popen, pclose

Purpose

     Initiates a pipe to or from a process.

Library

     Standard I/O Library (libc.a)

Syntax

     #include <stdio.h>

     FILE *popen (command, type)               int pclose (stream)
     char *command, *type;                     FILE *stream;
     Description

     The popen  subroutine creates a pipe  between the calling
     program and a shell command to be executed.

     The command parameter points  to a null-terminated string
     containing  a shell  command  line.   The type  parameter
     pointers to  a null-terminated  string containing  an I/O
     mode, either ""r"" for reading or ""w"" for writing.

     The popen subroutine  returns a pointer to  a FILE struc-
     ture for the stream.  If the type parameter is ""r"", you
     can  read from  the  standard output  of  the command  by
     reading from the  file stream.  If the  type parameter is
     ""w"", you can write to the standard input of the command
     by writing to the file stream.

     Use the  pclose subroutine to  close any stream  you have
     opened with the popen  subroutine.  The pclose subroutine
     waits for  the associated  process to terminate  and then
     returns the exit status of the command.

     Because open files  are shared, a type  ""r"" command can
     be used as an input filter  and a type ""w"" as an output
     filter.

     Warning:   If  the  original processes  and  the  process
     started with  popen concurrently  read or write  a common
     file, neither should  use buffered I/O.  If  they do, the
     results are unpredictable.

     Some problems with  an output filter can  be prevented by
     taking care to  flush the buffer with  the fflush subrou-
     tine (see "fclose, fflush").

     The popen subroutine  returns a NULL pointer  if files or
     processes cannot  be created, or  if the shell  cannot be
     accessed.

     The pclose subroutine returns -1 if stream is not associ-
     ated with a popen command.

     Related Information

     In this book:  "pipe,"  "wait," "fclose, fflush," "fopen,
     freopen, fdopen," "standard i/o library," and "system."