how to update a text file in C?

Hi, 
I was wondering if you could update a text file in C in read/write
mode (r+)
I want to do something like this
void uppercase(char *fname) {
  FILE *fp;
  int c;
  fp=fopen(fname,"r+");
  if(fp==NULL) {
    printf("Unable to open file %s\n",fname);
    exit(1);
  }
  while((c=fgetc(fp))!=EOF)
    fputc(toupper(c),fp);
  fclose(fp);
}
(I know this will not work, but is there anyway I can achieve this
functionality of updating a text file in r+ mode. I DONT WANT to copy
the contents of the text file to a temp file and then process the temp
file, rename the temp file to fname and finally delete the original
file.
Please help me. 
Thanks,
Rajiv
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