man2/msync.2

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  25 .TH MSYNC 2 2015-08-08 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 msync \- synchronize a file with a memory map
  28 .SH SYNOPSIS
  29 .B #include <sys/mman.h>
  30 .PP
  31 .BI "int msync(void *" addr ", size_t " length ", int " flags );
  32 .SH DESCRIPTION
  33 .BR msync ()
  34 flushes changes made to the in-core copy of a file that was mapped
  35 into memory using
  36 .BR mmap (2)
  37 back to the filesystem.
  38 Without use of this call,
  39 there is no guarantee that changes are written back before
  40 .BR munmap (2)
  41 is called.
  42 To be more precise, the part of the file that
  43 corresponds to the memory area starting at
  44 .I addr
  45 and having length
  46 .I length
  47 is updated.
  48 .PP
  49 The
  50 .I flags
  51 argument should specify exactly one of
  52 .BR MS_ASYNC
  53 and
  54 .BR MS_SYNC ,
  55 and may additionally include the
  56 .B MS_INVALIDATE
  57 bit.
  58 These bits have the following meanings:
  59 .TP
  60 .B MS_ASYNC
  61 Specifies that an update be scheduled, but the call returns immediately.
  62 .TP
  63 .B MS_SYNC
  64 Requests an update and waits for it to complete.
  65 .TP
  66 .B MS_INVALIDATE
  67 .\" Since Linux 2.4, this seems to be a no-op (other than the
  68 .\" EBUSY check for VM_LOCKED).
  69 Asks to invalidate other mappings of the same file
  70 (so that they can be updated with the fresh values just written).
  71 .SH RETURN VALUE
  72 On success, zero is returned.
  73 On error, \-1 is returned, and
  74 .I errno
  75 is set appropriately.
  76 .SH ERRORS
  77 .TP
  78 .B EBUSY
  79 .B MS_INVALIDATE
  80 was specified in
  81 .IR flags ,
  82 and a memory lock exists for the specified address range.
  83 .TP
  84 .B EINVAL
  85 .I addr
  86 is not a multiple of PAGESIZE; or any bit other than
  87 .BR MS_ASYNC " | " MS_INVALIDATE " | " MS_SYNC
  88 is set in
  89 .IR flags ;
  90 or both
  91 .B MS_SYNC
  92 and
  93 .B MS_ASYNC
  94 are set in
  95 .IR flags .
  96 .TP
  97 .B ENOMEM
  98 The indicated memory (or part of it) was not mapped.
  99 .SH CONFORMING TO
 100 POSIX.1-2001, POSIX.1-2008.
 101 .PP
 102 This call was introduced in Linux 1.3.21, and then used
 103 .B EFAULT
 104 instead of
 105 .BR ENOMEM .
 106 In Linux 2.4.19, this was changed to the POSIX value
 107 .BR ENOMEM .
 108 .SH AVAILABILITY
 109 On POSIX systems on which
 110 .BR msync ()
 111 is available, both
 112 .B _POSIX_MAPPED_FILES
 113 and
 114 .B _POSIX_SYNCHRONIZED_IO
 115 are defined in
 116 .I <unistd.h>
 117 to a value greater than 0.
 118 (See also
 119 .BR sysconf (3).)
 120 .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
 121 .\" -1: unavailable, 0: ask using sysconf().
 122 .\" glibc defines them to 1.
 123 .SH NOTES
 124 According to POSIX, either
 125 .BR MS_SYNC
 126 or
 127 .BR MS_ASYNC
 128 must be specified in
 129 .IR flags ,
 130 and indeed failure to include one of these flags will cause
 131 .BR msync ()
 132 to fail on some systems.
 133 However, Linux permits a call to
 134 .BR msync ()
 135 that specifies neither of these flags,
 136 with semantics that are (currently) equivalent to specifying
 137 .BR MS_ASYNC .
 138 (Since Linux 2.6.19,
 139 .\" commit 204ec841fbea3e5138168edbc3a76d46747cc987
 140 .BR MS_ASYNC
 141 is in fact a no-op, since the kernel properly tracks dirty
 142 pages and flushes them to storage as necessary.)
 143 Notwithstanding the Linux behavior,
 144 portable, future-proof applications should ensure that they specify either
 145 .BR MS_SYNC
 146 or
 147 .BR MS_ASYNC
 148 in
 149 .IR flags .
 150 .SH SEE ALSO
 151 .BR mmap (2)
 152 .PP
 153 B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.