1FSYNC(3P) POSIX Programmer's Manual FSYNC(3P)
2
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.
10
12 fsync — synchronize changes to a file
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15 #include <unistd.h>
16 int fsync(int fildes);
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20 The fsync() function shall request that all data for the open file
21 descriptor named by fildes is to be transferred to the storage device
22 associated with the file described by fildes. The nature of the trans‐
23 fer is implementation-defined. The fsync() function shall not return
24 until the system has completed that action or until an error is
25 detected.
26 If _POSIX_SYNCHRONIZED_IO is defined, the fsync() function shall force
28 all currently queued I/O operations associated with the file indicated
29 by file descriptor fildes to the synchronized I/O completion state. All
30 I/O operations shall be completed as defined for synchronized I/O file
31 integrity completion.
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34 Upon successful completion, fsync() shall return 0. Otherwise, -1 shall
35 be returned and errno set to indicate the error. If the fsync() func‐
36 tion fails, outstanding I/O operations are not guaranteed to have been
37 completed.
38
40 The fsync() function shall fail if:
41 EBADF The fildes argument is not a valid descriptor.
43 EINTR The fsync() function was interrupted by a signal.
45 EINVAL The fildes argument does not refer to a file on which this oper‐
47 ation is possible.
48 EIO An I/O error occurred while reading from or writing to the file
50 system.
51 In the event that any of the queued I/O operations fail, fsync() shall
53 return the error conditions defined for read() and write().
54 The following sections are informative.
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58 None.
59
61 The fsync() function should be used by programs which require modifica‐
62 tions to a file to be completed before continuing; for example, a pro‐
63 gram which contains a simple transaction facility might use it to
64 ensure that all modifications to a file or files caused by a transac‐
65 tion are recorded.
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68 The fsync() function is intended to force a physical write of data from
69 the buffer cache, and to assure that after a system crash or other
70 failure that all data up to the time of the fsync() call is recorded on
71 the disk. Since the concepts of ``buffer cache'', ``system crash'',
72 ``physical write'', and ``non-volatile storage'' are not defined here,
73 the wording has to be more abstract.
74 If _POSIX_SYNCHRONIZED_IO is not defined, the wording relies heavily on
76 the conformance document to tell the user what can be expected from the
77 system. It is explicitly intended that a null implementation is permit‐
78 ted. This could be valid in the case where the system cannot assure
79 non-volatile storage under any circumstances or when the system is
80 highly fault-tolerant and the functionality is not required. In the
81 middle ground between these extremes, fsync() might or might not actu‐
82 ally cause data to be written where it is safe from a power failure.
83 The conformance document should identify at least that one configura‐
84 tion exists (and how to obtain that configuration) where this can be
85 assured for at least some files that the user can select to use for
86 critical data. It is not intended that an exhaustive list is required,
87 but rather sufficient information is provided so that if critical data
88 needs to be saved, the user can determine how the system is to be con‐
89 figured to allow the data to be written to non-volatile storage.
90 It is reasonable to assert that the key aspects of fsync() are unrea‐
92 sonable to test in a test suite. That does not make the function any
93 less valuable, just more difficult to test. A formal conformance test
94 should probably force a system crash (power shutdown) during the test
95 for this condition, but it needs to be done in such a way that auto‐
96 mated testing does not require this to be done except when a formal
97 record of the results is being made. It would also not be unreasonable
98 to omit testing for fsync(), allowing it to be treated as a quality-of-
99 implementation issue.
100
102 None.
103
105 sync()
106 The Base Definitions volume of POSIX.1‐2017, <unistd.h>
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110 Portions of this text are reprinted and reproduced in electronic form
111 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
112 table Operating System Interface (POSIX), The Open Group Base Specifi‐
113 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
114 Electrical and Electronics Engineers, Inc and The Open Group. In the
115 event of any discrepancy between this version and the original IEEE and
116 The Open Group Standard, the original IEEE and The Open Group Standard
117 is the referee document. The original Standard can be obtained online
118 at http://www.opengroup.org/unix/online.html .
119 Any typographical or formatting errors that appear in this page are
121 most likely to have been introduced during the conversion of the source
122 files to man page format. To report such errors, see https://www.ker‐
123 nel.org/doc/man-pages/reporting_bugs.html .
124IEEE/The Open Group 2017 FSYNC(3P)