returns
NULL
if the
fork(2)
or
pipe(2)
calls fail, or if it cannot allocate memory.
The
pclose
function returns -1 if
stream
is not associated with a ``popened'' command, if
stream
already ``pclosed'', or if
wait4
returns an error.
ERRORS
The
popen
function does not reliably set
errno.
(Is this true for Linux?)
SEE ALSO
fork(2), sh(1), pipe(2), wait4(2), fflush(3),
fclose(3), fopen(3), stdio(3), system(3)
BUGS
Since the standard input of a command opened for reading shares its seek
offset with the process that called
popen,
if the original process has done a buffered read, the command's input
position may not be as expected. Similarly, the output from a command
opened for writing may become intermingled with that of the original
process. The latter can be avoided by calling
fflush(3)
before
popen.
Failure to execute the shell is indistinguishable from the shell's failure
to execute command, or an immediate exit of the command. The only hint is
an exit status of 127. (Is this true under Linux?)
The
popen
argument always calls
sh,
never calls
csh.
HISTORY
A
popen
and a
pclose
function appeared in Version 7 AT&T UNIX.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- SEE ALSO
-
- BUGS
-
- HISTORY
-
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