Interpretations

Answering questions that may arise related to the meaning of portions of an IEEE standard concerning specific applications.

IEEE Standards Interpretation for IEEE Std 1003.1™-2001 IEEE Standard Standard for Information Technology -- Portable Operating System Interface (POSIX®)

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Interpretation Request #53
Topic: asctime() tm_year gt 9999 Relevant Sections: XSH asctime() Page: 123 Line: 4407-4428

If asctime() is called with a tm structure whose tm_year field results in a year > 9999 (which is possible with 64-bit time_t), the current specification of asctime() would result in asctime() to overrunning a 26-character buffer; the specification says the sprintf() format for printing the year is "%d", and (eg) a 5-digit number would print 5 characters, overrunning the buffer.

Similarly, since the user can create the input struct tm, it is possible for the user to set the fields of the struct tm to values that are outside the normal bounds. In such a case, the sprintf() format given in the asctime() specification can result in a buffer overrun. For example, if tm_hour is 100, the sprintf() format ".2d" writes the string "100", which could result in a buffer overrun. The specification should be updated to state the algorithm can be used as long as the values of the tm struct are restricted to the normal bounds.

(Note TC2 already says asctime() can return NULL if unsuccessful.)

11 Solution proposed by the submitter (optional):

Action:
page 123 line 4487-4488 section asctime() objection
Change: The asctime() function shall convert the broken-down time in the structure pointed to by timeptr into a string in the form to The asctime() function shall convert the broken-down time in the structure pointed to by timeptr into a string in the form, provided the broken-down time in the fields of the structure pointed to by timeptr contain values that are within the normal ranges (see ), and the calculated year does not exceed four digits:

page 123 line 4508 section asctime() objection
After line 4508, add:
Otherwise, if any of the fields of the tm structure pointed to by timeptr contain values that are outside the normal ranges, asctime() behavior is undefined. If the calculated year exceeds four digits, asctime() shall either return NULL and set errno to EOVERFLOW, or write no more than four unspecified characters to the year field.

page 123 line 4526 section asctime() objection
Change: No errors are defined to The asctime() and asctime_r() functions may fail if: [EOVERFLOW] The result cannot be represented.

Interpretation Response #53
The standards states the requirements for asctime(), and conforming implementations must conform to this. However, concerns have been raised about this which are being referred to the sponsor."

Rationale for Interpretation
None.