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 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_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.
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.
ERRORS¶
- EFAULT
- pipefd is 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.
- ENFILE
- The user hard limit on memory that can be allocated for pipes has been
reached and the caller is not privileged; see pipe(7).
VERSIONS¶
pipe2() was added to Linux in version 2.6.27; glibc support is available
starting with version 2.9.
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)
COLOPHON¶
This page is part of release 4.10 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/.