данные сращивания с / на трубу (splice data to/from a pipe)
Имя (Name)
splice - splice data to/from a pipe
Синопсис (Synopsis)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
ssize_t splice(int fd_in, off64_t *off_in, int fd_out,
off64_t *off_out, size_t len, unsigned int flags);
Описание (Description)
splice() moves data between two file descriptors without copying
between kernel address space and user address space. It
transfers up to len bytes of data from the file descriptor fd_in
to the file descriptor fd_out, where one of the file descriptors
must refer to a pipe.
The following semantics apply for fd_in and off_in:
* If fd_in refers to a pipe, then off_in must be NULL.
* If fd_in does not refer to a pipe and off_in is NULL, then
bytes are read from fd_in starting from the file offset, and
the file offset is adjusted appropriately.
* If fd_in does not refer to a pipe and off_in is not NULL, then
off_in must point to a buffer which specifies the starting
offset from which bytes will be read from fd_in; in this case,
the file offset of fd_in is not changed.
Analogous statements apply for fd_out and off_out.
The flags argument is a bit mask that is composed by ORing
together zero or more of the following values:
SPLICE_F_MOVE
Attempt to move pages instead of copying. This is only a
hint to the kernel: pages may still be copied if the
kernel cannot move the pages from the pipe, or if the pipe
buffers don't refer to full pages. The initial
implementation of this flag was buggy: therefore starting
in Linux 2.6.21 it is a no-op (but is still permitted in a
splice() call); in the future, a correct implementation
may be restored.
SPLICE_F_NONBLOCK
Do not block on I/O. This makes the splice pipe
operations nonblocking, but splice() may nevertheless
block because the file descriptors that are spliced
to/from may block (unless they have the O_NONBLOCK flag
set).
SPLICE_F_MORE
More data will be coming in a subsequent splice. This is
a helpful hint when the fd_out refers to a socket (see
also the description of MSG_MORE in send(2), and the
description of TCP_CORK in tcp(7)).
SPLICE_F_GIFT
Unused for splice(); see vmsplice(2).
Возвращаемое значение (Return value)
Upon successful completion, splice() returns the number of bytes
spliced to or from the pipe.
A return value of 0 means end of input. If fd_in refers to a
pipe, then this means that there was no data to transfer, and it
would not make sense to block because there are no writers
connected to the write end of the pipe.
On error, splice() returns -1 and errno is set to indicate the
error.
Ошибки (Error)
EAGAIN SPLICE_F_NONBLOCK was specified in flags or one of the
file descriptors had been marked as nonblocking
(O_NONBLOCK), and the operation would block.
EBADF One or both file descriptors are not valid, or do not have
proper read-write mode.
EINVAL The target filesystem doesn't support splicing.
EINVAL The target file is opened in append mode.
EINVAL Neither of the file descriptors refers to a pipe.
EINVAL An offset was given for nonseekable device (e.g., a pipe).
EINVAL fd_in and fd_out refer to the same pipe.
ENOMEM Out of memory.
ESPIPE Either off_in or off_out was not NULL, but the
corresponding file descriptor refers to a pipe.
Версии (Versions)
The splice() system call first appeared in Linux 2.6.17; library
support was added to glibc in version 2.5.
Стандарты (Conforming to)
This system call is Linux-specific.
Примечание (Note)
The three system calls splice(), vmsplice(2), and tee(2), provide
user-space programs with full control over an arbitrary kernel
buffer, implemented within the kernel using the same type of
buffer that is used for a pipe. In overview, these system calls
perform the following tasks:
• splice() moves data from the buffer to an arbitrary file
descriptor, or vice versa, or from one buffer to another.
• tee(2) "copies" the data from one buffer to another.
• vmsplice(2) "copies" data from user space into the buffer.
Though we talk of copying, actual copies are generally avoided.
The kernel does this by implementing a pipe buffer as a set of
reference-counted pointers to pages of kernel memory. The kernel
creates "copies" of pages in a buffer by creating new pointers
(for the output buffer) referring to the pages, and increasing
the reference counts for the pages: only pointers are copied, not
the pages of the buffer.
In Linux 2.6.30 and earlier, exactly one of fd_in and fd_out was
required to be a pipe. Since Linux 2.6.31, both arguments may
refer to pipes.
Примеры (Examples)
See tee(2).
Смотри также (See also)
copy_file_range(2), sendfile(2), tee(2), vmsplice(2), pipe(7)
