POPEN(3)                  386BSD 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 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 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 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().

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

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

SEE ALSO
     fork(2),  sh(1),  pipe(2),  wait4(2),  fflush(3),  fclose(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
     failure to execute command, or an immediate exit of the command.  The
     only hint is an exit status of 127.

     The popen() argument always calls sh,  never calls csh.

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

BSD Experimental                April 30, 1991                               2