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 descriptors must refer to a pipe.
refers to a pipe, then off_in
must be NULL. If
does not refer to a pipe and off_in
is NULL, then bytes
are read from fd_in
starting from the current file offset, and the
current 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
; in this case, the current file offset of fd_in
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 flag set).
- 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))
- 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 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 referred to by fd_in
On error, splice
() returns -1 and errno
is set to indicate the
- One or both file descriptors are not valid, or do not have
proper read-write mode.
- Target file system doesn't support splicing; target file is
opened in append mode; neither of the descriptors refers to a pipe; or
offset given for nonseekable device.
- 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
, 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:
- moves data from the buffer to an arbitrary file descriptor,
or vice versa, or from one buffer to another.
- "copies" the data from one buffer to
- "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.
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found