Is it possible to distinguish the error returned by fgets

Is there a way to distinguish which of the two cases caused the error?

Yes, use feof() and ferror() to distinguish. @Nothing Nothing

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Yet it is important to use correctly. Consider the two codes:

char buf[100];
fgets(s, sizeof s, stream);
if (feof(stream)) return "End-of-file occurred";
if (ferror(stream)) return "Input error occurred"; 


if (fgets(s, sizeof s, stream) == NULL) {
  if (feof(stream)) return "End-of-file occurred";
  if (ferror(stream)) return "Input error occurred"; 
  return "Should never get here";
}

The second properly tests the return value against NULL, as suggested by OP.

The first can encounter a rare problem. The ferror(stream) tests a flag. This flag may have been set by a prior I/O function call on stream so this fgets() is not necessarily the cause of the error. Best to check the result of fgets() to see if this function failed.

If code is to continue using stream after an error detected, be sure to clear the error before continuing - like maybe to attempt a re-try.

if (ferror(stream)) {   
  clearerr(stream);
  return "Input error occurred");
}

Note that clearerr() clears both the error and end-of-file flags.

The same applies for feof(), yet most code is written to quit using stream once an end-of-file is true.

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There is a 3rd pathological way to receive NULL and neither feof() nor ferror() returns NULL as detailed in Is fgets() returning NULL with a short buffer compliant?. Careful reading of the C spec has 3 "ifs", of which it is possible that not of them are true as so the spec is lacking - which implies UB.

If the failure has been caused by end-of-file condition, additionally sets the eof indicator (see feof()) on stream. The contents of the array pointed to by str are not altered in this case. If the failure has been caused by some other error, sets the error indicator (see ferror()) on stream. The contents of the array pointed to by str are indeterminate (it may not even be null-terminated).

Therefore, you would need to check for feof() and ferror() in order to determine the error.

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