11-27-2001
Look at the popen() subroutine in the man pages.
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POPEN(3) Linux Programmer's Manual POPEN(3)
NAME
popen, pclose - pipe stream to or from a process
SYNOPSIS
#include <stdio.h>
FILE *popen(const char *command, const char *type);
int pclose(FILE *stream);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
popen(), pclose():
_POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_SOURCE
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 type argument is a pointer to a null-terminated string which must contain
either the letter 'r' for reading or the letter 'w' for writing. Since glibc 2.9, this argument can additionally include the letter 'e',
which causes the close-on-exec flag (FD_CLOEXEC) to be set on the underlying file descriptor; see the description of the O_CLOEXEC flag in
open(2) for reasons why this may be useful.
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(3). 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(2).
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 wait4(2) returns an error, or some other error is detected.
ERRORS
The popen() function does not set errno if memory allocation fails. If the underlying fork(2) or pipe(2) fails, errno is set appropri-
ately. If the type argument is invalid, and this condition is detected, errno is set to EINVAL.
If pclose() cannot obtain the child status, errno is set to ECHILD.
CONFORMING TO
POSIX.1-2001.
The 'e' value for type is a Linux extension.
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.
SEE ALSO
sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3), stdio(3), system(3)
COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
GNU
2010-02-03 POPEN(3)