pipe(2) System Calls Manual pipe(2)
NAME
pipe, pipe2 - create pipe
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int pipe(int pipefd[2]);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h> /* Definition of O_* constants */
#include <unistd.h>
int pipe2(int pipefd[2], int flags);
/* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64, pipe() has the
following prototype; see NOTES */
#include <unistd.h>
struct fd_pair {
long fd[2];
};
struct fd_pair pipe(void);
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 de-
tails, see pipe(7).
If flags is 0, then pipe2() is the same as pipe(). The following val-
ues 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 de-
scriptors. 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 follow-
ing points:
• Writes of greater than PIPE_BUF bytes (see pipe(7)) will be
split into multiple packets. The constant PIPE_BUF is de-
fined 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 spec-
ifies 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.
Since Linux 4.5, it is possible to change the O_DIRECT setting
of a pipe file descriptor using fcntl(2).
O_NONBLOCK
Set the O_NONBLOCK file status flag on the open file descrip-
tions referred to by the new file descriptors. Using this flag
saves extra calls to fcntl(2) to achieve the same result.
O_NOTIFICATION_PIPE
Since Linux 5.8, general notification mechanism is built on the
top of the pipe where kernel splices notification messages into
pipes opened by user space. The owner of the pipe has to tell
the kernel which sources of events to watch and filters can also
be applied to select which subevents should be placed into the
pipe.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, errno is set
to indicate the error, and pipefd is left unchanged.
On Linux (and other systems), pipe() does not modify pipefd on failure.
A requirement standardizing this behavior was added in POSIX.1-2008
TC2. 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).
ENOPKG (pipe2()) O_NOTIFICATION_PIPE was passed in flags and support
for notifications (CONFIG_WATCH_QUEUE) is not compiled into the
kernel.
VERSIONS
pipe2() was added in Linux 2.6.27; glibc support is available starting
with glibc 2.9.
STANDARDS
pipe(): POSIX.1-2001, POSIX.1-2008.
pipe2() is Linux-specific.
NOTES
The System V ABI on some architectures allows the use of more than one
register for returning multiple values; several architectures (namely,
Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64) (ab)use this feature in
order to implement the pipe() system call in a functional manner: the
call doesn't take any arguments and returns a pair of file descriptors
as the return value on success. The glibc pipe() wrapper function
transparently deals with this. See syscall(2) for information regard-
ing registers used for storing second file descriptor.
EXAMPLES
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 <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
int pipefd[2];
char buf;
pid_t cpid;
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), tee(2), vmsplice(2),
write(2), popen(3), pipe(7)
Linux man-pages 6.03 2023-02-05 pipe(2)
Generated by dwww version 1.15 on Sun Jun 2 04:15:56 CEST 2024.