NAME¶

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¶

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:

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

RETURN VALUE¶

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.

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).

VERSIONS¶

CONFORMING TO¶

pipe2() is Linux-specific.

EXAMPLE¶

#include <sys/types.h>
#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¶

COLOPHON¶