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	 type
       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
       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.

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 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.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)