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.\" Copyright (c) 1980, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" @(#)pipe.2 8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd March 31, 2013
.Dt PIPE 2
.Os
.Sh NAME
.Nm pipe
.Nd create descriptor pair for interprocess communication
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn pipe "int fildes[2]"
.Ft int
.Fn pipe2 "int fildes[2]" "int flags"
.Sh DESCRIPTION
The
.Fn pipe
system call
creates a
.Em pipe ,
which is an object allowing
bidirectional data flow,
and allocates a pair of file descriptors.
.Pp
The
.Fn pipe2
system call allows control over the attributes of the file descriptors
via the
.Fa flags
argument.
Values for
.Fa flags
are constructed by a bitwise-inclusive OR of flags from the following
list, defined in
.In fcntl.h :
.Bl -tag -width ".Dv O_NONBLOCK"
.It Dv O_CLOEXEC
Set the close-on-exec flag for the new file descriptors.
.It Dv O_NONBLOCK
Set the non-blocking flag for the ends of the pipe.
.El
.Pp
If the
.Fa flags
argument is 0, the behavior is identical to a call to
.Fn pipe .
.Pp
By convention, the first descriptor is normally used as the
.Em read end
of the pipe,
and the second is normally the
.Em write end ,
so that data written to
.Fa fildes[1]
appears on (i.e., can be read from)
.Fa fildes[0] .
This allows the output of one program to be
sent
to another program:
the source's standard output is set up to be
the write end of the pipe,
and the sink's standard input is set up to be
the read end of the pipe.
The pipe itself persists until all its associated descriptors are
closed.
.Pp
A pipe that has had an end closed is considered
.Em widowed .
Writing on such a pipe causes the writing process to receive
a
.Dv SIGPIPE
signal.
Widowing a pipe is the only way to deliver end-of-file to a reader:
after the reader consumes any buffered data, reading a widowed pipe
returns a zero count.
.Pp
The bidirectional nature of this implementation of pipes is not
portable to older systems, so it is recommended to use the convention
for using the endpoints in the traditional manner when using a
pipe in one direction.
.Sh RETURN VALUES
.Rv -std pipe
.Sh ERRORS
The
.Fn pipe
and
.Fn pipe2
system calls will fail if:
.Bl -tag -width Er
.It Bq Er EMFILE
Too many descriptors are active.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ENOMEM
Not enough kernel memory to establish a pipe.
.El
.Pp
The
.Fn pipe2
system call will also fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa flags
argument is invalid.
.El
.Sh SEE ALSO
.Xr sh 1 ,
.Xr fork 2 ,
.Xr read 2 ,
.Xr socketpair 2 ,
.Xr write 2
.Sh HISTORY
The
.Fn pipe
function appeared in
.At v3 .
.Pp
Bidirectional pipes were first used on
.At V.4 .
.Pp
The
.Fn pipe2
function appeared in
.Fx 10.0 .
|
