PIPE(2) System Calls Manual PIPE(2)

pipe, pipe2 — create descriptor pair for interprocess communication

Standard C Library (libc, -lc)

#include <unistd.h>

int
pipe(int fildes[2]);

#include <fcntl.h>
#include <unistd.h>

int
pipe2(int fildes[2], int flags);

The () 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 of the pipe, and the second connects to the , 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 . 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 () function behaves exactly like pipe() only it allows extra flags to be set on the returned file descriptors. The following flags are valid:

Set the “close-on-exec” property.
Set the “close-on-fork” property.
Sets non-blocking I/O.
Return EPIPE instead of raising SIGPIPE.

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.

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

[]
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.
[]
Too many descriptors are active.
[]
The system file table is full.
[]
Not enough kernel memory to establish a pipe.

pipe2() will also fail if:

[]
flags contains an invalid value.

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

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

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