Assigning constant values to long variables

Although type identification of constants follows explicit rules in C and C++, many programs use hexadecimal or unsuffixed constants as "typeless" variables and rely on a twos complement representation to truncate values that exceed the limits permitted on a 32-bit system.

As these large values are likely to be extended into a 64-bit long type in 64-bit mode, unexpected results can occur, generally at the following boundary areas:
  • constant > UINT_MAX
  • constant < INT_MIN
  • constant > INT_MAX
Some examples of unexpected boundary side effects are listed in the following table.
Table 1. Unexpected boundary results of constants assigned to long types
Constant assigned to long Equivalent value 32-bit mode 64-bit mode
–2,147,483,649 INT_MIN–1 +2,147,483,647 –2,147,483,649
+2,147,483,648 INT_MAX+1 –2,147,483,648 +2,147,483,648
+4,294,967,726 UINT_MAX+1 0 +4,294,967,296
0xFFFFFFFF UINT_MAX –1 +4,294,967,295
0x100000000 UINT_MAX+1 0 +4,294,967,296
0xFFFFFFFFFFFFFFFF ULONG_MAX –1 –1

Unsuffixed constants can lead to type ambiguities that can affect other parts of your program, such as when the results of sizeof operations are assigned to variables. For example, in 32-bit mode, the compiler types a number like 4294967295 (UINT_MAX) as an unsigned long and sizeof returns 4 bytes. In 64-bit mode, this same number becomes a signed long and sizeof returns 8 bytes. Similar problems occur when the compiler passes constants directly to functions.

You can avoid these problems by using the suffixes L (for long constants), UL (for unsigned long constants), LL (for long long constants), or ULL (for unsigned long long constants) to explicitly type all constants that have the potential of affecting assignment or expression evaluation in other parts of your program. In the example cited in the preceding paragraph, suffixing the number as 4294967295U forces the compiler to always recognize the constant as an unsigned int in 32-bit or 64-bit mode. These suffixes can also be applied to hexadecimal constants.