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## 6.3.3 FIFO Operations

I/O operations on a FIFO are essentially the same as for normal pipes, with once major exception. An ``open'' system call or library function should be used to physically open up a channel to the pipe. With half-duplex pipes, this is unnecessary, since the pipe resides in the kernel and not on a physical filesystem. In our examples, we will treat the pipe as a stream, opening it up with fopen(), and closing it with fclose().

Consider a simple server process:

    /*****************************************************************************
      Excerpt from "Linux Programmer's Guide - Chapter 6"
      (C)opyright 1994-1995, Scott Burkett
      *****************************************************************************
      MODULE: fifoserver.c
      *****************************************************************************/

     #include <stdio.h>
     #include <stdlib.h>
     #include <sys/stat.h>
     #include <unistd.h>

     #include <linux/stat.h>

     #define FIFO_FILE       "MYFIFO"

     int main(void)
     {
             FILE *fp;
             char readbuf[80];

             /* Create the FIFO if it does not exist */
             umask(0);
             mknod(FIFO_FILE, S_IFIFO|0666, 0);

             while(1)
             {
                     fp = fopen(FIFO_FILE, "r");
                     fgets(readbuf, 80, fp);
                     printf("Received string: %s\n", readbuf);
                     fclose(fp);
             }

             return(0);
     }

Since a FIFO blocks by default, run the server in the background after you compile it:

            $ fifoserver&

We will discuss a FIFO's blocking action in a moment. First, consider the following simple client frontend to our server:

    /*****************************************************************************
      Excerpt from "Linux Programmer's Guide - Chapter 6"
      (C)opyright 1994-1995, Scott Burkett
      *****************************************************************************
      MODULE: fifoclient.c
      *****************************************************************************/

     #include <stdio.h>
     #include <stdlib.h>

     #define FIFO_FILE       "MYFIFO"

     int main(int argc, char *argv[])
     {
             FILE *fp;

             if ( argc != 2 ) {
                     printf("USAGE: fifoclient [string]\n");
                     exit(1);
             }

             if((fp = fopen(FIFO_FILE, "w")) == NULL) {
                     perror("fopen");
                     exit(1);
             }

             fputs(argv[1], fp);

             fclose(fp);
             return(0);
     }