C file operation open file in a+ mode

Without knowing what YFILE and yfopen etc. actually do, it is just guesswork.
But, I can't help thinking that just from the names pYFILE should be a YFILE* rather than a YFILE

Don't think that will fix your problem, but it looks like you have some explaining to do...
Use the "Improve question" widget to edit your question and provide better information.

FILE *pFILE = fopen("c:\\TEST.txt", "a+");
ASSERT(0 != feof(pFILE));

feof[^] "The feof function returns a nonzero value after the first read operation that attempts to read past the end of the file. It returns 0 if the current position is not end of file."

You haven't done a read operation, so feof will return zero.

The code mearly cleans up after opening the file for read and write mode (append if the file exist).

Thus:
1) open the file for reading and writing append mode.
2) Check whether EOF... Looks like some code missing. ALSO: are you sure its yfeof
and not feof
3) close the stream.
4) Nullify pointer.

I would use the ASSERT in a while loop otherwise this code seems pointless.
E.g.

while ( ASSERT(0 != feof(pYFILE)) )
{
count = fread( buffer, sizeof( char ), 100, stream );
total += count;
}

MSDN states:
"a+"
Opens for reading and appending; the appending operation includes the removal of the EOF marker before new data is written to the file and the EOF marker is restored after writing is complete

So the assertion behavior is normal.

If you don't want EOF to be removed, then use "a" instead of "a+".