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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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pipe(2) System Calls Manual pipe(2)
pipe, pipe2 - create pipe
Standard C library (libc, -lc)
#include <unistd.h>
int pipe(int pipefd[2]);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h> /* Definition of O_* constants */
#include <unistd.h>
int pipe2(int pipefd[2], int flags);
/* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64, pipe() has the
following prototype; see VERSIONS */
#include <unistd.h>
struct fd_pair {
long fd[2];
};
struct fd_pair pipe(void);
pipe() creates a pipe, a unidirectional data channel that can be
used for interprocess communication. The array pipefd is used to
return two file descriptors referring to the ends of the pipe.
pipefd[0] refers to the read end of the pipe. pipefd[1] refers to
the write end of the pipe. Data written to the write end of the
pipe is buffered by the kernel until it is read from the read end
of the pipe. For further details, see pipe(7).
If flags is 0, then pipe2() is the same as pipe(). The following
values can be bitwise ORed in flags to obtain different behavior:
O_CLOEXEC
Set the close-on-exec (FD_CLOEXEC) flag on the two new file
descriptors. See the description of the same flag in
open(2) for reasons why this may be useful.
O_DIRECT (since Linux 3.4)
Create a pipe that performs I/O in "packet" mode. Each
write(2) to the pipe is dealt with as a separate packet,
and read(2)s from the pipe will read one packet at a time.
Note the following points:
• Writes of greater than PIPE_BUF bytes (see pipe(7)) will
be split into multiple packets. The constant PIPE_BUF
is defined in <limits.h>.
• If a read(2) specifies a buffer size that is smaller
than the next packet, then the requested number of bytes
are read, and the excess bytes in the packet are
discarded. Specifying a buffer size of PIPE_BUF will be
sufficient to read the largest possible packets (see the
previous point).
• Zero-length packets are not supported. (A read(2) that
specifies a buffer size of zero is a no-op, and returns
0.)
Older kernels that do not support this flag will indicate
this via an EINVAL error.
Since Linux 4.5, it is possible to change the O_DIRECT
setting of a pipe file descriptor using fcntl(2).
O_NONBLOCK
Set the O_NONBLOCK file status flag on the open file
descriptions referred to by the new file descriptors.
Using this flag saves extra calls to fcntl(2) to achieve
the same result.
O_NOTIFICATION_PIPE
Since Linux 5.8, general notification mechanism is built on
the top of the pipe where kernel splices notification
messages into pipes opened by user space. The owner of the
pipe has to tell the kernel which sources of events to
watch and filters can also be applied to select which
subevents should be placed into the pipe.
On success, zero is returned. On error, -1 is returned, errno is
set to indicate the error, and pipefd is left unchanged.
On Linux (and other systems), pipe() does not modify pipefd on
failure. A requirement standardizing this behavior was added in
POSIX.1-2008 TC2. The Linux-specific pipe2() system call likewise
does not modify pipefd on failure.
EFAULT pipefd is not valid.
EINVAL (pipe2()) Invalid value in flags.
EMFILE The per-process limit on the number of open file
descriptors has been reached.
ENFILE The system-wide limit on the total number of open files has
been reached.
ENFILE The user hard limit on memory that can be allocated for
pipes has been reached and the caller is not privileged;
see pipe(7).
ENOPKG (pipe2()) O_NOTIFICATION_PIPE was passed in flags and
support for notifications (CONFIG_WATCH_QUEUE) is not
compiled into the kernel.
The System V ABI on some architectures allows the use of more than
one register for returning multiple values; several architectures
(namely, Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64) (ab)use
this feature in order to implement the pipe() system call in a
functional manner: the call doesn't take any arguments and returns
a pair of file descriptors as the return value on success. The
glibc pipe() wrapper function transparently deals with this. See
syscall(2) for information regarding registers used for storing
second file descriptor.
pipe() POSIX.1-2008.
pipe2()
Linux.
pipe() POSIX.1-2001.
pipe2()
Linux 2.6.27, glibc 2.9.
The following program creates a pipe, and then fork(2)s to create
a child process; the child inherits a duplicate set of file
descriptors that refer to the same pipe. After the fork(2), each
process closes the file descriptors that it doesn't need for the
pipe (see pipe(7)). The parent then writes the string contained
in the program's command-line argument to the pipe, and the child
reads this string a byte at a time from the pipe and echoes it on
standard output.
Program source
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
int pipefd[2];
char buf;
pid_t cpid;
if (argc != 2) {
fprintf(stderr, "Usage: %s <string>\n", argv[0]);
exit(EXIT_FAILURE);
}
if (pipe(pipefd) == -1)
err(EXIT_FAILURE, "pipe");
cpid = fork();
if (cpid == -1)
err(EXIT_FAILURE, "fork");
if (cpid == 0) { /* Child reads from pipe */
if (close(pipefd[1]) == -1) /* Close unused write end */
err(EXIT_FAILURE, "close");
while (read(pipefd[0], &buf, 1) > 0) {
if (write(STDOUT_FILENO, &buf, 1) != 1)
err(EXIT_FAILURE, "write");
}
if (write(STDOUT_FILENO, "\n", 1) != 1)
err(EXIT_FAILURE, "write");
if (close(pipefd[0]) == -1)
err(EXIT_FAILURE, "close");
_exit(EXIT_SUCCESS);
} else { /* Parent writes argv[1] to pipe */
if (close(pipefd[0]) == -1) /* Close unused read end */
err(EXIT_FAILURE, "close");
if (write(pipefd[1], argv[1], strlen(argv[1])) != strlen(argv[1]))
err(EXIT_FAILURE, "write");
if (close(pipefd[1]) == -1) /* Reader will see EOF */
err(EXIT_FAILURE, "close");
if (wait(NULL) == -1) /* Wait for child */
err(EXIT_FAILURE, "wait");
exit(EXIT_SUCCESS);
}
}
fork(2), read(2), socketpair(2), splice(2), tee(2), vmsplice(2),
write(2), popen(3), pipe(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 pipe(2)
Pages that refer to this page: eventfd(2), fork(2), getrlimit(2), ioctl_pipe(2), socketpair(2), statfs(2), syscall(2), syscalls(2), io_uring_prep_pipe(3), pmda(3), pmdaconnect(3), __pmprocesspipe(3), popen(3), capabilities(7), fifo(7), inode(7), man-pages(7), pipe(7), signal-safety(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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