In your particular case, both brightness and maxval have a type smaller than int so they are promoted to int with the same value and the multiplication produces an int value. This requires a good understanding of the integer promotion and conversion rules, which vary from one language to another and are somewhat tricky in C, especially with operands mixing signed and unsigned types. It is the programmer's responsibility to ascertain that the range of the operands ensures that the multiplication does not overflow. In Python and some other languages, this is not an issue because integers do not have a restricted range, but in C, C++, java, javascript and many other languages, integer types have a fixed number of bits so the multiplication can exceed this range. The expression (brightness * maxval) / 100 computes an intermediary value brightness * maxval that may exceed the range of the type used to compute it. Your question raises an important issue in C programming and in programming in general: does the program behave as expected in all cases?
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