splice - splice data to/from a pipe
Standard C library (
libc,
-lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
ssize_t splice(int fd_in, off64_t *_Nullable off_in,
int fd_out, off64_t *_Nullable off_out,
size_t len, unsigned int flags);
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).
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.
- 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.
The
splice() system call first appeared in Linux 2.6.17; library support
was added in glibc 2.5.
This system call is Linux-specific.
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.
See
tee(2).
copy_file_range(2),
sendfile(2),
tee(2),
vmsplice(2),
pipe(7)