popen.3

POPEN(3)                    Linux Programmer's Manual                   POPEN(3)



NAME
       popen, pclose - pipe stream to or from a process

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 versions <= 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 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 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).

RETURN VALUE
       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.  In the event of an error, these functions set
       errno to indicate the cause of 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 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.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────┬───────────────┬─────────┐
       │Interface         Attribute     Value   │
       ├──────────────────┼───────────────┼─────────┤
       │popen(), pclose() │ Thread safety │ MT-Safe │
       └──────────────────┴───────────────┴─────────┘

CONFORMING TO
       POSIX.1-2001.

       The 'e' value for type is a Linux extension.

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 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)

COLOPHON
       This page is part of release 4.00 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.



GNU                                2015-03-29                           POPEN(3)