man2/splice.2

   1 .\" This manpage is Copyright (C) 2006 Jens Axboe
   2 .\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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  25 .\"
  26 .TH SPLICE 2 2019-05-09 "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 .PP
  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 file descriptors must refer to a pipe.
  50 .PP
  51 The following semantics apply for
  52 .I fd_in
  53 and
  54 .IR off_in :
  55 .IP * 3
  56 If
  57 .I fd_in
  58 refers to a pipe, then
  59 .I off_in
  60 must be NULL.
  61 .IP *
  62 If
  63 .I fd_in
  64 does not refer to a pipe and
  65 .I off_in
  66 is NULL, then bytes are read from
  67 .I fd_in
  68 starting from the file offset,
  69 and the file offset is adjusted appropriately.
  70 .IP *
  71 If
  72 .I fd_in
  73 does not refer to a pipe and
  74 .I off_in
  75 is not NULL, then
  76 .I off_in
  77 must point to a buffer which specifies the starting
  78 offset from which bytes will be read from
  79 .IR fd_in ;
  80 in this case, the file offset of
  81 .I fd_in
  82 is not changed.
  83 .PP
  84 Analogous statements apply for
  85 .I fd_out
  86 and
  87 .IR off_out .
  88 .PP
  89 The
  90 .I flags
  91 argument is a bit mask that is composed by ORing together
  92 zero or more of the following values:
  93 .TP
  94 .B SPLICE_F_MOVE
  95 Attempt to move pages instead of copying.
  96 This is only a hint to the kernel:
  97 pages may still be copied if the kernel cannot move the
  98 pages from the pipe, or if
  99 the pipe buffers don't refer to full pages.
 100 The initial implementation of this flag was buggy:
 101 therefore starting in Linux 2.6.21 it is a no-op
 102 (but is still permitted in a
 103 .BR splice ()
 104 call);
 105 in the future, a correct implementation may be restored.
 106 .TP
 107 .B SPLICE_F_NONBLOCK
 108 Do not block on I/O.
 109 This makes the splice pipe operations nonblocking, but
 110 .BR splice ()
 111 may nevertheless block because the file descriptors that
 112 are spliced to/from may block (unless they have the
 113 .B O_NONBLOCK
 114 flag set).
 115 .TP
 116 .B SPLICE_F_MORE
 117 More data will be coming in a subsequent splice.
 118 This is a helpful hint when
 119 the
 120 .I fd_out
 121 refers to a socket (see also the description of
 122 .B MSG_MORE
 123 in
 124 .BR send (2),
 125 and the description of
 126 .B TCP_CORK
 127 in
 128 .BR tcp (7)).
 129 .TP
 130 .B SPLICE_F_GIFT
 131 Unused for
 132 .BR splice ();
 133 see
 134 .BR vmsplice (2).
 135 .SH RETURN VALUE
 136 Upon successful completion,
 137 .BR splice ()
 138 returns the number of bytes
 139 spliced to or from the pipe.
 140 .PP
 141 A return value of 0 means end of input.
 142 If
 143 .I fd_in
 144 refers to a pipe, then this means that there was no data to transfer,
 145 and it would not make sense to block because there are no writers
 146 connected to the write end of the pipe.
 147 .PP
 148 On error,
 149 .BR splice ()
 150 returns \-1 and
 151 .I errno
 152 is set to indicate the error.
 153 .SH ERRORS
 154 .TP
 155 .B EAGAIN
 156 .B SPLICE_F_NONBLOCK
 157 was specified in
 158 .IR flags
 159 or one of the file descriptors had been marked as nonblocking
 160 .RB ( O_NONBLOCK ) ,
 161 and the operation would block.
 162 .TP
 163 .B EBADF
 164 One or both file descriptors are not valid,
 165 or do not have proper read-write mode.
