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 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 mode 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 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.

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                                        7 May 1998                                           POPEN(3)