pipe (2)

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NAME

pipe, pipe2 - create pipe

SYNOPSIS

#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);

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_NONBLOCK

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.

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.

RETURN VALUE

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

ERRORS

EFAULT

pipefd is not valid.

EINVAL

(pipe2()) Invalid value in flags.

EMFILE

Too many file descriptors are in use by the process.

ENFILE

The system limit on the total number of open files has been reached.

VERSIONS

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.

pipe2() is Linux-specific.

EXAMPLE

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 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.

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

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

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

    if (pipe(pipefd) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    cpid = fork();
    if (cpid == -1) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

    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);
        close(pipefd[0]);
        _exit(EXIT_SUCCESS);

    } 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 */
        exit(EXIT_SUCCESS);
    }
}

SEE ALSO

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

COLOPHON

This page is part of release 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

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