PIPE(2) | 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]);

`#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 IEEE Std 1003.1-1990 (“POSIX.1”). The `pipe2`() function conforms to IEEE Std 1003.1-2024 (“POSIX.1”).

# HISTORY

A `pipe`() function call appeared in Version 3 AT&T UNIX. Since Version 4 AT&T UNIX, it allocates two distinct file descriptors.

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

July 8, 2025 | NetBSD 11.0