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

NAME
     ppooppeenn, ppcclloossee - process I/O

LLIIBBRRAARRYY
     Standard C Library (libc, -lc)

SYNOPSIS
     ##iinncclluuddee <>

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

     int
     ppcclloossee(FILE *stream);

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

     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 -cc
     flag; interpretation, if any, is performed by the shell.

     The return value from ppooppeenn() is a normal standard I/O stream in all
     respects save that it must be closed with ppcclloossee() rather than ffcclloossee().
     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
     ppooppeenn(), 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 ppooppeenn().

     Note that output ppooppeenn() streams are fully buffered by default.

     The ppcclloossee() function waits for the associated process to terminate and
     returns the exit status of the command as returned by wait4(2).

RETURN VALUES
     The ppooppeenn() function returns NULL if the fork(2) or pipe(2) calls fail,
     or if it cannot allocate memory.

     The ppcclloossee() function returns -1 if stream is not associated with a
     ``popened'' command, if stream already ``pclosed'', or if wait4(2)
     returns an error.

EERRRROORRSS
     The ppooppeenn() 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 ppooppeenn(), 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 ppooppeenn().

     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 ppooppeenn() function always calls sh(1), never calls csh(1).

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

     Bidirectional functionality was added in FreeBSD 2.2.6.

BSD                               May 3, 1995                              BSD