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_CLOFORK: Set the close-on-fork 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 value -1 is returned and the global variable errno is set to indicate the error.

ERRORS

The pipe() and pipe2() system calls will fail if:

[EFAULT]: fildes argument points to an invalid memory location.
[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.

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.

The O_CLOFORK flag appeared in FreeBSD 15.0.