1 .\" Hey Emacs! This file is -*- nroff -*- source.
3 .\" This manpage is Copyright (C) 2006 Jens Axboe
4 .\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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26 .TH SPLICE 2 2008-09-26 "Linux" "Linux Programmer's Manual"
28 splice \- splice data to/from a pipe
31 .B #define _GNU_SOURCE
34 .BI "long splice(int " fd_in ", loff_t *" off_in ", int " fd_out ,
35 .BI " loff_t *" off_out ", size_t " len \
36 ", unsigned int " flags );
40 moves data between two file descriptors
41 without copying between kernel address space and user address space.
44 bytes of data from the file descriptor
46 to the file descriptor
48 where one of the descriptors must refer to a pipe.
52 refers to a pipe, then
57 does not refer to a pipe and
59 is NULL, then bytes are read from
61 starting from the current file offset,
62 and the current file offset is adjusted appropriately.
65 does not refer to a pipe and
69 must point to a buffer which specifies the starting
70 offset from which bytes will be read from
72 in this case, the current file offset of
75 Analogous statements apply for
82 argument is a bit mask that is composed by ORing together
83 zero or more of the following values:
86 Attempt to move pages instead of copying.
87 This is only a hint to the kernel:
88 pages may still be copied if the kernel cannot move the
89 pages from the pipe, or if
90 the pipe buffers don't refer to full pages.
91 The initial implementation of this flag was buggy:
92 therefore starting in Linux 2.6.21 it is a no-op
93 (but is still permitted in a
96 in the future, a correct implementation may be restored.
100 This makes the splice pipe operations non-blocking, but
102 may nevertheless block because the file descriptors that
103 are spliced to/from may block (unless they have the
108 More data will be coming in a subsequent splice.
109 This is a helpful hint when
112 refers to a socket (see also the description of
116 and the description of
127 Upon successful completion,
129 returns the number of bytes
130 spliced to or from the pipe.
131 A return value of 0 means that there was no data to transfer,
132 and it would not make sense to block, because there are no
133 writers connected to the write end of the pipe referred to by
140 is set to indicate the error.
144 One or both file descriptors are not valid,
145 or do not have proper read-write mode.
148 Target file system doesn't support splicing;
149 neither of the descriptors refers to a pipe; or
150 offset given for non-seekable device.
160 was not NULL, but the corresponding file descriptor refers to a pipe.
164 system call first appeared in Linux 2.6.17.
166 This system call is Linux-specific.
168 The three system calls
173 provide userspace programs with full control over an arbitrary
174 kernel buffer, implemented within the kernel using the same type
175 of buffer that is used for a pipe.
176 In overview, these system calls perform the following tasks:
179 moves data from the buffer to an arbitrary file descriptor, or vice versa,
180 or from one buffer to another.
183 "copies" the data from one buffer to another.
186 "copies" data from user space into the buffer.
188 Though we talk of copying, actual copies are generally avoided.
189 The kernel does this by implementing a pipe buffer as a set
190 of reference-counted pointers to pages of kernel memory.
191 The kernel creates "copies" of pages in a buffer by creating new
192 pointers (for the output buffer) referring to the pages,
193 and increasing the reference counts for the pages:
194 only pointers are copied, not the pages of the buffer.
196 .\" Linus: Now, imagine using the above in a media server, for example.
197 .\" Let's say that a year or two has passed, so that the video drivers
198 .\" have been updated to be able to do the splice thing, and what can
201 .\" - splice from the (mpeg or whatever - let's just assume that the video
202 .\" input is either digital or does the encoding on its own - like they
203 .\" pretty much all do) video input into a pipe (remember: no copies - the
204 .\" video input will just DMA directly into memory, and splice will just
205 .\" set up the pages in the pipe buffer)
206 .\" - tee that pipe to split it up
207 .\" - splice one end to a file (ie "save the compressed stream to disk")
208 .\" - splice the other end to a real-time video decoder window for your
209 .\" real-time viewing pleasure.
211 .\" Linus: Now, the advantage of splice()/tee() is that you can
212 .\" do zero-copy movement of data, and unlike sendfile() you can
213 .\" do it on _arbitrary_ data (and, as shown by "tee()", it's more
214 .\" than just sending the data to somebody else: you can duplicate
215 .\" the data and choose to forward it to two or more different
216 .\" users - for things like logging etc.).
225 .BR feature_test_macros (7)