PIPE

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:

*

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.

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

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

CONFORMING TO

pipe2() is Linux-specific.  

EXAMPLE

Program source

#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

Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

VERSIONS

CONFORMING TO

EXAMPLE

Program source

SEE ALSO