pipe
Section: System Calls (2)
Updated: 2023-02-05
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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 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.
-
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 descriptions
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 regarding 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)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- STANDARDS
-
- NOTES
-
- EXAMPLES
-
- Program source
-
- SEE ALSO
-
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Time: 11:31:28 GMT, April 24, 2024