pipe man page

pipe, pipe2 — create pipe


#include <unistd.h>

int pipe(int pipefd[2]);

#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h> /* Obtain O_* constant definitions */
#include <unistd.h>

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


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:

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.

Set the O_NONBLOCK file status flag on the two new open file descriptions. Using this flag saves extra calls to fcntl(2) to achieve the same result.

Return Value

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

On Linux (and other systems), pipe() does not modify pipefd on failure. A requirement standardizing this behavior was added in POSIX.1-2016. The Linux-specific pipe2() system call likewise does not modify pipefd on failure.


pipefd is not valid.
(pipe2()) Invalid value in flags.
The per-process limit on the number of open file descriptors has been reached.
The system-wide limit on the total number of open files has been reached.


pipe2() was added to Linux in version 2.6.27; glibc support is available starting with version 2.9.

Conforming to

pipe(): POSIX.1-2001, POSIX.1-2008.

pipe2() is Linux-specific.


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 <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

main(int argc, char *argv[])
    int pipefd[2];
    pid_t cpid;
    char buf;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <string>\n", argv[0]);

    if (pipe(pipefd) == -1) {

    cpid = fork();
    if (cpid == -1) {

    if (cpid == 0) {    /* Child reads from pipe */
        close(pipefd[1]);          /* Close unused write end */

        while (read(pipefd[0], &buf, 1) > 0)
            write(STDOUT_FILENO, &buf, 1);

        write(STDOUT_FILENO, "\n", 1);

    } else {            /* Parent writes argv[1] to pipe */
        close(pipefd[0]);          /* Close unused read end */
        write(pipefd[1], argv[1], strlen(argv[1]));
        close(pipefd[1]);          /* Reader will see EOF */
        wait(NULL);                /* Wait for child */

See Also

fork(2), read(2), socketpair(2), splice(2), write(2), popen(3), pipe(7)

Referenced By

aclient(1), capabilities(7), eventfd(2), eventtest(6), explain(1), explain(3), explain_pipe(3), explain_pipe2(3), explain_pipe2_or_die(3), explain_pipe_or_die(3), fifo(7), fork(2), getrlimit(2), guestunmount(1), iv_event_raw(3), ksh(1), man-pages(7), mksh(1), pipe(7), PMDA(3), pmdaConnect(3), popen(3), sec(1), socketpair(2), stat(2), statfs(2), syscalls(2), tcsh(1).

Explore man page connections for pipe(2).

pipe2(2) is an alias of pipe(2).