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Conversions from unsigned types with greater precision to unsigned types with lesser precision require only the upper bounds to be checked.

Noncompliant Code Example (time_t Return Value)

The time() function returns a (time_t)(-1) to indicate that the calendar time is not available. The C Standard requires only that the time_t type is an arithmetic type capable of representing time. It is left to the implementor to decide the best arithmetic type to use to represent time. If time_t is implemented as an unsigned integer type smaller than a signed int, the return value of time() will never compare equal to the integer literal -1.

#include <time.h>
 
void func(void) {
  time_t now = time(NULL);
  if (now != -1) {
    /* Continue processing */
  }
}

Compliant Solution (time_t Return Value)

To ensure the comparison is properly performed, the return value of time() should be compared against -1 cast to type time_t:

#include <time.h>
 
void func(void) {
  time_t now = time(NULL);
  if (now != (time_t)-1) {
    /* Continue processing */
  }
}

This solution is in accordance with INT18-C. Evaluate integer expressions in a larger size before comparing or assigning to that size.

Exceptions

INT31-EX1EX0: The C Standard defines minimum ranges for standard integer types. For example, the minimum range for an object of type unsigned short int is 0 to 65,535, whereas the minimum range for int is −32,767 to +32,767. Consequently, it is not always possible to represent all possible values of an unsigned short int as an int. However, on the IA-32 architecture, for example, the actual integer range is from −2,147,483,648 to +2,147,483,647, meaning that it is quite possible to represent all the values of an unsigned short int as an int for this architecture. As a result, it is not necessary to provide a test for this conversion on IA-32. It is not possible to make assumptions about conversions without knowing the precision of the underlying types. If these tests are not provided, assumptions concerning precision must be clearly documented, as the resulting code cannot be safely ported to a system where these assumptions are invalid. A good way to document these assumptions is to use static assertions. (See DCL03-C. Use a static assertion to test the value of a constant expression.)

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