splice - splice data to/from a pipe
#define _GNU_SOURCE /* See feature_test_macros(7) */
ssize_t splice(int fd_in, loff_t *off_in, int fd_out,
loff_t *off_out, size_t len, unsigned int flags);
() 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
, where one of the file descriptors must refer to a pipe.
The following semantics apply for fd_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
argument is a bit mask that is composed by ORing together zero
or more of the following values:
- 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.
- 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
- 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
- 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
- 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.
- One or both file descriptors are not valid, or do not have proper
- The target filesystem doesn't support splicing.
- The target file is opened in append mode.
- Neither of the file descriptors refers to a pipe.
- An offset was given for nonseekable device (e.g., a pipe).
- fd_in and fd_out refer to the same pipe.
- Out of memory.
- Either off_in or off_out was not NULL, but the corresponding
file descriptor refers to a pipe.
() system call first appeared in Linux 2.6.17; library support
was added to glibc in version 2.5.
This system call is Linux-specific.
The three system calls splice
(2), and tee
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
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
required to be a pipe. Since Linux 2.6.31, both arguments may refer to pipes.