man2/splice.2

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   3 .\" This manpage is Copyright (C) 2006 Jens Axboe
   4 .\" and Copyright (C) 2006 Michael Kerrisk <mtk-manpages@gmx.net>
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  25 .\"
  26 .TH SPLICE 2 2006-04-28 "Linux 2.6.17" "Linux Programmer's Manual"
  27 .SH NAME
  28 splice \- splice data to/from a pipe
  29 .SH SYNOPSIS
  30 .nf
  31 .B #define _GNU_SOURCE
  32 .B #include <fcntl.h>
  33
  34 .BI "long splice(int " fd_in ", off_t *" off_in ", int " fd_out ,
  35 .BI "            off_t *" off_out ", size_t " len \
  36 ", unsigned int " flags );
  37 .fi
  38 .SH DESCRIPTION
  39 .BR splice ()
  40 moves data between two file descriptors
  41 without copying between kernel address space and user address space.
  42 It transfers up to
  43 .I len
  44 bytes of data from the file descriptor
  45 .I fd_in
  46 to the file descriptor
  47 .IR fd_out ,
  48 where one of the descriptors must refer to a pipe.
  49
  50 If
  51 .I fd_in
  52 refers to a pipe, then
  53 .I off_in
  54 must be NULL.
  55 If
  56 .I fd_in
  57 does not refer to a pipe and
  58 .I off_in
  59 is NULL, then bytes are read from
  60 .I fd_in
  61 starting from the current file offset,
  62 and the current file offset is adjusted appropriately.
  63 If
  64 .I fd_in
  65 does not refer to a pipe and
  66 .I off_in
  67 is not NULL, then
  68 .I off_in
  69 must point to a buffer which specifies the starting
  70 offset from which bytes will be read from
  71 .IR fd_in ;
  72 in this case, the current file offset of
  73 .IR fd_in
  74 is not changed.
  75 Analogous statements apply for
  76 .I out_fd
  77 and
  78 .IR off_out .
  79
  80 The
  81 .I flags
  82 argument is a bit mask that is composed by ORing together
  83 zero or more of the following values:
  84 .TP 1.9i
  85 .B SPLICE_F_MOVE
  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 .TP
  92 .B SPLICE_F_NONBLOCK
  93 Do not block on I/O.
  94 This makes the splice pipe operations non-blocking, but
  95 .BR splice ()
  96 may nevertheless block because the file descriptors that
  97 are spliced to/from may block (unless they have the
  98 .BR O_NONBLOCK
  99 flag set).
 100 .TP
 101 .B SPLICE_F_MORE
 102 More data will be coming in a subsequent splice.
 103 This is a helpful hint when
 104 the
 105 .I fd_out
 106 refers to a socket (see also the description of
 107 .B MSG_MORE
 108 in
 109 .BR send (2),
 110 and the description of
 111 .B TCP_CORK
 112 in
 113 .BR tcp (7))
 114 .TP
 115 .B SPLICE_F_GIFT
 116 Unused for
 117 .BR splice ();
 118 see
 119 .BR vmsplice (2).
 120 .SH RETURN VALUE
 121 Upon successful completion,
 122 .BR splice ()
 123 returns the number of bytes
 124 spliced to or from the pipe.
 125 A return value of 0 means that there was no data to transfer,
 126 and it would not make sense to block, because there are no
 127 writers connected to the write end of the pipe referred to by
 128 .IR fd_in .
 129
 130 On error,
 131 .BR splice ()
 132 returns \-1 and
 133 .I errno
 134 is set to indicate the error.
 135 .SH ERRORS
 136 .TP
 137 .B EBADF
 138 One or both file descriptors are not valid,
 139 or do not have proper read-write mode.
 140 .TP
 141 .B EINVAL
 142 Target file system doesn't support splicing;
 143 neither of the descriptors refers to a pipe; or
 144 offset given for non-seekable device.
 145 .TP
 146 .B ENOMEM
 147 Out of memory.
 148 .TP
 149 .B ESPIPE
 150 Either
 151 .I off_in
 152 or
 153 .I off_out
 154 was not NULL, but the corresponding file descriptor refers to a pipe.
 155 .SH VERSIONS
 156 The
 157 .BR splice (2)
 158 system call first appeared in Linux 2.6.17.
 159 .SH "CONFORMING TO"
 160 This system call is Linux specific.
 161 .SH NOTES
 162 The three system calls
 163 .BR splice (2),
 164 .BR vmsplice (2),
 165 and
 166 .BR tee (2)),
 167 provide userspace programs with full control over an arbitrary
 168 kernel buffer, implemented within the kernel using the same type
 169 of buffer that is used for a pipe.
 170 In overview, these system calls perform the following tasks:
 171 .TP 1.2i
 172 .BR splice ()
 173 moves data from the buffer to an arbitrary file descriptor, or vice versa,
 174 or from one buffer to another.
 175 .TP
 176 .BR tee (2)
 177 "copies" the data from one buffer to another.
 178 .TP
 179 .BR vmsplice (2)
 180 "copies" data from user space into the buffer.
 181 .PP
 182 Though we talk of copying, actual copies are generally avoided.
 183 The kernel does this by implementing a pipe buffer as a set
 184 of reference-counted pointers to pages of kernel memory.
 185 The kernel creates "copies" of pages in a buffer by creating new
 186 pointers (for the output buffer) referring to the pages,
 187 and increasing the reference counts for the pages:
 188 only pointers are copied, not the pages of the buffer.
 189 .\"
 190 .\" Linus: Now, imagine using the above in a media server, for example.
 191 .\" Let's say that a year or two has passed, so that the video drivers
 192 .\" have been updated to be able to do the splice thing, and what can
 193 .\" you do? You can:
 194 .\"
 195 .\" - splice from the (mpeg or whatever - let's just assume that the video
 196 .\"   input is either digital or does the encoding on its own - like they
 197 .\"   pretty much all do) video input into a pipe (remember: no copies - the
 198 .\"   video input will just DMA directly into memory, and splice will just
 199 .\"   set up the pages in the pipe buffer)
 200 .\" - tee that pipe to split it up
 201 .\" - splice one end to a file (ie "save the compressed stream to disk")
 202 .\" - splice the other end to a real-time video decoder window for your
 203 .\"   real-time viewing pleasure.
 204 .\"
 205 .\" Linus: Now, the advantage of splice()/tee() is that you can
 206 .\" do zero-copy movement of data, and unlike sendfile() you can
 207 .\" do it on _arbitrary_ data (and, as shown by "tee()", it's more
 208 .\" than just sending the data to somebody else: you can duplicate
 209 .\" the data and choose to forward it to two or more different
 210 .\" users - for things like logging etc).
 211 .\"
 212 .SH EXAMPLE
 213 See
 214 .BR tee (2).
 215 .SH SEE ALSO
 216 .BR sendfile (2),
 217 .BR splice (2),
 218 .BR tee (2),
 219 .BR feature_test_macros (7)