Re: recommended programming practices for writing (was Linux 2.6.29)

[Bernd Petrovitsch <bernd@firmix.at>]

On Thu, 2009-03-26 at 15:03 -0400, Adam Turk wrote:
> I have been reading the Linux 2.6.29 thread with interest.  I have
> written several (10 or so) C programs that write large amounts of data
> (between 1 and 2 GB file sizes are common).  A snippet of code looks
> like this:
> 
> if((fptr = fopen(outfilename,"w")) == NULL) {
>   printf("File %s could not be created\n", outfilename);
> }
> else {
>   fprintf(fptr,"%s\n",datablock);
>   while(!writeouput(datablock,amount,tax)) {
>     getnext(dtablock)
>     fprintf(fptr,"%s\n",datablock);
>   }
>   fclose(fptr);
> }
> 
> I learned C about 15 years ago and there was no mention of a fsync.
> My C book doesn't mention fsync either.  Granted I have written only

Probably because fsync() is a system call (and not a standard C lib
function).

[...]
> From what Linus posted about git and checking the return from fclose I
> think I going to start doing that.  I also think I am going to start
> checking the return from fprintf and maybe write to a /tmp/file and
> then rename it.

For a really robust app it's probably not wrong. At least one gets an
error (e.g. "disk full") immediately and not only at fclose() time after
(trying to) write 2GB data.

> So is there a C fsync that I should add before my fclose?

"fflush(fptr);" flushes all of the buffers (managed by the C-lib) of
fptr and also delivers an error.
fsync() is to flush the in-kernel cached pages of that file.

> What is the proper way to write to files?

Basically just as you do above.

	Bernd
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