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Upon looking at the ISO C11 standard for fgets §7.21.7.2, 3, the return value is stated regarding the synopsis code:
#include <stdio.h>
char *fgets(char* restrict s, int n, FILE* restrict stream);
The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned.
The standard says that a null pointer is returned for either an end-of-file and no characters have been read in or a read error occurs. My question is, just from fgets, and the returned null pointer, is there a way to distinguish which of the two cases caused the error?
634
The fgets() function returns a pointer to the string buffer if successful. A NULL return value indicates an error or an end-of-file condition. Use the feof() or ferror() functions to determine whether the NULL value indicates an error or the end of the file. In either case, the value of the string is unchanged.
Upon successful completion, fgets() returns s. If the stream is at end-of-file, the end-of-file indicator for the stream is set and fgets() returns a null pointer. If a read error occurs, the error indicator for the stream is set, fgets() returns a null pointer and sets errno to indicate the error.
Returned value If successful, fgets() returns a pointer to the string buffer. If unsuccessful, fgets() returns NULL to indicate failure. If n is less than or equal to 0, it indicates a domain error; errno is set to EDOM to indicate the cause of the failure. When n equals 1, it indicates a valid result.
fgets terminates at the newline character but appends it at the end of the string str . The function also appends the terminating null character at the end of the passed string.
Is there a way to distinguish which of the two cases caused the error?
Yes, use feof() and ferror() to distinguish. @Nothing Nothing
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.
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.
145
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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