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.\"     @(#)pipe.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: releng/12.2/lib/libc/sys/pipe.2 326437 2017-12-01 22:48:20Z imp $
.\"
.Dd December 1, 2017
.Dt PIPE 2
.Os
.Sh NAME
.Nm pipe ,
.Nm pipe2
.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
function
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 IMPLEMENTATION NOTES
The
.Fn pipe
function calls the
.Fn pipe2
system call.
As a result, system call traces such as those captured by
.Xr dtrace 1
or
.Xr ktrace 1
will show calls to
.Fn pipe2 .
.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 EFAULT
.Ar fildes
argument points to an invalid memory location.
.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 .
.Pp
The
.Fn pipe
function became a wrapper around
.Fn pipe2
in
.Fx 11.0 .