man2/vmsplice.2

   1 .\" Hey Emacs! This file is -*- nroff -*- source.
   2 .\"
   3 .\" This manpage is Copyright (C) 2006 Jens Axboe
   4 .\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
   5 .\"
   6 .\" Permission is granted to make and distribute verbatim copies of this
   7 .\" manual provided the copyright notice and this permission notice are
   8 .\" preserved on all copies.
   9 .\"
  10 .\" Permission is granted to copy and distribute modified versions of this
  11 .\" manual under the conditions for verbatim copying, provided that the
  12 .\" entire resulting derived work is distributed under the terms of a
  13 .\" permission notice identical to this one.
  14 .\"
  15 .\" Since the Linux kernel and libraries are constantly changing, this
  16 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  17 .\" responsibility for errors or omissions, or for damages resulting from
  18 .\" the use of the information contained herein.  The author(s) may not
  19 .\" have taken the same level of care in the production of this manual,
  20 .\" which is licensed free of charge, as they might when working
  21 .\" professionally.
  22 .\"
  23 .\" Formatted or processed versions of this manual, if unaccompanied by
  24 .\" the source, must acknowledge the copyright and authors of this work.
  25 .\"
  26 .TH VMSPLICE 2 2006-04-28 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 vmsplice \- splice user pages into a pipe
  29 .SH SYNOPSIS
  30 .nf
  31 .B #define _GNU_SOURCE
  32 .B #include <fcntl.h>
  33 .B #include <sys/uio.h>
  34
  35 .BI "long vmsplice(int " fd ", const struct iovec *" iov ,
  36 .BI "              unsigned long " nr_segs ", unsigned int " flags );
  37 .fi
  38 .SH DESCRIPTION
  39 .\" Linus: vmsplice() system call to basically do a "write to
  40 .\" the buffer", but using the reference counting and VM traversal
  41 .\" to actually fill the buffer. This means that the user needs to
  42 .\" be careful not to re-use the user-space buffer it spliced into
  43 .\" the kernel-space one (contrast this to "write()", which copies
  44 .\" the actual data, and you can thus re-use the buffer immediately
  45 .\" after a successful write), but that is often easy to do.
  46 The
  47 .BR vmsplice ()
  48 system call maps
  49 .I nr_segs
  50 ranges of user memory described by
  51 .I iov
  52 into a pipe.
  53 The file descriptor
  54 .I fd
  55 must refer to a pipe.
  56
  57 The pointer
  58 .I iov
  59 points to an array of
  60 .I iovec
  61 structures as defined in
  62 .IR <sys/uio.h> :
  63
  64 .in +0.25i
  65 .nf
  66 struct iovec {
  67     void  *iov_base;            /* Starting address */
  68     size_t iov_len;             /* Number of bytes */
  69 };
  70 .in -0.25i
  71 .fi
  72
  73 The
  74 .I flags
  75 argument is a bit mask that is composed by ORing together
  76 zero or more of the following values:
  77 .TP 1.9i
  78 .B SPLICE_F_MOVE
  79 Unused for
  80 .BR vmsplice ();
  81 see
  82 .BR splice (2).
  83 .TP
  84 .B SPLICE_F_NONBLOCK
  85 .\" Not used for vmsplice
  86 .\" May be in the future -- therefore EAGAIN
  87 Do not block on I/O; see
  88 .BR splice (2)
  89 for further details.
  90 .TP
  91 .B SPLICE_F_MORE
  92 Currently has no effect for
  93 .BR vmsplice (),
  94 but may be implemented in the future; see
  95 .BR splice (2).
  96 .TP
  97 .B SPLICE_F_GIFT
  98 The user pages are a gift to the kernel.
  99 The application may not modify this memory ever,
 100 .\" FIXME Explain the following line in a little more detail:
 101 or page cache and on-disk data may differ.
 102 Gifting pages to the kernel means that a subsequent
 103 .BR splice (2)
 104 .B SPLICE_F_MOVE
 105 can successfully move the pages;
 106 if this flag is not specified, then a subsequent
 107 .BR splice (2)
 108 .B SPLICE_F_MOVE
 109 must copy the pages.
 110 Data must also be properly page aligned, both in memory and length.
 111 .\" .... if we expect to later SPLICE_F_MOVE to the cache.
 112 .SH RETURN VALUE
 113 Upon successful completion,
 114 .BR vmsplice ()
 115 returns the number of bytes transferred to the pipe.
 116 On error,
 117 .BR vmsplice ()
 118 returns \-1 and
 119 .I errno
 120 is set to indicate the error.
 121 .SH ERRORS
 122 .TP
 123 .B EBADF
 124 .I fd
 125 either not valid, or doesn't refer to a pipe.
 126 .TP
 127 .B EINVAL
 128 .I nr_segs
 129 is 0 or greater than
 130 .BR IOV_MAX ;
 131 or memory not aligned if
 132 .B SPLICE_F_GIFT
 133 set.
 134 .TP
 135 .B ENOMEM
 136 Out of memory.
 137 .SH VERSIONS
 138 The
 139 .BR vmsplice (2)
 140 system call first appeared in Linux 2.6.17.
 141 .SH "CONFORMING TO"
 142 This system call is Linux specific.
 143 .SH NOTES
 144 .BR vmsplice ()
 145 follows the other vectorized read/write type functions when it comes to
 146 limitations on number of segments being passed in.
 147 This limit is
 148 .B IOV_MAX
 149 as defined in
 150 .IR <limits.h> .
 151 At the time of this writing, that limit is 1024.
 152 .SH SEE ALSO
 153 .BR splice (2),
 154 .BR tee (2),
 155 .BR feature_test_macros (7)