1 .\" This manpage is Copyright (C) 2006 Jens Axboe
2 .\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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26 .TH SPLICE 2 2012-05-04 "Linux" "Linux Programmer's Manual"
28 splice \- splice data to/from a pipe
31 .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
34 .BI "ssize_t splice(int " fd_in ", loff_t *" off_in ", int " fd_out ,
35 .BI " loff_t *" off_out ", size_t " len \
36 ", unsigned int " flags );
37 .\" Return type was long before glibc 2.7
41 moves data between two file descriptors
42 without copying between kernel address space and user address space.
45 bytes of data from the file descriptor
47 to the file descriptor
49 where one of the descriptors must refer to a pipe.
53 refers to a pipe, then
58 does not refer to a pipe and
60 is NULL, then bytes are read from
62 starting from the current file offset,
63 and the current file offset is adjusted appropriately.
66 does not refer to a pipe and
70 must point to a buffer which specifies the starting
71 offset from which bytes will be read from
73 in this case, the current file offset of
76 Analogous statements apply for
83 argument is a bit mask that is composed by ORing together
84 zero or more of the following values:
87 Attempt to move pages instead of copying.
88 This is only a hint to the kernel:
89 pages may still be copied if the kernel cannot move the
90 pages from the pipe, or if
91 the pipe buffers don't refer to full pages.
92 The initial implementation of this flag was buggy:
93 therefore starting in Linux 2.6.21 it is a no-op
94 (but is still permitted in a
97 in the future, a correct implementation may be restored.
101 This makes the splice pipe operations nonblocking, but
103 may nevertheless block because the file descriptors that
104 are spliced to/from may block (unless they have the
109 More data will be coming in a subsequent splice.
110 This is a helpful hint when
113 refers to a socket (see also the description of
117 and the description of
128 Upon successful completion,
130 returns the number of bytes
131 spliced to or from the pipe.
132 A return value of 0 means that there was no data to transfer,
133 and it would not make sense to block, because there are no
134 writers connected to the write end of the pipe referred to by
141 is set to indicate the error.
145 One or both file descriptors are not valid,
146 or do not have proper read-write mode.
149 Target filesystem doesn't support splicing;
150 target file is opened in append mode;
151 .\" The append-mode error is given since 2.6.27; in earlier kernels,
152 .\" splice() in append mode was broken
153 neither of the descriptors refers to a pipe; or
154 offset given for nonseekable device.
164 was not NULL, but the corresponding file descriptor refers to a pipe.
168 system call first appeared in Linux 2.6.17;
169 library support was added to glibc in version 2.5.
171 This system call is Linux-specific.
173 The three system calls
178 provide user-space programs with full control over an arbitrary
179 kernel buffer, implemented within the kernel using the same type
180 of buffer that is used for a pipe.
181 In overview, these system calls perform the following tasks:
184 moves data from the buffer to an arbitrary file descriptor, or vice versa,
185 or from one buffer to another.
188 "copies" the data from one buffer to another.
191 "copies" data from user space into the buffer.
193 Though we talk of copying, actual copies are generally avoided.
194 The kernel does this by implementing a pipe buffer as a set
195 of reference-counted pointers to pages of kernel memory.
196 The kernel creates "copies" of pages in a buffer by creating new
197 pointers (for the output buffer) referring to the pages,
198 and increasing the reference counts for the pages:
199 only pointers are copied, not the pages of the buffer.
201 .\" Linus: Now, imagine using the above in a media server, for example.
202 .\" Let's say that a year or two has passed, so that the video drivers
203 .\" have been updated to be able to do the splice thing, and what can
206 .\" - splice from the (mpeg or whatever - let's just assume that the video
207 .\" input is either digital or does the encoding on its own - like they
208 .\" pretty much all do) video input into a pipe (remember: no copies - the
209 .\" video input will just DMA directly into memory, and splice will just
210 .\" set up the pages in the pipe buffer)
211 .\" - tee that pipe to split it up
212 .\" - splice one end to a file (ie "save the compressed stream to disk")
213 .\" - splice the other end to a real-time video decoder window for your
214 .\" real-time viewing pleasure.
216 .\" Linus: Now, the advantage of splice()/tee() is that you can
217 .\" do zero-copy movement of data, and unlike sendfile() you can
218 .\" do it on _arbitrary_ data (and, as shown by "tee()", it's more
219 .\" than just sending the data to somebody else: you can duplicate
220 .\" the data and choose to forward it to two or more different
221 .\" users - for things like logging etc.).
231 This page is part of release 3.65 of the Linux
234 A description of the project,
235 and information about reporting bugs,
237 \%http://www.kernel.org/doc/man\-pages/.