Having issues with a .dat file

I am opening a Courses.dat file and need to read specific bytes on each line. For some reason, after opening the file, I get a segmentation fault on ANYTHING I do to it. I have tried opening with "r" and "rb", but nothing works. This might not be enough code to determine the error but I am just wondering what the possible cause could be for a seg fault immediately after opening a file.

I have checked that the file exists, the file name is correct, I can open and close the file successfully, but as soon as I do anything with it, it fails.

Code:

    if((datP = fopen(fname, "r")) == NULL) {              /*open .dat file*/
      fprintf(stderr,"Error opening .dat file.\n");   exit(1);
    }
  fseek(datP,0L,SEEK_END);
  fileLen = ftell(datP);
  fseek(datP,0L,SEEK_SET);
  printf("fileLen: %d",fileLen);

Perhaps the error lies before this code snippet.

You should post a complete program that has the segfault if you want help. We basically have to debug to find the problem.

As usual, I spend 30min trying to figure out whats going wrong. Then I post something and find the error 5 minutes later. It turns out the error was about 15 lines after this code. But since it wasnt printing out any of the output in between it was throwing me off.