man2/splice.2

   1 .\" This manpage is Copyright (C) 2006 Jens Axboe
   2 .\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .TH splice 2 (date) "Linux man-pages (unreleased)"
   7 .SH NAME
   8 splice \- splice data to/from a pipe
   9 .SH LIBRARY
  10 Standard C library
  11 .RI ( libc ", " \-lc )
  12 .SH SYNOPSIS
  13 .nf
  14 .BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
  15 .B #include <fcntl.h>
  16 .PP
  17 .BI "ssize_t splice(int " fd_in ", off64_t *" off_in ", int " fd_out ,
  18 .BI "               off64_t *" off_out ", size_t " len \
  19 ", unsigned int " flags );
  20 .\" Return type was long before glibc 2.7
  21 .fi
  22 .SH DESCRIPTION
  23 .BR splice ()
  24 moves data between two file descriptors
  25 without copying between kernel address space and user address space.
  26 It transfers up to
  27 .I len
  28 bytes of data from the file descriptor
  29 .I fd_in
  30 to the file descriptor
  31 .IR fd_out ,
  32 where one of the file descriptors must refer to a pipe.
  33 .PP
  34 The following semantics apply for
  35 .I fd_in
  36 and
  37 .IR off_in :
  38 .IP \(bu 3
  39 If
  40 .I fd_in
  41 refers to a pipe, then
  42 .I off_in
  43 must be NULL.
  44 .IP \(bu
  45 If
  46 .I fd_in
  47 does not refer to a pipe and
  48 .I off_in
  49 is NULL, then bytes are read from
  50 .I fd_in
  51 starting from the file offset,
  52 and the file offset is adjusted appropriately.
  53 .IP \(bu
  54 If
  55 .I fd_in
  56 does not refer to a pipe and
  57 .I off_in
  58 is not NULL, then
  59 .I off_in
  60 must point to a buffer which specifies the starting
  61 offset from which bytes will be read from
  62 .IR fd_in ;
  63 in this case, the file offset of
  64 .I fd_in
  65 is not changed.
  66 .PP
  67 Analogous statements apply for
  68 .I fd_out
  69 and
  70 .IR off_out .
  71 .PP
  72 The
  73 .I flags
  74 argument is a bit mask that is composed by ORing together
  75 zero or more of the following values:
  76 .TP
  77 .B SPLICE_F_MOVE
  78 Attempt to move pages instead of copying.
  79 This is only a hint to the kernel:
  80 pages may still be copied if the kernel cannot move the
  81 pages from the pipe, or if
  82 the pipe buffers don't refer to full pages.
  83 The initial implementation of this flag was buggy:
  84 therefore starting in Linux 2.6.21 it is a no-op
  85 (but is still permitted in a
  86 .BR splice ()
  87 call);
  88 in the future, a correct implementation may be restored.
  89 .TP
  90 .B SPLICE_F_NONBLOCK
  91 Do not block on I/O.
  92 This makes the splice pipe operations nonblocking, but
  93 .BR splice ()
  94 may nevertheless block because the file descriptors that
  95 are spliced to/from may block (unless they have the
  96 .B O_NONBLOCK
  97 flag set).
  98 .TP
  99 .B SPLICE_F_MORE
 100 More data will be coming in a subsequent splice.
 101 This is a helpful hint when
 102 the
 103 .I fd_out
 104 refers to a socket (see also the description of
 105 .B MSG_MORE
 106 in
 107 .BR send (2),
 108 and the description of
 109 .B TCP_CORK
 110 in
 111 .BR tcp (7)).
 112 .TP
 113 .B SPLICE_F_GIFT
 114 Unused for
 115 .BR splice ();
 116 see
 117 .BR vmsplice (2).
 118 .SH RETURN VALUE
 119 Upon successful completion,
 120 .BR splice ()
 121 returns the number of bytes
 122 spliced to or from the pipe.
 123 .PP
 124 A return value of 0 means end of input.
 125 If
 126 .I fd_in
 127 refers to a pipe, then this means that there was no data to transfer,
 128 and it would not make sense to block because there are no writers
 129 connected to the write end of the pipe.
 130 .PP
 131 On error,
 132 .BR splice ()
 133 returns \-1 and
 134 .I errno
 135 is set to indicate the error.
 136 .SH ERRORS
 137 .TP
 138 .B EAGAIN
 139 .B SPLICE_F_NONBLOCK
 140 was specified in
 141 .I flags
 142 or one of the file descriptors had been marked as nonblocking
 143 .RB ( O_NONBLOCK ) ,
 144 and the operation would block.
