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 || _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 a "popened" 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.

ERRORS

       The  popen()  function  does  not set errno if memory allocation fails.	If the underlying fork(2) or pipe(2) fails, errno is set appropri-
       ately.  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 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 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2010-02-03								  POPEN(3)