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FSYNC(2)                   Linux Programmer's ManualFSYNC(2)NAME       fsync,  fdatasync - synchronize a file's in-core state with storage de-       viceSYNOPSIS       #include <unistd.h>       int fsync(int fd);       int fdatasync(int fd);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):       fsync():           Glibc 2.16 and later:               No feature test macros need be defined           Glibc up to and including 2.15:               _BSD_SOURCE || _XOPEN_SOURCE                   || /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L       fdatasync():           _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500DESCRIPTION       fsync() transfers ("flushes") all modified in-core data of (i.e., modi-       fied  buffer cache pages for) the file referred to by the file descrip-       tor fd to the disk device (or other permanent storage device)  so  that       all  changed information can be retrieved even if the system crashes or       is rebooted.  This includes writing through or flushing a disk cache if       present.   The  call  blocks until the device reports that the transfer       has completed.       As well as flushing the file data, fsync() also  flushes  the  metadata       information associated with the file (seeinode(7)).       Calling  fsync()  does not necessarily ensure that the entry in the di-       rectory containing the file has also reached disk.   For  that  an  ex-       plicit 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  re-       trieval  to  be correctly handled.  For example, changes to st_atime or       st_mtime (respectively, time of last access and time of last  modifica-       tion; seeinode(7)) do not require flushing because they are not neces-       sary for a subsequent data read to be handled correctly.  On the  other       hand, a change to the file size (st_size, as made by sayftruncate(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.  This error may relate              to  data written to some other file descriptor on the same file.              Since Linux 4.13, errors from write-back will be reported to all              file  descriptors  that  might have written the data which trig-              gered the error.  Some filesystems (e.g., NFS) keep close  track              of  which data came through which file descriptor, and give more              precise reporting.  Other filesystems (e.g., most local filesys-              tems)  will report errors to all file descriptors that were open              on the file when the error was recorded.       ENOSPC Disk space was exhausted while synchronizing.       EROFS, EINVAL              fd is bound to a special file (e.g., a pipe,  FIFO,  or  socket)              which does not support synchronization.       ENOSPC, EDQUOT              fd  is  bound  to a file on NFS or another filesystem which does              not allocate space at the time of awrite(2)  system  call,  and              some previous write failed due to insufficient storage space.CONFORMING TO       POSIX.1-2001, POSIX.1-2008, 4.3BSD.AVAILABILITY       On  POSIX  systems  on  which fdatasync() is available, _POSIX_SYNCHRO-       NIZED_IO is defined in <unistd.h> to a value greater than 0.  (See alsosysconf(3).)NOTES       On  some  UNIX  systems (but not Linux), fd must be a writable file de-       scriptor.       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 filesys-       tems do not know how to flush disk caches.  In these cases disk  caches       need  to be disabled usinghdparm(8) orsdparm(8) to guarantee safe op-       eration.SEE ALSOsync(1),bdflush(2),open(2),posix_fadvise(2),pwritev(2),sync(2),sync_file_range(2),fflush(3),fileno(3),hdparm(8),mount(8)COLOPHON       This  page  is  part of release 5.05 of the Linux man-pages project.  A       description of the project, information about reporting bugs,  and  the       latest     version     of     this    page,    can    be    found    at       https://www.kernel.org/doc/man-pages/.Linux                             2019-03-06FSYNC(2)