 166 .TP
 167 .B EINVAL
 168 The target filesystem doesn't support splicing.
 169 .TP
 170 .B EINVAL
 171 The target file is opened in append mode.
 172 .\" The append-mode error is given since 2.6.27; in earlier kernels,
 173 .\" splice() in append mode was broken
 174 .TP
 175 .B EINVAL
 176 Neither of the file descriptors refers to a pipe.
 177 .TP
 178 .B EINVAL
 179 An offset was given for nonseekable device (e.g., a pipe).
 180 .TP
 181 .B EINVAL
 182 .I fd_in
 183 and
 184 .I fd_out
 185 refer to the same pipe.
 186 .TP
 187 .B ENOMEM
 188 Out of memory.
 189 .TP
 190 .B ESPIPE
 191 Either
 192 .I off_in
 193 or
 194 .I off_out
 195 was not NULL, but the corresponding file descriptor refers to a pipe.
 196 .SH VERSIONS
 197 The
 198 .BR splice ()
 199 system call first appeared in Linux 2.6.17;
 200 library support was added to glibc in version 2.5.
 201 .SH CONFORMING TO
 202 This system call is Linux-specific.
 203 .SH NOTES
 204 The three system calls
 205 .BR splice (),
 206 .BR vmsplice (2),
 207 and
 208 .BR tee (2),
 209 provide user-space programs with full control over an arbitrary
 210 kernel buffer, implemented within the kernel using the same type
 211 of buffer that is used for a pipe.
 212 In overview, these system calls perform the following tasks:
 213 .TP 1.2i
 214 .BR splice ()
 215 moves data from the buffer to an arbitrary file descriptor, or vice versa,
 216 or from one buffer to another.
 217 .TP
 218 .BR tee (2)
 219 "copies" the data from one buffer to another.
 220 .TP
 221 .BR vmsplice (2)
 222 "copies" data from user space into the buffer.
 223 .PP
 224 Though we talk of copying, actual copies are generally avoided.
 225 The kernel does this by implementing a pipe buffer as a set
 226 of reference-counted pointers to pages of kernel memory.
 227 The kernel creates "copies" of pages in a buffer by creating new
 228 pointers (for the output buffer) referring to the pages,
 229 and increasing the reference counts for the pages:
 230 only pointers are copied, not the pages of the buffer.
 231 .\"
 232 .\" Linus: Now, imagine using the above in a media server, for example.
 233 .\" Let's say that a year or two has passed, so that the video drivers
 234 .\" have been updated to be able to do the splice thing, and what can
 235 .\" you do? You can:
 236 .\"
 237 .\" - splice from the (mpeg or whatever - let's just assume that the video
 238 .\"   input is either digital or does the encoding on its own - like they
 239 .\"   pretty much all do) video input into a pipe (remember: no copies - the
 240 .\"   video input will just DMA directly into memory, and splice will just
 241 .\"   set up the pages in the pipe buffer)
 242 .\" - tee that pipe to split it up
 243 .\" - splice one end to a file (ie "save the compressed stream to disk")
 244 .\" - splice the other end to a real-time video decoder window for your
 245 .\"   real-time viewing pleasure.
 246 .\"
 247 .\" Linus: Now, the advantage of splice()/tee() is that you can
 248 .\" do zero-copy movement of data, and unlike sendfile() you can
 249 .\" do it on _arbitrary_ data (and, as shown by "tee()", it's more
 250 .\" than just sending the data to somebody else: you can duplicate
 251 .\" the data and choose to forward it to two or more different
 252 .\" users - for things like logging etc.).
 253 .\"
 254 .PP
 255 In Linux 2.6.30 and earlier,
 256 exactly one of
 257 .I fd_in
 258 and
 259 .I fd_out
 260 was required to be a pipe.
 261 Since Linux 2.6.31,
 262 .\" commit 7c77f0b3f9208c339a4b40737bb2cb0f0319bb8d
 263 both arguments may refer to pipes.
 264 .SH EXAMPLE
 265 See
 266 .BR tee (2).
 267 .SH SEE ALSO
 268 .BR copy_file_range (2),
 269 .BR sendfile (2),
 270 .BR tee (2),
 271 .BR vmsplice (2),
 272 .BR pipe (7)