popen

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  contain-
       ing  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 writ-
       ing.  Since glibc 2.9, this argument can additionally include the  let-
       ter  '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  stan-
       dard  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.

       Both  functions  set  errno  to  an appropriate value in the case of an
       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, POSIX.1-2008.

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

NOTES
       Note: 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 writ-
       ing 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 4.15 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
       https://www.kernel.org/doc/man-pages/.

GNU                               2017-09-15                          POPEN(3)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2021 Hurricane Electric. All Rights Reserved.