popen, pclose - pipe stream to or from a process
FILE *popen(const char *command, const char *type);
int pclose(FILE *stream);
Feature Test Macro Requirements for glibc (see feature_test_macros
_POSIX_C_SOURCE >= 2
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
() function opens a process by creating a pipe, forking, and
invoking the shell. Since a pipe is by definition unidirectional, the
argument may specify only reading or writing, not both; the
resulting stream is correspondingly read-only or write-only.
argument is a pointer to a null-terminated string containing
a shell command line. This command is passed to /bin/sh
flag; interpretation, if any, is performed by the shell.
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
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
(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
(), 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
Note that output popen
() streams are block buffered by default.
() function waits for the associated process to terminate and
returns the exit status of the command as returned by wait4
(): 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
fail, or if the function cannot allocate memory, NULL is returned.
(): on success, returns the exit status of the command; if
(2) returns an error, or some other error is detected, -1 is
Both functions set errno
to an appropriate value in the case of an error.
() 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
() cannot obtain the child status, errno
is set to
For an explanation of the terms used in this section, see attributes
|popen (), pclose ()
The 'e' value for type
is a Linux extension.
: carefully read Caveats in system
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
(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.