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fdatasync(2) [centos man page]

FSYNC(2)						     Linux Programmer's Manual							  FSYNC(2)

fsync, fdatasync - synchronize a file's in-core state with storage device SYNOPSIS
#include <unistd.h> int fsync(int fd); int fdatasync(int fd); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): fsync(): _BSD_SOURCE || _XOPEN_SOURCE || /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L fdatasync(): _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500 DESCRIPTION
fsync() transfers ("flushes") all modified in-core data of (i.e., modified buffer cache pages for) the file referred to by the file descriptor fd to the disk device (or other permanent storage device) so that all changed information can be retrieved even after the system crashed or was rebooted. This includes writing through or flushing a disk cache if present. The call blocks until the device reports that the transfer has completed. It also flushes metadata information associated with the file (see stat(2)). Calling fsync() does not necessarily ensure that the entry in the directory containing the file has also reached disk. For that an explicit fsync() on a file descriptor for the directory is also needed. fdatasync() is similar to fsync(), but does not flush modified metadata unless that metadata is needed in order to allow a subsequent data retrieval to be correctly handled. For example, changes to st_atime or st_mtime (respectively, time of last access and time of last modi- fication; see stat(2)) do not require flushing because they are not necessary for a subsequent data read to be handled correctly. On the other hand, a change to the file size (st_size, as made by say ftruncate(2)), would require a metadata flush. The aim of fdatasync() is to reduce disk activity for applications that do not require all metadata to be synchronized with the disk. RETURN VALUE
On success, these system calls return zero. On error, -1 is returned, and errno is set appropriately. ERRORS
EBADF fd is not a valid open file descriptor. EIO An error occurred during synchronization. EROFS, EINVAL fd is bound to a special file which does not support synchronization. CONFORMING TO
On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRONIZED_IO is defined in <unistd.h> to a value greater than 0. (See also sysconf(3).) NOTES
On some UNIX systems (but not Linux), fd must be a writable file descriptor. In Linux 2.2 and earlier, fdatasync() is equivalent to fsync(), and so has no performance advantage. The fsync() implementations in older kernels and lesser used filesystems does not know how to flush disk caches. In these cases disk caches need to be disabled using hdparm(8) or sdparm(8) to guarantee safe operation. SEE ALSO
bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8), mount(8), sync(8), update(8) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at Linux 2012-02-27 FSYNC(2)

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SYNC(2) 						     Linux Programmer's Manual							   SYNC(2)

sync, syncfs - commit buffer cache to disk SYNOPSIS
#include <unistd.h> void sync(void); int syncfs(int fd); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): sync(): _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED syncfs(): _GNU_SOURCE DESCRIPTION
sync() causes all buffered modifications to file metadata and data to be written to the underlying file systems. syncfs() is like sync(), but synchronizes just the file system containing file referred to by the open file descriptor fd. RETURN VALUE
syncfs() returns 0 on success; on error, it returns -1 and sets errno to indicate the error. ERRORS
sync() is always successful. syncfs() can fail for at least the following reason: EBADF fd is not a valid file descriptor. VERSIONS
syncfs() first appeared in Linux 2.6.39; library support was added to glibc in version 2.14. CONFORMING TO
sync(): SVr4, 4.3BSD, POSIX.1-2001. syncfs() is Linux-specific. NOTES
Since glibc 2.2.2 the Linux prototype for sync() is as listed above, following the various standards. In libc4, libc5, and glibc up to 2.2.1 it was "int sync(void)", and sync() always returned 0. BUGS
According to the standard specification (e.g., POSIX.1-2001), sync() schedules the writes, but may return before the actual writing is done. However, since version 1.3.20 Linux does actually wait. (This still does not guarantee data integrity: modern disks have large caches.) SEE ALSO
bdflush(2), fdatasync(2), fsync(2), sync(8), update(8) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at Linux 2012-05-04 SYNC(2)
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