1MSYNC(3P) POSIX Programmer's Manual MSYNC(3P)
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6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
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12 msync - synchronize memory with physical storage
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15 #include <sys/mman.h>
16 int msync(void *addr, size_t len, int flags);
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21 The msync() function shall write all modified data to permanent storage
22 locations, if any, in those whole pages containing any part of the
23 address space of the process starting at address addr and continuing
24 for len bytes. If no such storage exists, msync() need not have any
25 effect. If requested, the msync() function shall then invalidate cached
26 copies of data.
27 The implementation shall require that addr be a multiple of the page
29 size as returned by sysconf().
30 For mappings to files, the msync() function shall ensure that all write
32 operations are completed as defined for synchronized I/O data integrity
33 completion. It is unspecified whether the implementation also writes
34 out other file attributes. When the msync() function is called on
35 MAP_PRIVATE mappings, any modified data shall not be written to the
36 underlying object and shall not cause such data to be made visible to
37 other processes. It is unspecified whether data in MAP_PRIVATE map‐
38 pings has any permanent storage locations. The effect of msync() on a
39 shared memory object or a typed memory object is unspecified. The
40 behavior of this function is unspecified if the mapping was not estab‐
41 lished by a call to mmap().
42 The flags argument is constructed from the bitwise-inclusive OR of one
44 or more of the following flags defined in the <sys/mman.h> header:
45 Symbolic Constant Description
47 MS_ASYNC Perform asynchronous writes.
48 MS_SYNC Perform synchronous writes.
49 MS_INVALIDATE Invalidate cached data.
50 When MS_ASYNC is specified, msync() shall return immediately once all
52 the write operations are initiated or queued for servicing; when
53 MS_SYNC is specified, msync() shall not return until all write opera‐
54 tions are completed as defined for synchronized I/O data integrity com‐
55 pletion. Either MS_ASYNC or MS_SYNC is specified, but not both.
56 When MS_INVALIDATE is specified, msync() shall invalidate all cached
58 copies of mapped data that are inconsistent with the permanent storage
59 locations such that subsequent references shall obtain data that was
60 consistent with the permanent storage locations sometime between the
61 call to msync() and the first subsequent memory reference to the data.
62 If msync() causes any write to a file, the file's st_ctime and st_mtime
64 fields shall be marked for update.
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67 Upon successful completion, msync() shall return 0; otherwise, it shall
68 return -1 and set errno to indicate the error.
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71 The msync() function shall fail if:
72 EBUSY Some or all of the addresses in the range starting at addr and
74 continuing for len bytes are locked, and MS_INVALIDATE is speci‐
75 fied.
76 EINVAL The value of flags is invalid.
78 EINVAL The value of addr is not a multiple of the page size {PAGESIZE}.
80 ENOMEM The addresses in the range starting at addr and continuing for
82 len bytes are outside the range allowed for the address space of
83 a process or specify one or more pages that are not mapped.
84 The following sections are informative.
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89 None.
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92 The msync() function is only supported if the Memory Mapped Files
93 option and the Synchronized Input and Output option are supported, and
94 thus need not be available on all implementations.
95 The msync() function should be used by programs that require a memory
97 object to be in a known state; for example, in building transaction
98 facilities.
99 Normal system activity can cause pages to be written to disk. There‐
101 fore, there are no guarantees that msync() is the only control over
102 when pages are or are not written to disk.
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105 The msync() function writes out data in a mapped region to the perma‐
106 nent storage for the underlying object. The call to msync() ensures
107 data integrity of the file.
108 After the data is written out, any cached data may be invalidated if
110 the MS_INVALIDATE flag was specified. This is useful on systems that do
111 not support read/write consistency.
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114 None.
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117 mmap(), sysconf(), the Base Definitions volume of IEEE Std 1003.1-2001,
118 <sys/mman.h>
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121 Portions of this text are reprinted and reproduced in electronic form
122 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
123 -- Portable Operating System Interface (POSIX), The Open Group Base
124 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
125 Electrical and Electronics Engineers, Inc and The Open Group. In the
126 event of any discrepancy between this version and the original IEEE and
127 The Open Group Standard, the original IEEE and The Open Group Standard
128 is the referee document. The original Standard can be obtained online
129 at http://www.opengroup.org/unix/online.html .
130IEEE/The Open Group 2003 MSYNC(3P)