popen(3) - Linux manual page (original) (raw)
popen(3) Library Functions Manual popen(3)
NAME top
popen, pclose - pipe stream to or from a processLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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 <= 2.19: */ _BSD_SOURCE || _SVID_SOURCEDESCRIPTION top
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)](../man2/open.2.html) 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)](../man3/fclose.3.html). 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)](../man2/wait4.2.html).RETURN VALUE top
**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)](../man2/fork.2.html) or [pipe(2)](../man2/pipe.2.html)
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)](../man2/wait4.2.html) returns an error, or some other error is detected, -1 is
returned.
On failure, both functions set _[errno](../man3/errno.3.html)_ to indicate the error.ERRORS top
The **popen**() function does not set _[errno](../man3/errno.3.html)_ if memory allocation
fails. If the underlying [fork(2)](../man2/fork.2.html) or [pipe(2)](../man2/pipe.2.html) fails, _[errno](../man3/errno.3.html)_ is set
to indicate the error. If the _type_ argument is invalid, and this
condition is detected, _[errno](../man3/errno.3.html)_ is set to **EINVAL**.
If **pclose**() cannot obtain the child status, _[errno](../man3/errno.3.html)_ is set to
**ECHILD**.ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ **Interface** │ **Attribute** │ **Value** │
├──────────────────────────────────────┼───────────────┼─────────┤
│ **popen**(), **pclose**() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘VERSIONS top
The 'e' value for _type_ is a Linux extension.STANDARDS top
POSIX.1-2008.HISTORY top
POSIX.1-2001.CAVEATS top
Carefully read Caveats in [system(3)](../man3/system.3.html).BUGS top
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)](../man3/fflush.3.html) before **popen**().
Failure to execute the shell is indistinguishable from the shell's
failure to execute the command, or an immediate exit of the
command. The only hint is an exit status of 127.SEE ALSO top
**sh**(1), [fork(2)](../man2/fork.2.html), [pipe(2)](../man2/pipe.2.html), [wait4(2)](../man2/wait4.2.html), [fclose(3)](../man3/fclose.3.html), [fflush(3)](../man3/fflush.3.html), [fopen(3)](../man3/fopen.3.html),
[stdio(3)](../man3/stdio.3.html), [system(3)](../man3/system.3.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 popen(3)
Pages that refer to this page:pipe(2), getexeccon(3), __pmprocessexec(3), __pmprocesspipe(3)