| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | BUGS | SEE ALSO | COLOPHON | The Linux Programming Interface |
POPEN(3) Linux Programmer's Manual POPEN(3)
popen, pclose - pipe stream to or from a process
#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
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).
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.
The popen() function does not set errno if memory allocation fails. If the
underlying fork(2) or pipe(2) fails, errno is set appropriately. 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.
POSIX.1-2001.
The 'e' value for type is a Linux extension.
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.
sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3), stdio(3),
system(3)
This page is part of release 3.32 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)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface