fsync(2)

[Eugene Grosbein]

13.07.2020 2:05, Konstantin Belousov wrote:

> On Mon, Jul 13, 2020 at 01:49:22AM +0700, Eugene Grosbein wrote:
>> Hi!
>>
>> Assume we have parent process that created a file and keeps it open not writing anything there.
>> The parent spawns a child passing file name and the child opens it,
>> fills it with data and exits without fsync()'ing the file.
>>
>> In case of UFS there is upto 30 seconds time gap when file size is not updated,
>> so if crash occurs, the file ends up empty.
>>
>> The question: will fsync() in parent work for such still open file descriptor?
> 
> fsync() syncs the vnode, not the file or file descriptor. So fsync() on
> any file descriptor referencing the same vnode, is enough to ensure that
> the data is written to the underlying volume.

Thanks! This seems to be undocumented.

https://pubs.opengroup.org/onlinepubs/009695399/functions/fsync.html does not mention
implementation details (and could not) but our manual page could be improved, could it?