Named Pipes: FIFOs

FIFO

A named pipe is a special type of file (remember that everything in Linux is a file!) that exists as a name in the file system but behaves like the unnamed
pipes that we’ve met already.

We can create named pipes from the command line and from within a program. Historically, the command line program for creating them was mknod:

$ mknod filename p

However, the mknod command is not in the X/Open command list, so it may not be available on all UNIX-like systems. The preferred command line method is to use

$ mkfifo filename

Some older versions of UNIX only had the mknod command. X/Open Issue 4 Version 2 has the mknod function call, but not the command line program. Linux, friendly as ever, supplies both mknod and mkfifo.

From inside a program, we can use two different calls:

#include <sys/types.h>
#include <sys/stat.h>
int mkfifo(const char *filename, mode_t mode);
int mknod(const char *filename, mode_t mode | S_IFIFO, (dev_t) 0);

Like the mknod command, you can use the mknod function for making many special types of files. Using a dev_t value of 0 and ORing the file access mode with S_IFIFO is the only portable use of this function that creates a named pipe. We’ll use the simpler mkfifo function in our examples


#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
int main()
{
int res = mkfifo(“/tmp/my_fifo”, 0777);
if (res == 0) printf(“FIFO created\n”);
exit(EXIT_SUCCESS);
}



We can look for the pipe with

$ ls -lF /tmp/my_fifo

prwxr-xr-x 1 vineet embedyourcareer  0 march 18 14:55 /tmp/my_fifo|

Notice that the first character of output is a p, indicating a pipe. The | symbol at the end is added by the ls command’s -F option and also indicates a pipe.

How It Works

The program uses the mkfifo function to create a special file. Although we ask for a mode of 0777, this is altered by the user mask (umask) setting (in this case 022), just as in normal file creation, so the resulting file has mode 755. If your umask is set differently, for example to 0002, you will see different permissions on the created file.
We can remove the FIFO just like a conventional file by using the rm command, or from within a program by using the unlink system call.