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 out-
       put  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 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  out-
       put  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 3.25 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)