pipe(2)

NAME
pipe, pipe2 - create pipe

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#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);

DESCRIPTION
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.

RETURN VALUE
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.

ERRORS

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.

VERSIONS
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.

STANDARDS
POSIX.1-2024.

HISTORY

pipe()
    POSIX.1-2001.

pipe2()
    POSIX.1-2024. Linux 2.6.27, glibc 2.9.

EXAMPLES
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);
        }
    }

SEE ALSO
fork(2), read(2), socketpair(2), splice(2), tee(2), vmsplice(2), write(2), popen(3), pipe(7)