man2/pipe.2

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.\" Modified 2005, mtk: added an example program
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.TH PIPE 2 2008-01-09 "Linux" "Linux Programmer's Manual"
.SH NAME
pipe \- create pipe
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.BI "int pipe(int " pipefd "[2]);"
.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).
.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 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 "CONFORMING TO"
POSIX.1-2001.
.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)