badbasser98

Sorry to revive this thread, but I have a question that I think should be included in this thread.

The client utility (mentioned in first post) that the users will have writes to a single file. With 90+ users it is inevitable that multiple people could be trying to write (add service call) to the file at the same time. Before I deploy this program, I would like to add a function that will check the .DAT file to see if it is currently being edited before the actual write process begins.

I would greatly appreciate any info that can be provided on the creation of this function. Links, tutorials, etc.

Thanks,
-BB98

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The Dark

How are you opening the file at the moment in the utility?
If you are using ofstreams, you could add a sharing parameter to the open call or to the constructor call. _SH_DENYWR will stop others from opening the file for write at the same time. You would then just have to handle the case where the utility couldn't open the file, e.g. by waiting for a bit and then retrying.
Note: I haven't tried this, so it could be all wrong.

badbasser98

I'm just using fopen, fwrite, and fclose. Should I be using a different method?

Thanks,
-BB98

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Zap

Some programs won't write to files properly if another program already has a file open. Forger told me that.

Prm753

Could you just write a function with fopen() to see if the .dat file is open before you allow a user to write to it, or do I misunderstand you?

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The Dark

The fopen function doesn't have an option for file locking. What you can do, without having to rewrite your existing code, is to use the sopen() function with a third parameter of _SH_DENYWR, this will give you a file descriptor and then use fdopen() to convert this file descriptor to a FILE pointer.

badbasser98

The client does not keep the file open, it is opened for read only when the client is first started to verify the file exists and is within the size limit. The next time the file is opened is when they submit the entry, then it is opened for appending. I just want to prevent having multiple people trying to write to the file at the same time.

I think The Dark's solution will work, I just need to finish editing the code. The only question I have so far is, do I have to turn off the write lock? Or will that happen when the file is closed?

Thanks for the help.
-BB98

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The Dark

It will happen when the file is closed.

badbasser98

I got this to work as I wanted by initiating the lock immediatly after the program writes to the file for a short period of time. I noticed through trial and error that this was the vulnerable time when data loss happened during very close consecutive writes. I have tested this method quite extensively and have not experienced any data loss thus far with the lock in place.

Please let me know if this might present other problems I am not aware of.

Thanks for the help The Dark.

-BB98

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