создать трубу (create pipe)
Имя (Name)
pipe, pipe2 - create pipe
Синопсис (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);
Описание (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.
Возвращаемое значение (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.
Ошибки (Error)
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).
Версии (Versions)
pipe2() was added to Linux in version 2.6.27; glibc support is
available starting with version 2.9.
Стандарты (Conforming to)
pipe(): POSIX.1-2001, POSIX.1-2008.
pipe2() is Linux-specific.
Примечание (Note)
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.
Примеры (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 <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);
}
}
Смотри также (See also)
fork(2), read(2), socketpair(2), splice(2), tee(2), vmsplice(2),
write(2), popen(3), pipe(7)
