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.Nm pipe2 37.Nd create descriptor pair for interprocess communication 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In unistd.h 42.Ft int 43.Fn pipe "int fildes[2]" 44.Ft int 45.Fn pipe2 "int fildes[2]" "int flags" 46.Sh DESCRIPTION 47The 48.Fn pipe 49system call 50creates a 51.Em pipe , 52which is an object allowing 53bidirectional data flow, 54and allocates a pair of file descriptors. 55.Pp 56The 57.Fn pipe2 58system call allows control over the attributes of the file descriptors 59via the 60.Fa flags 61argument. 62Values for 63.Fa flags 64are constructed by a bitwise-inclusive OR of flags from the following 65list, defined in 66.In fcntl.h : 67.Bl -tag -width ".Dv O_NONBLOCK" 68.It Dv O_CLOEXEC 69Set the close-on-exec flag for the new file descriptors. 70.It Dv O_NONBLOCK 71Set the non-blocking flag for the ends of the pipe. 72.El 73.Pp 74If the 75.Fa flags 76argument is 0, the behavior is identical to a call to 77.Fn pipe . 78.Pp 79By convention, the first descriptor is normally used as the 80.Em read end 81of the pipe, 82and the second is normally the 83.Em write end , 84so that data written to 85.Fa fildes[1] 86appears on (i.e., can be read from) 87.Fa fildes[0] . 88This allows the output of one program to be 89sent 90to another program: 91the source's standard output is set up to be 92the write end of the pipe, 93and the sink's standard input is set up to be 94the read end of the pipe. 95The pipe itself persists until all its associated descriptors are 96closed. 97.Pp 98A pipe that has had an end closed is considered 99.Em widowed . 100Writing on such a pipe causes the writing process to receive 101a 102.Dv SIGPIPE 103signal. 104Widowing a pipe is the only way to deliver end-of-file to a reader: 105after the reader consumes any buffered data, reading a widowed pipe 106returns a zero count. 107.Pp 108The bidirectional nature of this implementation of pipes is not 109portable to older systems, so it is recommended to use the convention 110for using the endpoints in the traditional manner when using a 111pipe in one direction. 112.Sh RETURN VALUES 113.Rv -std pipe 114.Sh ERRORS 115The 116.Fn pipe 117and 118.Fn pipe2 119system calls will fail if: 120.Bl -tag -width Er 121.It Bq Er EMFILE 122Too many descriptors are active. 123.It Bq Er ENFILE 124The system file table is full. 125.It Bq Er ENOMEM 126Not enough kernel memory to establish a pipe. 127.El 128.Pp 129The 130.Fn pipe2 131system call will also fail if: 132.Bl -tag -width Er 133.It Bq Er EINVAL 134The 135.Fa flags 136argument is invalid. 137.El 138.Sh SEE ALSO 139.Xr sh 1 , 140.Xr fork 2 , 141.Xr read 2 , 142.Xr socketpair 2 , 143.Xr write 2 144.Sh HISTORY 145The 146.Fn pipe 147function appeared in 148.At v3 . 149.Pp 150Bidirectional pipes were first used on 151.At V.4 . 152.Pp 153The 154.Fn pipe2 155function appeared in 156.Fx 10.0 . 157