POPEN(3)                 BSD Library Functions Manual                 POPEN(3)

NAME
     pclose, popen -- process I/O

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

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

     int
     pclose(FILE *stream);

DESCRIPTION
     The popen() function ``opens'' a process by creating a bidirectional
     pipe, forking, and invoking the shell.  Any streams opened by previous
     popen() calls in the parent process are closed in the new child process.
     Historically, popen() was implemented with a unidirectional pipe; hence,
     many implementations of popen() only allow the mode argument to specify
     reading or writing, not both.  Because popen() is now implemented using a
     bidirectional pipe, the mode argument may request a bidirectional data
     flow.  The mode argument is a pointer to a null-terminated string which
     must be `r' for reading, `w' for writing, or `r+' for reading and writ-
     ing.

     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 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, read-
     ing 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; it
     returns the exit status of the command, as returned by wait4(2).

RETURN VALUES
     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 stream is not associated with a
     ``popened'' command, if stream already ``pclosed'', or if wait4(2)
     returns an error.

ERRORS
     The popen() function does not reliably set errno.

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

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 fail-
     ure to execute command, or an immediate exit of the command.  The only
     hint is an exit status of 127.

     The popen() function always calls sh(1), never calls csh(1).

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

     Bidirectional functionality was added in FreeBSD 2.2.6.

BSD                               May 3, 1995                              BSD