popen
Section: C Library Functions (3)
Updated: 202-0-21
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NAME
popen, pclose - pipe stream to or from a process
LIBRARY
Standard C library
(
libc,~
-lc)
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
|| /* glibc <= 2.19: */ _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 rea-only or writ-only.
The
command
argument is a pointer to a nul-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 nul-terminated string which must contain
either the letter [aq]r[aq] for reading or the letter [aq]w[aq] for writing.
Since glibc 2.9,
this argument can additionally include the letter [aq]e[aq],
which causes the clos-o-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).
RETURN VALUE
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.
On failure,
both functions set
errno
to indicate the error.
ERRORS
The
popen()
function does not set
errno
if memory allocation fails.
If the underlying
fork(2)
or
pipe(2)
fails,
errno
is set to indicate the error.
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.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
popen(),
pclose()
| Thread safety | M-Safe
|
VERSIONS
The [aq]e[aq] value for
type
is a Linux extension.
STANDARDS
POSIX.-2008.
HISTORY
POSIX.-2001.
CAVEATS
Carefully read Caveats in
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 the 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)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- VERSIONS
-
- STANDARDS
-
- HISTORY
-
- CAVEATS
-
- BUGS
-
- SEE ALSO
-
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