Understanding the Linux Kernel

18.2. FIFOs

Although pipes are a simple, flexible, and efficient communication mechanism, they have one main drawback, namely, that there is no way to open an already existing pipe. This makes it impossible for two arbitrary processes to share the same pipe, unless the pipe was created by a common ancestor process.

This drawback is substantial for many application programs. Consider, for instance, a database engine server, which continuously polls client processes wishing to issue some queries and which sends back to them the results of the database lookups. Each interaction between the server and a given client might be handled by a pipe. However, client processes are usually created on demand by a command shell when a user explicitly queries the database; server and client processes thus cannot easily share a pipe.

In order to address such limitations, Unix systems introduce a special file type called a named pipe or FIFO (which stands for "first in, first out": the first byte written into the special file is also the first byte that will be read).^[5]

^[5] Starting with System V Release 3, FIFOs are implemented as full-duplex (bidirectional) objects.

FIFO files are similar to device files: they have a disk inode, but they do not make use of data blocks. Thanks to the disk inode, a FIFO can be accessed by any process, since the FIFO filename is included in the system's directory tree. In addition to having a filename, FIFOs are similar to unnamed pipes in that they also include ...