pipe(2) - Linux manual page (original) (raw)
pipe(2) System Calls Manual pipe(2)
NAME top
pipe, pipe2 - create pipeLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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 VERSIONS */
**#include <unistd.h>**
**struct fd_pair {**
**long fd[2];**
**};**
**struct fd_pair pipe(void);**DESCRIPTION top
**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)](../man7/pipe.7.html).
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)](../man2/open.2.html) 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)](../man2/write.2.html) to the pipe is dealt with as a separate packet,
and [read(2)](../man2/read.2.html)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)](../man7/pipe.7.html)) will
be split into multiple packets. The constant **PIPE_BUF**
is defined in _<limits.h>_.
• If a [read(2)](../man2/read.2.html) 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)](../man2/read.2.html) 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)](../man2/fcntl.2.html).
**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)](../man2/fcntl.2.html) 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 top
On success, zero is returned. On error, -1 is returned, _[errno](../man3/errno.3.html)_ 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 top
**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)](../man7/pipe.7.html).
**ENOPKG** (**pipe2**()) **O_NOTIFICATION_PIPE** was passed in _flags_ and
support for notifications (**CONFIG_WATCH_QUEUE**) is not
compiled into the kernel.VERSIONS top
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)](../man2/syscall.2.html) for information regarding registers used for storing
second file descriptor.STANDARDS top
**pipe**() POSIX.1-2008.
**pipe2**()
Linux.HISTORY top
**pipe**() POSIX.1-2001.
**pipe2**()
Linux 2.6.27, glibc 2.9.EXAMPLES top
The following program creates a pipe, and then [fork(2)](../man2/fork.2.html)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)](../man2/fork.2.html), each
process closes the file descriptors that it doesn't need for the
pipe (see [pipe(7)](../man7/pipe.7.html)). 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 <err.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/types.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)
err(EXIT_FAILURE, "pipe");
cpid = fork();
if (cpid == -1)
err(EXIT_FAILURE, "fork");
if (cpid == 0) { /* Child reads from pipe */
if (close(pipefd[1]) == -1) /* Close unused write end */
err(EXIT_FAILURE, "close");
while (read(pipefd[0], &buf, 1) > 0) {
if (write(STDOUT_FILENO, &buf, 1) != 1)
err(EXIT_FAILURE, "write");
}
if (write(STDOUT_FILENO, "\n", 1) != 1)
err(EXIT_FAILURE, "write");
if (close(pipefd[0]) == -1)
err(EXIT_FAILURE, "close");
_exit(EXIT_SUCCESS);
} else { /* Parent writes argv[1] to pipe */
if (close(pipefd[0]) == -1) /* Close unused read end */
err(EXIT_FAILURE, "close");
if (write(pipefd[1], argv[1], strlen(argv[1])) != strlen(argv[1]))
err(EXIT_FAILURE, "write");
if (close(pipefd[1]) == -1) /* Reader will see EOF */
err(EXIT_FAILURE, "close");
if (wait(NULL) == -1) /* Wait for child */
err(EXIT_FAILURE, "wait");
exit(EXIT_SUCCESS);
}
}SEE ALSO top
[fork(2)](../man2/fork.2.html), [read(2)](../man2/read.2.html), [socketpair(2)](../man2/socketpair.2.html), [splice(2)](../man2/splice.2.html), [tee(2)](../man2/tee.2.html), [vmsplice(2)](../man2/vmsplice.2.html),
[write(2)](../man2/write.2.html), [popen(3)](../man3/popen.3.html), [pipe(7)](../man7/pipe.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 pipe(2)
Pages that refer to this page:eventfd(2), fork(2), getrlimit(2), ioctl_pipe(2), socketpair(2), statfs(2), syscall(2), syscalls(2), pmda(3), pmdaconnect(3), __pmprocesspipe(3), popen(3), capabilities(7), fifo(7), inode(7), man-pages(7), pipe(7), signal-safety(7)