Manpage of PIPE

PIPE

Section: Linux Programmer's Manual (2)
Updated: 2016-03-15
Index
 

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 pipefdis 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 flagsis 0, then pipe2() is the same as pipe(). The following values can be bitwise ORed in flagsto 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_BUFbytes (see pipe(7)) will be split into multiple packets. The constant PIPE_BUFis 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_BUFwill 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 EINVALerror.
O_NONBLOCK
Set the O_NONBLOCKfile 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 errnois set appropriately.

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

ERRORS

EFAULT
pipefdis 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.
 

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, POSIX.1-2008.

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 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>

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), splice(2), write(2), popen(3), pipe(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
EXAMPLE
Program source
SEE ALSO

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Time: 22:27:43 GMT, June 20, 2016