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 May 1, 2013 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.Ft int 44.Fn pipe2 "int fildes[2]" "int flags" 45.Sh DESCRIPTION 46The 47.Fn pipe 48system call 49creates a 50.Em pipe , 51which is an object allowing 52bidirectional data flow, 53and allocates a pair of file descriptors. 54.Pp 55The 56.Fn pipe2 57system call allows control over the attributes of the file descriptors 58via the 59.Fa flags 60argument. 61Values for 62.Fa flags 63are constructed by a bitwise-inclusive OR of flags from the following 64list, defined in 65.In fcntl.h : 66.Bl -tag -width ".Dv O_NONBLOCK" 67.It Dv O_CLOEXEC 68Set the close-on-exec flag for the new file descriptors. 69.It Dv O_NONBLOCK 70Set the non-blocking flag for the ends of the pipe. 71.El 72.Pp 73If the 74.Fa flags 75argument is 0, the behavior is identical to a call to 76.Fn pipe . 77.Pp 78By convention, the first descriptor is normally used as the 79.Em read end 80of the pipe, 81and the second is normally the 82.Em write end , 83so that data written to 84.Fa fildes[1] 85appears on (i.e., can be read from) 86.Fa fildes[0] . 87This allows the output of one program to be 88sent 89to another program: 90the source's standard output is set up to be 91the write end of the pipe, 92and the sink's standard input is set up to be 93the read end of the pipe. 94The pipe itself persists until all its associated descriptors are 95closed. 96.Pp 97A pipe that has had an end closed is considered 98.Em widowed . 99Writing on such a pipe causes the writing process to receive 100a 101.Dv SIGPIPE 102signal. 103Widowing a pipe is the only way to deliver end-of-file to a reader: 104after the reader consumes any buffered data, reading a widowed pipe 105returns a zero count. 106.Pp 107The bidirectional nature of this implementation of pipes is not 108portable to older systems, so it is recommended to use the convention 109for using the endpoints in the traditional manner when using a 110pipe in one direction. 111.Sh RETURN VALUES 112.Rv -std pipe 113.Sh ERRORS 114The 115.Fn pipe 116and 117.Fn pipe2 118system calls will fail if: 119.Bl -tag -width Er 120.It Bq Er EMFILE 121Too many descriptors are active. 122.It Bq Er ENFILE 123The system file table is full. 124.It Bq Er ENOMEM 125Not enough kernel memory to establish a pipe. 126.El 127.Pp 128The 129.Fn pipe2 130system call will also fail if: 131.Bl -tag -width Er 132.It Bq Er EINVAL 133The 134.Fa flags 135argument is invalid. 136.El 137.Sh SEE ALSO 138.Xr sh 1 , 139.Xr fork 2 , 140.Xr read 2 , 141.Xr socketpair 2 , 142.Xr write 2 143.Sh HISTORY 144The 145.Fn pipe 146function appeared in 147.At v3 . 148.Pp 149Bidirectional pipes were first used on 150.At V.4 . 151.Pp 152The 153.Fn pipe2 154function appeared in 155.Fx 10.0 . 156