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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;
     }