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PIPE(2) FreeBSD System Calls Manual PIPE(2)
NAME
pipe, pipe2 - create descriptor pair for interprocess communication
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int
pipe(int fildes[2]);
int
pipe2(int fildes[2], int flags);
DESCRIPTION
The pipe() function creates a pipe, which is an object allowing
bidirectional data flow, and allocates a pair of file descriptors.
The pipe2() system call allows control over the attributes of the file
descriptors via the flags argument. Values for flags are constructed by
a bitwise-inclusive OR of flags from the following list, defined in
<fcntl.h>:
O_CLOEXEC Set the close-on-exec flag for the new file descriptors.
O_NONBLOCK Set the non-blocking flag for the ends of the pipe.
If the flags argument is 0, the behavior is identical to a call to
pipe().
By convention, the first descriptor is normally used as the read end of
the pipe, and the second is normally the write end, so that data written
to fildes[1] appears on (i.e., can be read from) 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.
A pipe that has had an end closed is considered widowed. Writing on such
a pipe causes the writing process to receive a 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.
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.
IMPLEMENTATION NOTES
The pipe() function calls the pipe2() system call. As a result, system
call traces such as those captured by dtrace(1) or ktrace(1) will show
calls to pipe2().
RETURN VALUES
The pipe() function returns the value 0 if successful; otherwise the
[EMFILE] Too many descriptors are active.
[ENFILE] The system file table is full.
[ENOMEM] Not enough kernel memory to establish a pipe.
The pipe2() system call will also fail if:
[EINVAL] The flags argument is invalid.
SEE ALSO
sh(1), fork(2), read(2), socketpair(2), write(2)
HISTORY
The pipe() function appeared in Version 3 AT&T UNIX.
Bidirectional pipes were first used on AT&T System V Release 4 UNIX.
The pipe2() function appeared in FreeBSD 10.0.
The pipe() function became a wrapper around pipe2() in FreeBSD 11.0.
FreeBSD 14.0-RELEASE-p6 December 1, 2017 FreeBSD 14.0-RELEASE-p6