SAS/C Library Reference, Vol. 1

pipe -- Create Unnamed Pipe

SYNOPSIS

 #include <unistd.h>

 int pipe(int fd[2]);

DESCRIPTION

pipe creates an I/O channel through which a process communicates with another process or with itself. fd points to a memory area where two file descriptors are stored. pipe stores the file descriptor for the output end of the pipe in fd[0], and it stores the file descriptor for the input end of the pipe in fd[2]. The first data written to the pipe are the first to be read. O_NONBLOCK and FD_CLOEXEC are turned off at both ends of the pipe.

RETURN VALUE

pipe returns 0 if it is successful, and - 1 if it is not successful.

EXAMPLE

This example invokes the ls shell command using fork and exec, and uses a pipe allocated to file descriptor 1 to obtain the output of ls and write it to stderr (which may be a non-HFS terminal or disk file if the example is run under MVS batch or TSO):

  #include <sys/types.h>
  #include <sys/wait.h>
  #include <unistd.h>
  #include <stdio.h>
  #include <signal.h>
  #include <lclib.h>

  static void do_ls(char * const []);

  main()
  {
     int pipefds[2];
     pid_t pid;
     char *const parmList[] = {"/bin/ls", "-l", "/u/userid/dirname",
                               NULL };
     char lsout[200];     /* buffer for out ls output              */
     int amt;
     int status;          /* status code from ls                   */

     fclose(stdout);      /* Avoid stdio interfering with fd 1.    */
     pipe(pipefds);       /* Create both ends of a pipe.           */

        /* Make write end of pipe fd 1.                            */
     dup2(pipefds[1],STDOUT_FILENO);

        /* Close write end.                                        */
     if (pipefds[1] != 1) close(pipefds[1]);

        /* In child process, invoke ls.                            */
     if ((pid = fork()) == 0) do_ls(parmList);
     close(1);            /* Close write end of pipe in parent.    */
     for(;;) {            /* Read from the pipe.                   */
        amt = read(pipefds[0], lsout, sizeof(lsout));
        if (amt <= 0) break;
        fwrite(lsout, 1, amt, stderr); /*Write ls output to stderr.*/
     }
     wait(&status);       /* Wait for ls to complete. */
     close(pipefds[0]);   /* Close pipe input end. */
     if (WIFEXITED(status)) exit(WEXITSTATUS(status));
     else          /* If ls failed, use kill to fail the same way. */
        kill(0, WTERMSIG(status));
  }

  static void do_ls(char *const parmList[]) {
     int rc;
     rc = execvp("ls", parmList);       /* Pass control to ls.     */
        /* execvp must have failed!                                */
     perror("execvp failure");
     abort();             /* Terminate process the hard way.       */
  }

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