NAME    [Toc]    [Back]

     popen, pclose - process I/O

SYNOPSIS    [Toc]    [Back]

     #include <stdio.h>

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

     int
     pclose(FILE *stream);

DESCRIPTION    [Toc]    [Back]

     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 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
 except 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(2).

RETURN VALUES    [Toc]    [Back]

     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    [Toc]    [Back]

     The popen() function does not reliably set errno.

SEE ALSO    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

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

BUGS    [Toc]    [Back]

     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(1); it never calls
csh(1).

OpenBSD      3.6                           June      4,      1993