1FSYNC(2) Linux Programmer's Manual FSYNC(2)
2
6 fsync, fdatasync - synchronize a file's in-core state with storage
7 device
8
10 #include <unistd.h>
11 int fsync(int fd);
13 int fdatasync(int fd);
15 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
17 fsync(): _BSD_SOURCE || _XOPEN_SOURCE
19 || /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L
20 fdatasync(): _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500
21
23 fsync() transfers ("flushes") all modified in-core data of (i.e., modi‐
24 fied buffer cache pages for) the file referred to by the file descrip‐
25 tor fd to the disk device (or other permanent storage device) so that
26 all changed information can be retrieved even after the system crashed
27 or was rebooted. This includes writing through or flushing a disk
28 cache if present. The call blocks until the device reports that the
29 transfer has completed. It also flushes metadata information associ‐
30 ated with the file (see stat(2)).
31 Calling fsync() does not necessarily ensure that the entry in the
33 directory containing the file has also reached disk. For that an
34 explicit fsync() on a file descriptor for the directory is also needed.
35 fdatasync() is similar to fsync(), but does not flush modified metadata
37 unless that metadata is needed in order to allow a subsequent data
38 retrieval to be correctly handled. For example, changes to st_atime or
39 st_mtime (respectively, time of last access and time of last modifica‐
40 tion; see stat(2)) do not require flushing because they are not neces‐
41 sary for a subsequent data read to be handled correctly. On the other
42 hand, a change to the file size (st_size, as made by say ftruncate(2)),
43 would require a metadata flush.
44 The aim of fdatasync() is to reduce disk activity for applications that
46 do not require all metadata to be synchronized with the disk.
47
49 On success, these system calls return zero. On error, -1 is returned,
50 and errno is set appropriately.
51
53 EBADF fd is not a valid open file descriptor.
54 EIO An error occurred during synchronization.
56 EROFS, EINVAL
58 fd is bound to a special file which does not support synchro‐
59 nization.
60
62 4.3BSD, POSIX.1-2001.
63
65 On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRO‐
66 NIZED_IO is defined in <unistd.h> to a value greater than 0. (See also
67 sysconf(3).)
68
70 On some UNIX systems (but not Linux), fd must be a writable file
71 descriptor.
72 In Linux 2.2 and earlier, fdatasync() is equivalent to fsync(), and so
74 has no performance advantage.
75 The fsync() implementations in older kernels and lesser used filesys‐
77 tems does not know how to flush disk caches. In these cases disk
78 caches need to be disabled using hdparm(8) or sdparm(8) to guarantee
79 safe operation.
80
82 bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8), mount(8),
83 sync(8), update(8)
84
86 This page is part of release 3.53 of the Linux man-pages project. A
87 description of the project, and information about reporting bugs, can
88 be found at http://www.kernel.org/doc/man-pages/.
89Linux 2012-02-27 FSYNC(2)