| pipe(2) - phpMan
PIPE(2) Linux Programmer's Manual PIPE(2)
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 pipefd is 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 ker‐
nel until it is read from the read end of the pipe. For further details, see pipe(7).
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
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
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
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.
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 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 argu‐
ment 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), write(2), popen(3), pipe(7)
COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2014-07-08 PIPE(2)
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