Perl Cookbook

Making a Process Look Like a File with Named Pipes

Problem

You want a process to intercept all access to a file. For instance, you want to make your ~/.plan file a program that returns a random quote.

Solution

Use named pipes. First create one, probably from your shell:

% mkfifo /path/to/named.pipe

Here’s a reader for it:

open(FIFO, "< /path/to/named.pipe")         or die $!;
while (<FIFO>) {
    print "Got: $_";
}
close(FIFO);

Here’s a writer for it:

open(FIFO, "> /path/to/named.pipe")         or die $!;
print FIFO "Smoke this.\n";
close(FIFO);

Discussion

A named pipe, or FIFO as they are also known, is a special file that acts as a buffer to connect processes on the same machine. Ordinary pipes also allow processes to communicate, but those processes must have inherited the filehandles from their parents. To use a named pipe, a process need know only the named pipe’s filename. In most cases, processes don’t even need to be aware that they’re reading from a FIFO.

Named pipes can be read from and written to just as though they were ordinary files (unlike Unix-domain sockets as discussed in Chapter 17). Data written into the FIFO is buffered up by the operating system, then read back in the order it was written in. Because a FIFO acts as a buffer to connect processes, opening one for reading will block until another process opens it for writing, and vice versa. If you open for read and write using the +< mode to open, you won’t block (on most systems) because your process could be both reader and writer. ...