1FSYNC(2) Linux Programmer's Manual FSYNC(2)
2
6 fsync, fdatasync - synchronize a file's in-core state with storage de‐
7 vice
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():
19 Glibc 2.16 and later:
20 No feature test macros need be defined
21 Glibc up to and including 2.15:
22 _BSD_SOURCE || _XOPEN_SOURCE
23 || /* Since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L
24 fdatasync():
26 _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500
27
29 fsync() transfers ("flushes") all modified in-core data of (i.e., modi‐
30 fied buffer cache pages for) the file referred to by the file descrip‐
31 tor fd to the disk device (or other permanent storage device) so that
32 all changed information can be retrieved even if the system crashes or
33 is rebooted. This includes writing through or flushing a disk cache if
34 present. The call blocks until the device reports that the transfer
35 has completed.
36 As well as flushing the file data, fsync() also flushes the metadata
38 information associated with the file (see inode(7)).
39 Calling fsync() does not necessarily ensure that the entry in the di‐
41 rectory containing the file has also reached disk. For that an ex‐
42 plicit fsync() on a file descriptor for the directory is also needed.
43 fdatasync() is similar to fsync(), but does not flush modified metadata
45 unless that metadata is needed in order to allow a subsequent data re‐
46 trieval to be correctly handled. For example, changes to st_atime or
47 st_mtime (respectively, time of last access and time of last modifica‐
48 tion; see inode(7)) do not require flushing because they are not neces‐
49 sary for a subsequent data read to be handled correctly. On the other
50 hand, a change to the file size (st_size, as made by say ftruncate(2)),
51 would require a metadata flush.
52 The aim of fdatasync() is to reduce disk activity for applications that
54 do not require all metadata to be synchronized with the disk.
55
57 On success, these system calls return zero. On error, -1 is returned,
58 and errno is set to indicate the error.
59
61 EBADF fd is not a valid open file descriptor.
62 EIO An error occurred during synchronization. This error may relate
64 to data written to some other file descriptor on the same file.
65 Since Linux 4.13, errors from write-back will be reported to all
66 file descriptors that might have written the data which trig‐
67 gered the error. Some filesystems (e.g., NFS) keep close track
68 of which data came through which file descriptor, and give more
69 precise reporting. Other filesystems (e.g., most local filesys‐
70 tems) will report errors to all file descriptors that were open
71 on the file when the error was recorded.
72 ENOSPC Disk space was exhausted while synchronizing.
74 EROFS, EINVAL
76 fd is bound to a special file (e.g., a pipe, FIFO, or socket)
77 which does not support synchronization.
78 ENOSPC, EDQUOT
80 fd is bound to a file on NFS or another filesystem which does
81 not allocate space at the time of a write(2) system call, and
82 some previous write failed due to insufficient storage space.
83
85 POSIX.1-2001, POSIX.1-2008, 4.3BSD.
86 On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRO‐
88 NIZED_IO is defined in <unistd.h> to a value greater than 0. (See also
89 sysconf(3).)
90
92 On some UNIX systems (but not Linux), fd must be a writable file de‐
93 scriptor.
94 In Linux 2.2 and earlier, fdatasync() is equivalent to fsync(), and so
96 has no performance advantage.
97 The fsync() implementations in older kernels and lesser used filesys‐
99 tems do not know how to flush disk caches. In these cases disk caches
100 need to be disabled using hdparm(8) or sdparm(8) to guarantee safe op‐
101 eration.
102
104 sync(1), bdflush(2), open(2), posix_fadvise(2), pwritev(2), sync(2),
105 sync_file_range(2), fflush(3), fileno(3), hdparm(8), mount(8)
106
108 This page is part of release 5.13 of the Linux man-pages project. A
109 description of the project, information about reporting bugs, and the
110 latest version of this page, can be found at
111 https://www.kernel.org/doc/man-pages/.
112Linux 2021-03-22 FSYNC(2)