popen

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



NAME
       popen, pclose - process I/O

SYNOPSIS
       #include <stdio.h>

       FILE *popen(const char *command, const char *type);

       int pclose(FILE *stream);

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  mode
       argument  is a pointer to a null-terminated string which must be either
       `r' for reading or `w' for writing.

       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().  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 pro-
       cess that called popen(), unless this is altered by the command itself.
       Conversely, reading from a ``popened'' 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 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.

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 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() or pipe() fails, errno is set appropriately.  If
       the mode 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.2

BUGS
       Since  the  standard  input  of a command opened for reading shares its
       seek offset with the process that called popen(), if the original  pro-
       cess  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.

HISTORY
       A popen() and a pclose() function appeared in Version 7 AT&T UNIX.

SEE ALSO
       fork(2), sh(1),  pipe(2),  wait4(2),  fflush(3),  fclose(3),  fopen(3),
       stdio(3), system(3)



BSD MANPAGE                       1998-05-07                          POPEN(3)