Section (2) splice
splice — splice data to/from a pipe
#define _GNU_SOURCE /* See feature_test_macros(7) */ #include <fcntl.h>
|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
the file descriptor
fd_out, where one of the file
descriptors must refer to a pipe.
The following semantics apply for
fd_inrefers to a pipe, then
off_inmust be NULL.
fd_indoes not refer to a pipe and
off_inis NULL, then bytes are read from
fd_instarting from the file offset, and the file offset is adjusted appropriately.
fd_indoes not refer to a pipe and
off_inis not NULL, then
off_inmust 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_inis not changed.
Analogous statements apply for
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_zsingle_quotesz_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
More data will be coming in a subsequent splice. This is a helpful hint when the
fd_outrefers to a socket (see also the description of
MSG_MOREin send(2), and the description of
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.
errno is set to
indicate the error.
SPLICE_F_NONBLOCKwas specified in
flagsor one of the file descriptors had been marked as nonblocking (
,and the operation would block.
One or both file descriptors are not valid, or do not have proper read-write mode.
The target filesystem doesn_zsingle_quotesz_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_outrefer to the same pipe.
Out of memory.
off_outwas not NULL, but the corresponding file descriptor refers to a pipe.
splice() system call
first appeared in Linux 2.6.17; library support was added to
glibc in version 2.5.
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:
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 another.
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_out was required to be a
pipe. Since Linux 2.6.31, both arguments may refer to
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and Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com>
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