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1 | .\" $NetBSD: fsync.2,v 1.4 1995/02/27 12:32:38 cgd Exp $ | |
2 | .\" | |
3 | .\" Copyright (c) 1983, 1993 | |
4 | .\" The Regents of the University of California. All rights reserved. | |
5 | .\" | |
6 | .\" Redistribution and use in source and binary forms, with or without | |
7 | .\" modification, are permitted provided that the following conditions | |
8 | .\" are met: | |
9 | .\" 1. Redistributions of source code must retain the above copyright | |
10 | .\" notice, this list of conditions and the following disclaimer. | |
11 | .\" 2. Redistributions in binary form must reproduce the above copyright | |
12 | .\" notice, this list of conditions and the following disclaimer in the | |
13 | .\" documentation and/or other materials provided with the distribution. | |
14 | .\" 3. All advertising materials mentioning features or use of this software | |
15 | .\" must display the following acknowledgement: | |
16 | .\" This product includes software developed by the University of | |
17 | .\" California, Berkeley and its contributors. | |
18 | .\" 4. Neither the name of the University nor the names of its contributors | |
19 | .\" may be used to endorse or promote products derived from this software | |
20 | .\" without specific prior written permission. | |
21 | .\" | |
22 | .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND | |
23 | .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE | |
24 | .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE | |
25 | .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE | |
26 | .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL | |
27 | .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS | |
28 | .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) | |
29 | .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT | |
30 | .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY | |
31 | .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF | |
32 | .\" SUCH DAMAGE. | |
33 | .\" | |
34 | .\" @(#)fsync.2 8.1 (Berkeley) 6/4/93 | |
35 | .\" | |
36 | .Dd June 4, 1993 | |
37 | .Dt FSYNC 2 | |
38 | .Os BSD 4.2 | |
39 | .Sh NAME | |
40 | .Nm fsync | |
41 | .Nd "synchronize a file's in-core state with that on disk" | |
42 | .Sh SYNOPSIS | |
43 | .Fd #include <unistd.h> | |
44 | .Ft int | |
45 | .Fo fsync | |
46 | .Fa "int fildes" | |
47 | .Fc | |
48 | .Sh DESCRIPTION | |
49 | .Fn fsync | |
50 | causes all modified data and attributes of | |
51 | .Fa fildes | |
52 | to be moved to a permanent storage device. | |
53 | This normally results in all in-core modified copies | |
54 | of buffers for the associated file to be written to a disk. | |
55 | .Pp | |
56 | Note that while | |
57 | .Fn fsync | |
58 | will flush all data from the host to the drive | |
59 | (i.e. the "permanent storage device"), | |
60 | the drive itself may not physically write the data | |
61 | to the platters for quite some time | |
62 | and it may be written in an out-of-order sequence. | |
63 | .Pp | |
64 | Specifically, if the drive loses power | |
65 | or the OS crashes, | |
66 | the application may find that only some or none of their data was written. | |
67 | The disk drive may also re-order the data | |
68 | so that later writes may be present, while earlier writes are not. | |
69 | .Pp | |
70 | This is not a theoretical edge case. | |
71 | This scenario is easily reproduced with real world workloads | |
72 | and drive power failures. | |
73 | .Pp | |
74 | For applications that require tighter guarantees | |
75 | about the integrity of their data, | |
76 | Mac OS X provides the F_FULLFSYNC fcntl. | |
77 | The F_FULLFSYNC fcntl asks the drive to flush all buffered data | |
78 | to permanent storage. | |
79 | Applications, such as databases, | |
80 | that require a strict ordering of writes | |
81 | should use F_FULLFSYNC to ensure that their data | |
82 | is written in the order they expect. | |
83 | Please see | |
84 | .Xr fcntl 2 | |
85 | for more detail. | |
86 | .Pp | |
87 | .Sh RETURN VALUES | |
88 | .Rv -std fsync | |
89 | .Sh ERRORS | |
90 | The | |
91 | .Fn fsync | |
92 | system call will fail if: | |
93 | .Bl -tag -width Er | |
94 | .\" ========== | |
95 | .It Bq Er EBADF | |
96 | .Fa fildes | |
97 | is not a valid descriptor. | |
98 | .\" ========== | |
99 | .It Bq Er EINTR | |
100 | Its execution is interrupted by a signal. | |
101 | .\" ========== | |
102 | .It Bq Er EINVAL | |
103 | .Fa fildes | |
104 | refers to a file type (e.g., a socket) | |
105 | that does not support this operation. | |
106 | .\" ========== | |
107 | .It Bq Er EIO | |
108 | An I/O error occurred while reading from or writing to the file system. | |
109 | .El | |
110 | .Pp | |
111 | If a queued I/O operation fails, | |
112 | .Fn fsync | |
113 | may fail with any of the errors defined for | |
114 | .Xr read 2 | |
115 | or | |
116 | .Xr write 2 . | |
117 | .Sh SEE ALSO | |
118 | .Xr fcntl 2 , | |
119 | .Xr read 2 , | |
120 | .Xr sync 2 , | |
121 | .Xr write 2 , | |
122 | .Xr sync 8 , | |
123 | .Xr update 8 | |
124 | .Sh HISTORY | |
125 | The | |
126 | .Fn fsync | |
127 | function call appeared in | |
128 | .Bx 4.2 . |