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### 15.2 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 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;
         }