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);
}