1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 4. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)pipe.2 8.1 (Berkeley) 6/4/93 29.\" $FreeBSD$ 30.\" 31.Dd January 30, 2006 32.Dt PIPE 2 33.Os 34.Sh NAME 35.Nm pipe 36.Nd create descriptor pair for interprocess communication 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In unistd.h 41.Ft int 42.Fn pipe "int fildes[2]" 43.Sh DESCRIPTION 44The 45.Fn pipe 46system call 47creates a 48.Em pipe , 49which is an object allowing 50bidirectional data flow, 51and allocates a pair of file descriptors. 52.Pp 53By convention, the first descriptor is normally used as the 54.Em read end 55of the pipe, 56and the second is normally the 57.Em write end , 58so that data written to 59.Fa fildes[1] 60appears on (i.e., can be read from) 61.Fa fildes[0] . 62This allows the output of one program to be 63sent 64to another program: 65the source's standard output is set up to be 66the write end of the pipe, 67and the sink's standard input is set up to be 68the read end of the pipe. 69The pipe itself persists until all its associated descriptors are 70closed. 71.Pp 72A pipe that has had an end closed is considered 73.Em widowed . 74Writing on such a pipe causes the writing process to receive 75a 76.Dv SIGPIPE 77signal. 78Widowing a pipe is the only way to deliver end-of-file to a reader: 79after the reader consumes any buffered data, reading a widowed pipe 80returns a zero count. 81.Pp 82The bidirectional nature of this implementation of pipes is not 83portable to older systems, so it is recommended to use the convention 84for using the endpoints in the traditional manner when using a 85pipe in one direction. 86.Sh RETURN VALUES 87.Rv -std pipe 88.Sh ERRORS 89The 90.Fn pipe 91system call will fail if: 92.Bl -tag -width Er 93.It Bq Er EMFILE 94Too many descriptors are active. 95.It Bq Er ENFILE 96The system file table is full. 97.It Bq Er ENOMEM 98Not enough kernel memory to establish a pipe. 99.El 100.Sh SEE ALSO 101.Xr sh 1 , 102.Xr fork 2 , 103.Xr read 2 , 104.Xr socketpair 2 , 105.Xr write 2 106.Sh HISTORY 107The 108.Fn pipe 109function appeared in 110.At v3 . 111.Pp 112Bidirectional pipes were first used on 113.At V.4 . 114