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.\"     @(#)pipe.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: src/lib/libc/sys/pipe.2,v 1.24.2.1.6.1 2010/12/21 17:09:25 kensmith Exp $
.\"
.Dd January 30, 2006
.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]"
.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
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
system 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 ENOMEM
Not enough kernel memory to establish a pipe.
.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 .