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).