fsync is a POSIX 1003.1b-1993 standard function which
flushes the write buffers associated with an open file
descriptor (filehandle). This node describes primarily the fsync found in the GNU C library, glibc, but most of the information may be applicable to other C libraries as well.
fsync's prototype (found in unistd.h) is:
int fsync(int fildes)
where fildes is the file descriptor whose write buffers are to be
flushed. fsync does not return until all data associated with the
file is written to the appropriate device; when it does, it
returns 0 on success, and returns -1 on error, as well as setting
- EBADF - fildes is not a valid file
- EINVAL - this function is unimplemented by the
Note that this function is a cancellation point for
multi-threaded programs; protect calls to fsync with
cancellation handlers if necessary.
When it is not necessary to flush metadata associated with the
file descriptor in question (for instance if the file is of a
fixed size), it is possible to use fdatasync
instead of fsync, which may be faster; fdatasync will be
emulated by a call to fsync if necessary. See also the function
sync, which does the same thing except flushing write buffers for the whole system, rather than just one descriptor.
(Most information paraphrased from the GNU C Library Reference
Manual, which is under the GNU Free Documentation