Section: Linux Programmer's Manual (2)Updated: 2009-09-15Local indexUp
splice - splice data to/from a pipe
#define _GNU_SOURCE#include <fcntl.h>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
bytes of data from the file descriptor
to the file descriptor
where one of the descriptors must refer to a pipe.
refers to a pipe, then
must be NULL.
does not refer to a pipe and
is NULL, then bytes are read from
starting from the current file offset,
and the current file offset is adjusted appropriately.
does not refer to a pipe and
is not NULL, then
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
is not changed.
Analogous statements apply for
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
in the future, a correct implementation may be restored.
Do not block on I/O.
This makes the splice pipe operations nonblocking, but
may nevertheless block because the file descriptors that
are spliced to/from may block (unless they have the
More data will be coming in a subsequent splice.
This is a helpful hint when
refers to a socket (see also the description of
and the description of
Upon successful completion,
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
returns -1 and
is set to indicate the error.
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.
was not NULL, but the corresponding file descriptor refers to a pipe.
system call first appeared in Linux 2.6.17.
This system call is Linux-specific.
The three system calls
provide userspace 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.
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.