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1 | .\" This manpage is Copyright (C) 2006 Jens Axboe | |
2 | .\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com> | |
3 | .\" | |
4 | .\" %%%LICENSE_START(VERBATIM) | |
5 | .\" Permission is granted to make and distribute verbatim copies of this | |
6 | .\" manual provided the copyright notice and this permission notice are | |
7 | .\" preserved on all copies. | |
8 | .\" | |
9 | .\" Permission is granted to copy and distribute modified versions of this | |
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11 | .\" entire resulting derived work is distributed under the terms of a | |
12 | .\" permission notice identical to this one. | |
13 | .\" | |
14 | .\" Since the Linux kernel and libraries are constantly changing, this | |
15 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
16 | .\" responsibility for errors or omissions, or for damages resulting from | |
17 | .\" the use of the information contained herein. The author(s) may not | |
18 | .\" have taken the same level of care in the production of this manual, | |
19 | .\" which is licensed free of charge, as they might when working | |
20 | .\" professionally. | |
21 | .\" | |
22 | .\" Formatted or processed versions of this manual, if unaccompanied by | |
23 | .\" the source, must acknowledge the copyright and authors of this work. | |
24 | .\" %%%LICENSE_END | |
25 | .\" | |
26 | .TH SPLICE 2 2012-05-04 "Linux" "Linux Programmer's Manual" | |
27 | .SH NAME | |
28 | splice \- splice data to/from a pipe | |
29 | .SH SYNOPSIS | |
30 | .nf | |
31 | .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" | |
32 | .B #include <fcntl.h> | |
33 | ||
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 | |
38 | .fi | |
39 | .SH DESCRIPTION | |
40 | .BR splice () | |
41 | moves data between two file descriptors | |
42 | without copying between kernel address space and user address space. | |
43 | It transfers up to | |
44 | .I len | |
45 | bytes of data from the file descriptor | |
46 | .I fd_in | |
47 | to the file descriptor | |
48 | .IR fd_out , | |
49 | where one of the descriptors must refer to a pipe. | |
50 | ||
51 | If | |
52 | .I fd_in | |
53 | refers to a pipe, then | |
54 | .I off_in | |
55 | must be NULL. | |
56 | If | |
57 | .I fd_in | |
58 | does not refer to a pipe and | |
59 | .I off_in | |
60 | is NULL, then bytes are read from | |
61 | .I fd_in | |
62 | starting from the current file offset, | |
63 | and the current file offset is adjusted appropriately. | |
64 | If | |
65 | .I fd_in | |
66 | does not refer to a pipe and | |
67 | .I off_in | |
68 | is not NULL, then | |
69 | .I off_in | |
70 | must point to a buffer which specifies the starting | |
71 | offset from which bytes will be read from | |
72 | .IR fd_in ; | |
73 | in this case, the current file offset of | |
74 | .I fd_in | |
75 | is not changed. | |
76 | Analogous statements apply for | |
77 | .I fd_out | |
78 | and | |
79 | .IR off_out . | |
80 | ||
81 | The | |
82 | .I flags | |
83 | argument is a bit mask that is composed by ORing together | |
84 | zero or more of the following values: | |
85 | .TP 1.9i | |
86 | .B SPLICE_F_MOVE | |
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 | |
95 | .BR splice () | |
96 | call); | |
97 | in the future, a correct implementation may be restored. | |
98 | .TP | |
99 | .B SPLICE_F_NONBLOCK | |
100 | Do not block on I/O. | |
101 | This makes the splice pipe operations nonblocking, but | |
102 | .BR splice () | |
103 | may nevertheless block because the file descriptors that | |
104 | are spliced to/from may block (unless they have the | |
105 | .B O_NONBLOCK | |
106 | flag set). | |
107 | .TP | |
108 | .B SPLICE_F_MORE | |
109 | More data will be coming in a subsequent splice. | |
110 | This is a helpful hint when | |
111 | the | |
112 | .I fd_out | |
113 | refers to a socket (see also the description of | |
114 | .B MSG_MORE | |
115 | in | |
116 | .BR send (2), | |
117 | and the description of | |
118 | .B TCP_CORK | |
119 | in | |
120 | .BR tcp (7)) | |
121 | .TP | |
122 | .B SPLICE_F_GIFT | |
123 | Unused for | |
124 | .BR splice (); | |
125 | see | |
126 | .BR vmsplice (2). | |
127 | .SH RETURN VALUE | |
128 | Upon successful completion, | |
129 | .BR splice () | |
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 | |
135 | .IR fd_in . | |
136 | ||
137 | On error, | |
138 | .BR splice () | |
139 | returns \-1 and | |
140 | .I errno | |
141 | is set to indicate the error. | |
142 | .SH ERRORS | |
143 | .TP | |
144 | .B EBADF | |
145 | One or both file descriptors are not valid, | |
146 | or do not have proper read-write mode. | |
147 | .TP | |
148 | .B EINVAL | |
149 | Target file system 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. | |
155 | .TP | |
156 | .B ENOMEM | |
157 | Out of memory. | |
158 | .TP | |
159 | .B ESPIPE | |
160 | Either | |
161 | .I off_in | |
162 | or | |
163 | .I off_out | |
164 | was not NULL, but the corresponding file descriptor refers to a pipe. | |
165 | .SH VERSIONS | |
166 | The | |
167 | .BR splice () | |
168 | system call first appeared in Linux 2.6.17; | |
169 | library support was added to glibc in version 2.5. | |
170 | .SH CONFORMING TO | |
171 | This system call is Linux-specific. | |
172 | .SH NOTES | |
173 | The three system calls | |
174 | .BR splice (), | |
175 | .BR vmsplice (2), | |
176 | and | |
177 | .BR tee (2), | |
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: | |
182 | .TP 1.2i | |
183 | .BR splice () | |
184 | moves data from the buffer to an arbitrary file descriptor, or vice versa, | |
185 | or from one buffer to another. | |
186 | .TP | |
187 | .BR tee (2) | |
188 | "copies" the data from one buffer to another. | |
189 | .TP | |
190 | .BR vmsplice (2) | |
191 | "copies" data from user space into the buffer. | |
192 | .PP | |
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. | |
200 | .\" | |
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 | |
204 | .\" you do? You can: | |
205 | .\" | |
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. | |
215 | .\" | |
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.). | |
222 | .\" | |
223 | .SH EXAMPLE | |
224 | See | |
225 | .BR tee (2). | |
226 | .SH SEE ALSO | |
227 | .BR sendfile (2), | |
228 | .BR tee (2), | |
229 | .BR vmsplice (2) |