freebsd-dev/lib/libc/sys/pipe.2

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.\"     @(#)pipe.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt PIPE 2
.Os BSD 4
.Sh NAME
.Nm pipe
.Nd create descriptor pair for interprocess communication
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn pipe "int *fildes"
.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
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
Pipes are really a special case of the 
.Xr socketpair 2
call and, in fact, are implemented as such in the system.
.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
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 [EMFILE]
.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 read 2 ,
.Xr write 2 ,
.Xr fork 2 ,
.Xr socketpair 2
.Sh HISTORY
A
.Nm
function call appeared in Version 6 AT&T UNIX.
.Pp
Bidirectional pipes were first used on Unix System V release 4.