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 2017-09-15 "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 and the operation would block.
 160 .TP
 161 .B EBADF
 162 One or both file descriptors are not valid,
 163 or do not have proper read-write mode.
 164 .TP
 165 .B EINVAL
 166 The target filesystem doesn't support splicing.
 167 .TP
 168 .B EINVAL
 169 The target file is opened in append mode.
 170 .\" The append-mode error is given since 2.6.27; in earlier kernels,
 171 .\" splice() in append mode was broken
 172 .TP
 173 .B EINVAL
 174 Neither of the file descriptors refers to a pipe.
 175 .TP
 176 .B EINVAL
 177 An offset was given for nonseekable device (e.g., a pipe).
 178 .TP
 179 .B EINVAL
 180 .I fd_in
 181 and
 182 .I fd_out
 183 refer to the same pipe.
 184 .TP
 185 .B ENOMEM
 186 Out of memory.
 187 .TP
 188 .B ESPIPE
 189 Either
 190 .I off_in
 191 or
 192 .I off_out
 193 was not NULL, but the corresponding file descriptor refers to a pipe.
 194 .SH VERSIONS
 195 The
 196 .BR splice ()
 197 system call first appeared in Linux 2.6.17;
 198 library support was added to glibc in version 2.5.
 199 .SH CONFORMING TO
 200 This system call is Linux-specific.
 201 .SH NOTES
 202 The three system calls
 203 .BR splice (),
 204 .BR vmsplice (2),
 205 and
 206 .BR tee (2),
 207 provide user-space programs with full control over an arbitrary
 208 kernel buffer, implemented within the kernel using the same type
 209 of buffer that is used for a pipe.
 210 In overview, these system calls perform the following tasks:
 211 .TP 1.2i
 212 .BR splice ()
 213 moves data from the buffer to an arbitrary file descriptor, or vice versa,
 214 or from one buffer to another.
 215 .TP
 216 .BR tee (2)
 217 "copies" the data from one buffer to another.
 218 .TP
 219 .BR vmsplice (2)
 220 "copies" data from user space into the buffer.
 221 .PP
 222 Though we talk of copying, actual copies are generally avoided.
 223 The kernel does this by implementing a pipe buffer as a set
 224 of reference-counted pointers to pages of kernel memory.
 225 The kernel creates "copies" of pages in a buffer by creating new
 226 pointers (for the output buffer) referring to the pages,
 227 and increasing the reference counts for the pages:
 228 only pointers are copied, not the pages of the buffer.
 229 .\"
 230 .\" Linus: Now, imagine using the above in a media server, for example.
 231 .\" Let's say that a year or two has passed, so that the video drivers
 232 .\" have been updated to be able to do the splice thing, and what can
 233 .\" you do? You can:
 234 .\"
 235 .\" - splice from the (mpeg or whatever - let's just assume that the video
 236 .\"   input is either digital or does the encoding on its own - like they
 237 .\"   pretty much all do) video input into a pipe (remember: no copies - the
 238 .\"   video input will just DMA directly into memory, and splice will just
 239 .\"   set up the pages in the pipe buffer)
 240 .\" - tee that pipe to split it up
 241 .\" - splice one end to a file (ie "save the compressed stream to disk")
 242 .\" - splice the other end to a real-time video decoder window for your
 243 .\"   real-time viewing pleasure.
 244 .\"
 245 .\" Linus: Now, the advantage of splice()/tee() is that you can
 246 .\" do zero-copy movement of data, and unlike sendfile() you can
 247 .\" do it on _arbitrary_ data (and, as shown by "tee()", it's more
 248 .\" than just sending the data to somebody else: you can duplicate
 249 .\" the data and choose to forward it to two or more different
 250 .\" users - for things like logging etc.).
 251 .\"
 252 .PP
 253 In Linux 2.6.30 and earlier,
 254 exactly one of
 255 .I fd_in
 256 and
 257 .I fd_out
 258 was required to be a pipe.
 259 Since Linux 2.6.31,
 260 .\" commit 7c77f0b3f9208c339a4b40737bb2cb0f0319bb8d
 261 both arguments may refer to pipes.
 262 .SH EXAMPLE
 263 See
 264 .BR tee (2).
 265 .SH SEE ALSO
 266 .BR copy_file_range (2),
 267 .BR sendfile (2),
 268 .BR tee (2),
 269 .BR vmsplice (2),
 270 .BR pipe (7)