pipe, pipe2 — create pipe
#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); |
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 kernel 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_CLOEXECSet 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 constantPIPE_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 EINVAL error.
O_NONBLOCKSet 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.
On success, zero is returned. On error, −1 is
returned, and errno is set
appropriately.
On Linux (and other systems), pipe() does not modify pipefd on failure. A
requirement standardizing this behavior was added in
POSIX.1-2016. The Linux-specific pipe2() system call likewise does not
modify pipefd on
failure.
pipefd is
not valid.
(pipe2()) Invalid
value in flags.
The per-process limit on the number of open file descriptors has been reached.
The system-wide limit on the total number of open files has been reached.
pipe2() was added to Linux
in version 2.6.27; glibc support is available starting with
version 2.9.
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.
#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);
}
}
This page is part of release 4.07 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
https://www.kernel.org/doc/man−pages/.
|
Copyright (C) 2005, 2008, Michael Kerrisk <mtk.manpagesgmail.com> (A few fragments remain from an earlier (1992) version by Drew Eckhardt <drewcs.colorado.edu>.) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified by Michael Haardt <michaelmoria.de> Modified 1993-07-23 by Rik Faith <faithcs.unc.edu> Modified 1996-10-22 by Eric S. Raymond <esrthyrsus.com> Modified 2004-06-17 by Michael Kerrisk <mtk.manpagesgmail.com> Modified 2005, mtk: added an example program Modified 2008-01-09, mtk: rewrote DESCRIPTION; minor additions to EXAMPLE text. 2008-10-10, mtk: add description of pipe2() |