Issue 0326: asctime()

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: The Austin Group, Stoughton (US)
Date: 2006-03-28
Reference document: AI-053.txt, DR 217
Submitted against: C99
Status: Fixed
Fixed in: C11
Cross-references: 0217
Converted from: summary-c99.htm, dr_326.htm

Summary: asctime() tm_year gt 9999

This is a potential defect forwarded from the Austin Group.

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.

Suggested Technical Corrigendum

Change 7.23.3.1 para 2 from:

The asctime function converts 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 as defined in <time.h>, and the calculated year does not exceed four digits:

(NB, see 7.23.1 para 4 for the specifications of the "normal ranges").

Also, add after the example code, and before the "Returns" section, the following new paragraph:

Otherwise, if any of the fields of the tm structure pointed to by timeptr contain values that are outside the normal ranges, the behavior of asctime() is undefined. If the calculated year exceeds four digits, the behavior is undefined.

Comment from WG14 on 2007-09-06:

Committee Discussion (for history only)

The proposed resolution invalidates code that strictly conforms to the C99 standard. Here is a contrived example (though there are some examples that are not contrived):

   #include <time.h>
   #include <stdio.h>

   struct tm tm;

   int
   main (void)
   {
     tm.tm_wday = 0;
     tm.tm_mon = 0;
     tm.tm_mday = -99;
     tm.tm_hour = 99;
     tm.tm_min = 99;
     tm.tm_sec = 99;
     tm.tm_year = -999 - 1900;
     printf ("%s\n", asctime (&tm));
     return 0;
   }

This code strictly conforms to C99, with well-defined behavior, and some implementations prints "Sun Jan-99 99:99:99 -999". The proposed resolution places extra constraints on asctime's arguments that would cause the above code to have undefined behavior.

The original interpretation request considered by the Austin Group contained an additional requirement, that the calculated year should not precede the Epoch (the date and time associated with (time_t)0). This restriction was removed in forwarding this to the C committee, since there is no C equivalent concept. However, if the calculated year is less than 1000, problems may occur, so perhaps the wording should be:

If the calculated year is less than 1000 or greater than 9999, the behavior is undefined.

Note: This appears to be a duplicate of DR 217, which advises no consensus / no change.

It was also pointed out that the Proposed Technical Corrigendum does not fix all of the issues, such as if tm_mon=4 and tm_mday=31, both valid numbers, but not a valid date.

Technical Corrigendum

Add after the example code, and before the "Returns" section, the following new paragraph:

If any of the fields of the tm structure pointed to by timeptr contain values that are outside the normal ranges*, the behavior of asctime() is undefined. If the calculated year exceeds four digits, or is less than the year 1000, the behavior is undefined.

Add footnote *:

See 7.23.1 para 4 for the specifications of the "normal ranges".