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

NAME

     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():
         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 >= 500

DESCRIPTION

     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 (see inode(7)).

     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 modifica-
     tion; see inode(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 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.  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 where 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 a write(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 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  filesys-
     tems  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.