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(libc.info)Creating a Pipe

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Creating a Pipe
===============

   The primitive for creating a pipe is the `pipe' function.  This
creates both the reading and writing ends of the pipe.  It is not very
useful for a single process to use a pipe to talk to itself.  In typical
use, a process creates a pipe just before it forks one or more child
processes (Note: Creating a Process).  The pipe is then used for
communication either between the parent or child processes, or between
two sibling processes.

   The `pipe' function is declared in the header file `unistd.h'.

 - Function: int pipe (int FILEDES[2])
     The `pipe' function creates a pipe and puts the file descriptors
     for the reading and writing ends of the pipe (respectively) into
     `FILEDES[0]' and `FILEDES[1]'.

     An easy way to remember that the input end comes first is that file
     descriptor `0' is standard input, and file descriptor `1' is
     standard output.

     If successful, `pipe' returns a value of `0'.  On failure, `-1' is
     returned.  The following `errno' error conditions are defined for
     this function:

    `EMFILE'
          The process has too many files open.

    `ENFILE'
          There are too many open files in the entire system.  Note:
          Error Codes, for more information about `ENFILE'.  This
          error never occurs in the GNU system.

   Here is an example of a simple program that creates a pipe.  This
program uses the `fork' function (Note: Creating a Process) to create
a child process.  The parent process writes data to the pipe, which is
read by the child process.

     #include <sys/types.h>
     #include <unistd.h>
     #include <stdio.h>
     #include <stdlib.h>
     
     /* Read characters from the pipe and echo them to `stdout'. */
     
     void
     read_from_pipe (int file)
     {
       FILE *stream;
       int c;
       stream = fdopen (file, "r");
       while ((c = fgetc (stream)) != EOF)
         putchar (c);
       fclose (stream);
     }
     
     /* Write some random text to the pipe. */
     
     void
     write_to_pipe (int file)
     {
       FILE *stream;
       stream = fdopen (file, "w");
       fprintf (stream, "hello, world!\n");
       fprintf (stream, "goodbye, world!\n");
       fclose (stream);
     }
     
     int
     main (void)
     {
       pid_t pid;
       int mypipe[2];
     
       /* Create the pipe. */
       if (pipe (mypipe))
         {
           fprintf (stderr, "Pipe failed.\n");
           return EXIT_FAILURE;
         }
     
       /* Create the child process. */
       pid = fork ();
       if (pid == (pid_t) 0)
         {
           /* This is the child process.
              Close other end first. */
           close (mypipe[1]);
           read_from_pipe (mypipe[0]);
           return EXIT_SUCCESS;
         }
       else if (pid < (pid_t) 0)
         {
           /* The fork failed. */
           fprintf (stderr, "Fork failed.\n");
           return EXIT_FAILURE;
         }
       else
         {
           /* This is the parent process.
              Close other end first. */
           close (mypipe[0]);
           write_to_pipe (mypipe[1]);
           return EXIT_SUCCESS;
         }
     }