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       popen, pclose - process I/O


       #include <stdio.h>

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

       int pclose(FILE *stream);


       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.


       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.


       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.




       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.


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


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