pipe, pipe2 - create pipe
Standard C library (
libc,
-lc)
#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);
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.
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.
- 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.
pipe2() was added in Linux 2.6.27; glibc support is available starting
with glibc 2.9.
pipe(): POSIX.1-2001, POSIX.1-2008.
pipe2() is Linux-specific.
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.
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 <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);
}
}
fork(2),
read(2),
socketpair(2),
splice(2),
tee(2),
vmsplice(2),
write(2),
popen(3),
pipe(7)