MSC05-A. Do not manipulate time_t typed values directly

time_t is specified as an "arithmetic type capable of representing times." However, how time is encoded within this arithmetic type is unspecified. Because the encoding is unspecified, there is no safe way to manually perform arithmetic on the type, and, as a result, the values should not be modified directly.

Non-Compliant Code Example

This non-compliant code example attempts to execute do_some_work() multiple times until at least seconds_to_work has passed. However, because the encoding is not defined, there is no guarantee that adding start to seconds_to_work will result in adding seconds_to_work seconds.

int do_work(int seconds_to_work) {
  time_t start = time(NULL);

  if (start == (time_t)(-1)) {
    /* Handle error */
  }
  while (time(NULL) < start + seconds_to_work) {
    /* ... */
  }
}

Compliant Solution

This compliant solution uses difftime() to determine the difference between two time_t values. difftime() returns the number of seconds from the second parameter until the first parameter and returns the result as a double.

int do_work(int seconds_to_work) {
  time_t start = time(NULL);
  time_t current = start;

  if (start == (time_t)(-1)) {
    /* Handle error */
  }
  while (difftime(current, start) < seconds_to_work) {
    current = time(NULL);
    if (current == (time_t)(-1)) {
       /* Handle error */
    }
    /* ... */
  }
}

Note that this loop may still not exit, because the range of time_t may not be able to represent two times seconds_to_work apart.

Risk Assessment

Using time_t incorrectly can lead to broken logic that could place a program in an infinite loop or cause an expected logic branch to not actually execute.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[Kettlewell 02]] Section 4.1, "time_t"
[[ISO/IEC 9899:1999]] Section 7.23, "Date and time <time.h>"

MSC04-A. Use comments consistently and in a readable fashion      13. Miscellaneous (MSC)       MSC06-A. Be aware of compiler optimization when dealing with sensitive data