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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13 msync — synchronize memory with physical storage
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16 #include <sys/mman.h>
17 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 may require that addr be a multiple of the page size
29 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 mappings
38 has any permanent storage locations. The effect of msync() on a shared
39 memory object or a typed memory object is unspecified. The behavior of
40 this function is unspecified if the mapping was not established by a
41 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 ┌──────────────────┬──────────────────────────────┐
47 │Symbolic Constant │ Description │
48 ├──────────────────┼──────────────────────────────┤
49 │MS_ASYNC │ Perform asynchronous writes. │
50 │MS_SYNC │ Perform synchronous writes. │
51 │MS_INVALIDATE │ Invalidate cached data. │
52 └──────────────────┴──────────────────────────────┘
53 When MS_ASYNC is specified, msync() shall return immediately once all
54 the write operations are initiated or queued for servicing; when
55 MS_SYNC is specified, msync() shall not return until all write opera‐
56 tions are completed as defined for synchronized I/O data integrity com‐
57 pletion. Either MS_ASYNC or MS_SYNC shall be specified, but not both.
58 When MS_INVALIDATE is specified, msync() shall invalidate all cached
60 copies of mapped data that are inconsistent with the permanent storage
61 locations such that subsequent references shall obtain data that was
62 consistent with the permanent storage locations sometime between the
63 call to msync() and the first subsequent memory reference to the data.
64 If msync() causes any write to a file, the file's last data modifica‐
66 tion and last file status change timestamps shall be marked for update.
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69 Upon successful completion, msync() shall return 0; otherwise, it shall
70 return −1 and set errno to indicate the error.
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73 The msync() function shall fail if:
74 EBUSY Some or all of the addresses in the range starting at addr and
76 continuing for len bytes are locked, and MS_INVALIDATE is speci‐
77 fied.
78 EINVAL The value of flags is invalid.
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 msync() function may fail if:
86 EINVAL The value of addr is not a multiple of the page size as returned
88 by sysconf().
89 The following sections are informative.
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93 None.
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96 The msync() function is only supported if the Synchronized Input and
97 Output option is supported, and thus need not be available on all
98 implementations.
99 The msync() function should be used by programs that require a memory
101 object to be in a known state; for example, in building transaction
102 facilities.
103 Normal system activity can cause pages to be written to disk. There‐
105 fore, there are no guarantees that msync() is the only control over
106 when pages are or are not written to disk.
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109 The msync() function writes out data in a mapped region to the perma‐
110 nent storage for the underlying object. The call to msync() ensures
111 data integrity of the file.
112 After the data is written out, any cached data may be invalidated if
114 the MS_INVALIDATE flag was specified. This is useful on systems that do
115 not support read/write consistency.
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118 None.
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121 mmap(), sysconf()
122 The Base Definitions volume of POSIX.1‐2008, <sys_mman.h>
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126 Portions of this text are reprinted and reproduced in electronic form
127 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
128 -- Portable Operating System Interface (POSIX), The Open Group Base
129 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
130 cal and Electronics Engineers, Inc and The Open Group. (This is
131 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
132 event of any discrepancy between this version and the original IEEE and
133 The Open Group Standard, the original IEEE and The Open Group Standard
134 is the referee document. The original Standard can be obtained online
135 at http://www.unix.org/online.html .
136 Any typographical or formatting errors that appear in this page are
138 most likely to have been introduced during the conversion of the source
139 files to man page format. To report such errors, see https://www.ker‐
140 nel.org/doc/man-pages/reporting_bugs.html .
141IEEE/The Open Group 2013 MSYNC(3P)