**Named pipes on Unix**

''Adapted (read; ripped with some minor editing) from a Linux man-page: pipe (7)''

Pipes and FIFOs (also known as named pipes) provide a unidirectional interprocess communication channel.  A pipe has a read end and a write end.  Data written to the write end of a pipe can be read from the read end of the pipe.

A FIFO (short for First In First Out) has a name within the file system, and is opened as a regular file.  Any process may open a FIFO, assuming the file permissions allow it, and may then read or write to it as appropriate.

***I/O on Pipes and FIFOs***

The only difference between pipes and FIFOs is the manner in which they are created and opened.  Once these tasks have been accomplished, I/O on pipes and FIFOs has exactly the same semantics.

The communication channel provided by a pipe is a byte stream: there is no concept of message boundaries.

***Pipe Capacity***

A pipe has a limited capacity.  If the pipe is full, then a write will block or fail, depending on whether the pipe was opened in non-blocking mode.  Different implementations have different limits for  the pipe capacity.  Applications should not rely on a particular capacity: an application should be designed so that a reading process consumes data as soon as it is available, so that a writing process does not remain blocked.

In Linux versions before 2.6.11, the capacity of a pipe was the same as the system page size (e.g., 4096 bytes on x86).  Since Linux 2.6.11, the pipe capacity is 65536 bytes.

***Portability notes***

On some systems (but not Linux), pipes are bidirectional: data can be transmitted in both directions between the pipe ends.  According to POSIX.1, pipes only need to be unidirectional.  Portable applications should avoid reliance on bidirectional pipe semantics.

**Named pipes on Windows**

Windows also supports named pipes, albeit with different semantics. [TWAPI] provides commands [http://twapi.magicsplat.com/namedpipe.html] for communicating over named pipes from Tcl.

**Named pipes on Clusenix**

http://www.ibiblio.org/Dave/Dr-Fun/df9707/df970710.jpg

**See also:**
   * [fifo]

<<categories>> File | Interprocess Communication | Channel