Two types of pipes? Post: 88106

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Top Forums UNIX for Dummies Questions & Answers Two types of pipes? Post 88106 by Perderabo on Monday 31st of October 2005 05:11:56 PM

10-31-2005

Administrator Emeritus

Your first type of pipe actually is a pipe. That is, it is created by the pipe() system call. That system call returns a pair of fd's in an array. You write stuff on fd[1] and read stuff on fd[0]. Pipes persist across both fork() and exec(). You can wind up with several processes writing to fd[1] or reading from fd[0] (not that multiple readers make that much sense). But all processes using the pipe are desended from the process that issued pipe() in the first place (I'm ignoring a very arcane exception here). Mostly though you have one process writing on fd[1] and another process reading from fd[0]. I just checked the Posix standard, and this is all that the standard officially mandates. Virually every modern version of unix implements an extention: writing on fd[0] and reading from fd[1]. This results in two independent data streams. This emulates the behavior of socketpair() and this extension arose on BSD when pipes were implemented as socket styles objects. (The first implementation was a as a screwy file system object.)

Your second style of pipe was originally called a "named pipe". It is politically incorrect to call them "named pipes" today. (Which is exactly why I call them "named pipes".) You are supposed to call them fifos today. (F. I. F. O == first in, first out). It is not possible for named pipes to be bi-directional like a socketpair() style pipe. There is a single data stream. Very often there will be a single reader and multiple writers. The processes need not be closely related to each other nor to the creator of the named pipe. Permission to read and write is controlled by file system permission. Unlike pipes, named pipes must be opened with open(). open() is expensive relative to pipe().

Which one is faster at transferring data depends on your kernel and how you built it. With HP-UX, for example, you can build a kernel that uses Streams-based pipes, but HP-UX does not support Streams-based fifos. This mostly affects non-standard features like using a zero length write to send a zero length message.

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fifo

FIFO(7) 						     Linux Programmer's Manual							   FIFO(7)

NAME

       fifo - first-in first-out special file, named pipe

DESCRIPTION

       A FIFO special file (a named pipe) is similar to a pipe, except that it is accessed as part of the file system.	It can be opened by multi-
       ple processes for reading or writing.  When processes are exchanging data via the FIFO, the kernel passes all data internally without writ-
       ing it to the file system.  Thus, the FIFO special file has no contents on the file system; the file system entry merely serves as a refer-
       ence point so that processes can access the pipe using a name in the file system.

       The kernel maintains exactly one pipe object for each FIFO special file that is opened by at least one process.	The FIFO must be opened on
       both ends (reading and writing) before data can be passed.  Normally, opening the FIFO blocks until the other end is opened also.

       A  process  can	open  a FIFO in nonblocking mode.  In this case, opening for read-only will succeed even if no-one has opened on the write
       side yet, opening for write-only will fail with ENXIO (no such device or address) unless the other end has already been opened.

       Under Linux, opening a FIFO for read and write will succeed both in blocking and nonblocking mode.  POSIX leaves this  behavior	undefined.
       This can be used to open a FIFO for writing while there are no readers available.  A process that uses both ends of the connection in order
       to communicate with itself should be very careful to avoid deadlocks.

NOTES

       When a process tries to write to a FIFO that is not opened for read on the other side, the process is sent a SIGPIPE signal.

       FIFO special files can be created by mkfifo(3), and are indicated by ls -l with the file type 'p'.

SEE ALSO

       mkfifo(1), open(2), pipe(2), sigaction(2), signal(2), socketpair(2), mkfifo(3), pipe(7)

COLOPHON

       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2008-12-03								   FIFO(7)