PIPE(2)

NAME
pipe, pipe2 - create descriptor pair for interprocess communication

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
int
pipe(int fildes[2]);
#include <fcntl.h>
#include <unistd.h>
int
pipe2(int fildes[2], int flags);

DESCRIPTION
The pipe() function creates a pipe, which is an object allowing unidirectional data flow, and allocates a pair of file descriptors. The first descriptor connects to the read end of the pipe, and the second connects to 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 whose read or write end has been 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 pipe2() function behaves exactly like pipe() only it allows extra flags to be set on the returned file descriptors. The following flags are valid:

O_CLOEXEC
    Set the “close-on-exec” property.

O_CLOFORK
    Set the “close-on-fork” property.

O_NONBLOCK
    Sets non-blocking I/O.

O_NOSIGPIPE
    Return EPIPE instead of raising SIGPIPE.

RETURN VALUES
On successful creation of the pipe, zero is returned. Otherwise, a value of -1 is returned and the variable errno set to indicate the error.

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

[EFAULT]
    The 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.

[EMFILE]
    Too many descriptors are active.

[ENFILE]
    The system file table is full.

[ENOMEM]
    Not enough kernel memory to establish a pipe.

pipe2() will also fail if:

[EINVAL]
    flags contains an invalid value.

SEE ALSO
sh(1), close(2), fcntl(2), fork(2), read(2), socketpair(2), write(2)

STANDARDS
The pipe() function conforms to p1003.1-90. The pipe2() function conforms to p1003.1-2024.

HISTORY
A pipe() function call appeared in AT&T UNIX. Since AT&T UNIX, it allocates two distinct file descriptors.

The pipe2() function is inspired from Linux and appeared in NetBSD 6.0.