Creating and Opening Pipes and FIFOs

A named pipe, also called a FIFO, is a pipe identified by an entry in a file system's name space. FIFOs are created using mknod(2), mkfifo(1M), or the mknod(1M) command. They are removed using unlink(2) or the rm(1) command.

FIFOs look like regular file system nodes, but are distinguished from them by a p in the first column when the ls -l command is run.

	/usr/sbin/mknod xxx pls -l xxx
prw-r--r-- 1 guest other 0 Aug 26 10:55 xxx
echoput> hello.world>xxx &put>
[1] 8733
cat xxx
hello world
[1]  + Done
rm xxx

FIFOs are unidirectional: that is, one end of the FIFO is used for writing data, the other for reading data. FIFOs allow simple one-way interprocess communication between unrelated processes. Modules may be pushed onto a FIFO. Data written to the FIFO is passed down the write side of the module and back up the read side as shown in Figure 6–1.

Figure 6-1  Pushing Modules on a STREAMS-based FIFO

FIFOs are opened in the same manner as other file system nodes with open(2). Any data written to the FIFO can be read from the same file descriptor in the first-in, first-out manner (serial, sequentially). Modules can also be pushed on the FIFO. See open(2) for the restrictions that apply when opening a FIFO. If O_NDELAY or O_NONBLOCK is not specified, an open on a FIFO blocks until both a reader and a writer are present.

Named or mounted streams provide a more powerful interface for interprocess communications than does a FIFO. See Named Streams for details.

A STREAMS-based pipe, also referred to as an anonymous pipe, is created using pipe(2), which returns two file descriptors, fd[0] and fd[1], each with its own stream head. The ends of the pipe are constructed so that data written to either end of a pipe may be read from the opposite end.

STREAMS modules can be added to a pipe with I_PUSH ioctl(2). A module can be pushed onto one or both ends of the pipe (see Figure 6–2 ). However, if a module is pushed onto one end of the pipe, that module cannot be popped from the other end.

Figure 6-2  Pushing Modules on a STREAMS-based Pipe