man-pages/man2/pipe.2

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.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified 2005, mtk: added an example program
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.\"     to EXAMPLE text.
.\" 2008-10-10, mtk: add description of pipe2()
.\"
.TH PIPE 2 2008-10-10 "Linux" "Linux Programmer's Manual"
.SH NAME
pipe, pipe2 \- create pipe
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.sp
.BI "int pipe(int " pipefd "[2]);"
.sp
.B #define _GNU_SOURCE
.B #include <unistd.h>
.sp
.BI "int pipe2(int " pipefd "[2], " flags );
.fi
.SH DESCRIPTION
.BR pipe ()
creates a pipe, a unidirectional data channel that
can be used for interprocess communication.
The array
.IR pipefd
is used to return two file descriptors referring to the ends of the pipe.
.IR pipefd[0]
refers to the read end of the pipe.
.IR 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
.BR pipe (7).

If
.IR flags
is 0, then
.BR pipe2 ()
is the same as
.BR pipe ().
The following values can be bitwise ORed in
.IR flags
to obtain different behavior:
.TP 12
.B O_NONBLOCK
Set the
.BR O_NONBLOCK
file status flag on the two new file descriptors.
Using this flag saves extra calls to
.BR fcntl ()
to achieve the same result.
.TP
.B O_CLOEXEC
Set the close-on-exec flag on the two new file descriptors.
See the description of the same flag in
.BR open (2)
for reasons why this may be useful.
.SH "RETURN VALUE"
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EFAULT
.I pipefd
is not valid.
.TP
.B EINVAL
.RB ( pipe2 ())
Invalid value in
.IR flags .
.TP
.B EMFILE
Too many file descriptors are in use by the process.
.TP
.B ENFILE
The system limit on the total number of open files has been reached.
.SH VERSIONS
.BR pipe2 ()
was added to Linux in version 2.6.27;
glibc support is available starting with
version 2.9.
.SH "CONFORMING TO"
.BR pipe ():
POSIX.1-2001.

.BR pipe2 ()
is Linux-specific.
.SH EXAMPLE
.\" fork.2 refers to this example program.
The following program creates a pipe, and then
.BR 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
.BR fork (2),
each process closes the descriptors that it doesn't need for the pipe
(see
.BR 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.
.nf

#include <sys/wait.h>
#include <assert.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;

    assert(argc == 2);

    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);
    }
}
.fi
.SH "SEE ALSO"
.BR fork (2),
.BR read (2),
.BR socketpair (2),
.BR write (2),
.BR popen (3),
.BR pipe (7)