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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 || _XOPEN_SOURCE || _POSIX_SOURCE
       _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-termi-
       nated 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 a "popened"  stream  reads  the  com-
       mand'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.

ERRORS
       The  popen() function does not set errno if memory allocation fails.  If the under-
       lying 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.

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 exe-
       cute 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), sys-
       tem(3)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



GNU                               2008-10-10                          POPEN(3)

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