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Assuming that the precision of  signed char is 7 bits, and the precision of unsigned char is 8 bits, this operation is perfectly safe.  However, if the compiler represents the signed and unsigned char types using 31 and 32 bit precision (respectively), the variable uc would need be converted to unsigned int instead of signed int.  As a result of the usual arithmetic conversions, the signed int would then be converted to unsigned and the addition would take place between the two unsigned int values. Also, because uc is equal to UCHAR_MAX which is equal to UINT_MAX in this example, the addition will result in an overflow. The resulting value is then zero-extended to fit into the 64-bit storage allocated by sll.

Addition

Non-compliant Code Example

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The following compliant solution tests the suspect addition operation to guarantee there is no possibility of unsigned overflow. In this particular case, an overflow condition is present and the error_handler() method is invoked.

unsigned int sum;
unsigned int ui1 = UINT_MAX;
unsigned int ui2 = 1;

if (~ui1 < ui2){
  error_handler("Overflow Error", NULL, EOVERFLOW);
}
sum = ui1 + ui2;