Is fgets() returning NULL with a short buffer compliant?

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The behavior is different in newer releases of glibc, for n == 1, it returns s which indicates success, this is not an unreasonable reading of 7.19.7.2 The fgets function paragraph 2 which says (it is the same in both C99 and C11, emphasis mine):

char *fgets(char * restrict s, int n, FILE * restrict stream);

The fgets function reads at most one less than the number of characters specified by n
from the stream pointed to by stream into the array pointed to by s. No additional
characters are read after a new-line character (which is retained) or after end-of-file. A null character is written immediately after the last character read into the array.

Not terribly useful but does not violate anything said in the standard, it will read at most 0 characters and null-terminate. So the results you are seeing looks like a bug that was fixed in later releases of glibc. It also clearly not an end of file nor a read error as covered in paragraph 3:

[…]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.

As far as the final case where n == 0 this looks like it is simply undefined behavior. The draft C99 standard section 4. Conformance paragraph 2 says (emphasis mine):

If a ‘‘shall’’ or ‘‘shall not’’ requirement that appears outside of a constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words ‘‘undefined behavior’’ or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe ‘‘behavior that is undefined’’.

The wording is the same in C11. It is impossible to read at most -1 characters and it is neither an end of file nor a read error. So we have no explicit definition of the behavior in this case. Looks like a defect but I cannot find any defect reports that cover this.

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