The following is my code:

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
   char buf[128];
   memset(buf, 0, sizeof(buf));

   FILE *fp = fopen("prepatch.log", "wb+");
   fwrite(buf, 1, sizeof(buf), fp);
   fseek(fp, 0, SEEK_SET);

#if 1
   fread(buf, 1, 8, fp);
#else
   fseek(fp, 8, SEEK_CUR);
#endif

   fwrite("test", 1, 4, fp);
   fclose(fp);
   return 0;
}

I cannot write "test" to the file if I read it.
But fseek does correct.

I find that 'FILE::_flag' is not set correctly.
So _flush does not flush the buffer.

VC2003 and VC2005 is same.