 145 .TP
 146 .B EBADF
 147 One or both file descriptors are not valid,
 148 or do not have proper read-write mode.
 149 .TP
 150 .B EINVAL
 151 The target filesystem doesn't support splicing.
 152 .TP
 153 .B EINVAL
 154 The target file is opened in append mode.
 155 .\" The append-mode error is given since 2.6.27; in earlier kernels,
 156 .\" splice() in append mode was broken
 157 .TP
 158 .B EINVAL
 159 Neither of the file descriptors refers to a pipe.
 160 .TP
 161 .B EINVAL
 162 An offset was given for nonseekable device (e.g., a pipe).
 163 .TP
 164 .B EINVAL
 165 .I fd_in
 166 and
 167 .I fd_out
 168 refer to the same pipe.
 169 .TP
 170 .B ENOMEM
 171 Out of memory.
 172 .TP
 173 .B ESPIPE
 174 Either
 175 .I off_in
 176 or
 177 .I off_out
 178 was not NULL, but the corresponding file descriptor refers to a pipe.
 179 .SH VERSIONS
 180 The
 181 .BR splice ()
 182 system call first appeared in Linux 2.6.17;
 183 library support was added to glibc in version 2.5.
 184 .SH STANDARDS
 185 This system call is Linux-specific.
 186 .SH NOTES
 187 The three system calls
 188 .BR splice (),
 189 .BR vmsplice (2),
 190 and
 191 .BR tee (2),
 192 provide user-space programs with full control over an arbitrary
 193 kernel buffer, implemented within the kernel using the same type
 194 of buffer that is used for a pipe.
 195 In overview, these system calls perform the following tasks:
 196 .TP
 197 .BR splice ()
 198 moves data from the buffer to an arbitrary file descriptor, or vice versa,
 199 or from one buffer to another.
 200 .TP
 201 .BR tee (2)
 202 "copies" the data from one buffer to another.
 203 .TP
 204 .BR vmsplice (2)
 205 "copies" data from user space into the buffer.
 206 .PP
 207 Though we talk of copying, actual copies are generally avoided.
 208 The kernel does this by implementing a pipe buffer as a set
 209 of reference-counted pointers to pages of kernel memory.
 210 The kernel creates "copies" of pages in a buffer by creating new
 211 pointers (for the output buffer) referring to the pages,
 212 and increasing the reference counts for the pages:
 213 only pointers are copied, not the pages of the buffer.
 214 .\"
 215 .\" Linus: Now, imagine using the above in a media server, for example.
 216 .\" Let's say that a year or two has passed, so that the video drivers
 217 .\" have been updated to be able to do the splice thing, and what can
 218 .\" you do? You can:
 219 .\"
 220 .\" - splice from the (mpeg or whatever - let's just assume that the video
 221 .\"   input is either digital or does the encoding on its own - like they
 222 .\"   pretty much all do) video input into a pipe (remember: no copies - the
 223 .\"   video input will just DMA directly into memory, and splice will just
 224 .\"   set up the pages in the pipe buffer)
 225 .\" - tee that pipe to split it up
 226 .\" - splice one end to a file (ie "save the compressed stream to disk")
 227 .\" - splice the other end to a real-time video decoder window for your
 228 .\"   real-time viewing pleasure.
 229 .\"
 230 .\" Linus: Now, the advantage of splice()/tee() is that you can
 231 .\" do zero-copy movement of data, and unlike sendfile() you can
 232 .\" do it on _arbitrary_ data (and, as shown by "tee()", it's more
 233 .\" than just sending the data to somebody else: you can duplicate
 234 .\" the data and choose to forward it to two or more different
 235 .\" users - for things like logging etc.).
 236 .\"
 237 .PP
 238 In Linux 2.6.30 and earlier,
 239 exactly one of
 240 .I fd_in
 241 and
 242 .I fd_out
 243 was required to be a pipe.
 244 Since Linux 2.6.31,
 245 .\" commit 7c77f0b3f9208c339a4b40737bb2cb0f0319bb8d
 246 both arguments may refer to pipes.
 247 .SH EXAMPLES
 248 See
 249 .BR tee (2).
 250 .SH SEE ALSO
 251 .BR copy_file_range (2),
 252 .BR sendfile (2),
 253 .BR tee (2),
 254 .BR vmsplice (2),
 255 .BR pipe (7)