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(libc.info)Pipe to a Subprocess

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Pipe to a Subprocess
====================

   A common use of pipes is to send data to or receive data from a
program being run as a subprocess.  One way of doing this is by using a
combination of `pipe' (to create the pipe), `fork' (to create the
subprocess), `dup2' (to force the subprocess to use the pipe as its
standard input or output channel), and `exec' (to execute the new
program).  Or, you can use `popen' and `pclose'.

   The advantage of using `popen' and `pclose' is that the interface is
much simpler and easier to use.  But it doesn't offer as much
flexibility as using the low-level functions directly.

 - Function: FILE * popen (const char *COMMAND, const char *MODE)
     The `popen' function is closely related to the `system' function;
     see Note: Running a Command.  It executes the shell command
     COMMAND as a subprocess.  However, instead of waiting for the
     command to complete, it creates a pipe to the subprocess and
     returns a stream that corresponds to that pipe.

     If you specify a MODE argument of `"r"', you can read from the
     stream to retrieve data from the standard output channel of the
     subprocess.  The subprocess inherits its standard input channel
     from the parent process.

     Similarly, if you specify a MODE argument of `"w"', you can write
     to the stream to send data to the standard input channel of the
     subprocess.  The subprocess inherits its standard output channel
     from the parent process.

     In the event of an error `popen' returns a null pointer.  This
     might happen if the pipe or stream cannot be created, if the
     subprocess cannot be forked, or if the program cannot be executed.

 - Function: int pclose (FILE *STREAM)
     The `pclose' function is used to close a stream created by `popen'.
     It waits for the child process to terminate and returns its status
     value, as for the `system' function.

   Here is an example showing how to use `popen' and `pclose' to filter
output through another program, in this case the paging program `more'.

     #include <stdio.h>
     #include <stdlib.h>
     
     void
     write_data (FILE * stream)
     {
       int i;
       for (i = 0; i < 100; i++)
         fprintf (stream, "%d\n", i);
       if (ferror (stream))
         {
           fprintf (stderr, "Output to stream failed.\n");
           exit (EXIT_FAILURE);
         }
     }
     
     int
     main (void)
     {
       FILE *output;
     
       output = popen ("more", "w");
       if (!output)
         {
           fprintf (stderr,
                    "incorrect parameters or too many files.\n");
           return EXIT_FAILURE;
         }
       write_data (output);
       if (pclose (output) != 0)
         {
           fprintf (stderr,
                    "Could not run more or other error.\n");
         }
       return EXIT_SUCCESS;
     }