.\"	$OpenBSD: pipe.2,v 1.15 2013/07/17 05:42:11 schwarze Exp $
.\"	$NetBSD: pipe.2,v 1.6 1995/02/27 12:35:27 cgd Exp $
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.\"     @(#)pipe.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: July 17 2013 $
.Dt PIPE 2
.Os
.Sh NAME
.Nm pipe
.Nd create descriptor pair for interprocess communication
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn pipe "int fildes[2]"
.Sh DESCRIPTION
The
.Fn pipe
function creates a
.Em pipe ,
which is an object allowing unidirectional data flow,
and allocates a pair of file descriptors.
The first descriptor connects to the
.Em read end
of the pipe,
and the second connects to 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 whose read or write end has been 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.
.Sh RETURN VALUES
On successful creation of the pipe, zero is returned.
Otherwise, a value of \-1 is returned and the variable
.Va errno
set to indicate the error.
.Sh ERRORS
The
.Fn pipe
call 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 EFAULT
The
.Fa fildes
buffer is in an invalid area of the process's address space.
.El
.Sh SEE ALSO
.Xr sh 1 ,
.Xr fork 2 ,
.Xr read 2 ,
.Xr socketpair 2 ,
.Xr write 2
.Sh STANDARDS
The
.Fn pipe
function conforms to
.St -p1003.1-2008 .
.Pp
As an extension, the pipe provided is actually capable of moving
data bidirectionally.
This is compatible with SVR4.
However, this is non-POSIX behaviour which should not be relied on,
for reasons of portability.
.Sh HISTORY
A
.Fn pipe
function call appeared in
.At v3 .
Since
.At v4 ,
it allocates two distict file descriptors.