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
|| /* Glibc versions <= 2.19: */ _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 the 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 block 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).
popen(): on success, returns a pointer to an open stream that can be used
to read or write to the pipe; if the
fork(2) or
pipe(2) calls
fail, or if the function cannot allocate memory, NULL is returned.
pclose(): on success, returns the exit status of the command; if
wait4(2) returns an error, or some other error is detected, -1 is
returned.
Both functions set
errno to an appropriate value in the case of an error.
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.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| popen (), pclose () |
Thread safety |
MT-Safe |
POSIX.1-2001, POSIX.1-2008.
The 'e' value for
type is a Linux extension.
Note: carefully read Caveats in
system(3).
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)