pipes/pipe_ls_wc.c

+/* pipe_ls_wc.c
+
+   Demonstrate the use of a pipe to connect two filters. We use fork()
+   to create two children. The first one execs ls(1), which writes to
+   the pipe, the second execs wc(1) to read from the pipe.
+*/
 #include <sys/wait.h>
 #include "tlpi_hdr.h"
 
 int
 main(int argc, char *argv[])
 {
     int pfd[2];                                     /* Pipe file descriptors */
 
     if (pipe(pfd) == -1)                            /* Create pipe */
         errExit("pipe");
 
     switch (fork()) {
     case -1:
         errExit("fork");
 
     case 0:             /* First child: exec 'ls' to write to pipe */
         if (close(pfd[0]) == -1)                    /* Read end is unused */
             errExit("close 1");
 
         /* Duplicate stdout on write end of pipe; close duplicated descriptor */
 
         if (pfd[1] != STDOUT_FILENO) {              /* Defensive check */
             if (dup2(pfd[1], STDOUT_FILENO) == -1)
                 errExit("dup2 1");
             if (close(pfd[1]) == -1)
                 errExit("close 2");
         }
 
         execlp("ls", "ls", (char *) NULL);          /* Writes to pipe */
         errExit("execlp ls");
 
     default:            /* Parent falls through to create next child */
         break;
     }
 
     switch (fork()) {
     case -1:
         errExit("fork");
 
     case 0:             /* Second child: exec 'wc' to read from pipe */
         if (close(pfd[1]) == -1)                    /* Write end is unused */
             errExit("close 3");
 
         /* Duplicate stdin on read end of pipe; close duplicated descriptor */
 
         if (pfd[0] != STDIN_FILENO) {               /* Defensive check */
             if (dup2(pfd[0], STDIN_FILENO) == -1)
                 errExit("dup2 2");
             if (close(pfd[0]) == -1)
                 errExit("close 4");
         }
 
         execlp("wc", "wc", "-l", (char *) NULL);
         errExit("execlp wc");
 
     default: /* Parent falls through */
         break;
     }
 
     /* Parent closes unused file descriptors for pipe, and waits for children */
 
     if (close(pfd[0]) == -1)
         errExit("close 5");
     if (close(pfd[1]) == -1)
         errExit("close 6");
     if (wait(NULL) == -1)
         errExit("wait 1");
     if (wait(NULL) == -1)
         errExit("wait 2");
 
     exit(EXIT_SUCCESS);
 }