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.\"     @(#)pipe.2	8.1 (Berkeley) 6/4/93
.\"
.Dd November 27, 2020
.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]"
.In fcntl.h
.In unistd.h
.Ft int
.Fn pipe2 "int fildes[2]" "int flags"
.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.
.Pp
The
.Fn pipe2
function
behaves exactly like
.Fn pipe
only it allows extra
.Fa flags
to be set on the returned file descriptor.
The following flags are valid:
.Bl -tag -width O_NONBLOCK -offset indent
.It Dv O_CLOEXEC
Set the
.Dq close-on-exec
property.
.It Dv O_NONBLOCK
Sets non-blocking I/O.
.It Dv O_NOSIGPIPE
Return
.Er EPIPE
instead of raising
.Dv SIGPIPE .
.El
.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
and
.Fn pipe2
calls will fail if:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa fildes
buffer is in an invalid area of the process's address space.
The reliable detection of this error cannot be guaranteed; when not
detected, a signal may be delivered to the process, indicating an
address violation.
.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
.Fn pipe2
will also fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa flags
contains an invalid value.
.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 -p1003.1-90 .
.Sh HISTORY
A
.Fn pipe
function call appeared in
.At v3 .
Since
.At v4 ,
it allocates two distinct file descriptors.
.Pp
The
.Fn pipe2
function is inspired from Linux and appeared in
.Nx 6.0 .