andy5995 andy5995 - 1 year ago 64
C Question

Is using type unsigned char acceptable when only a small number is needed?

If writing a program, and I need a number that is lower than 255, is it ok to use type unsigned char for use as a number, such as if I need a number to make a loop? Is there any reason, including sticking to tradition and convention, to stick to declaring numbers with int and float, etc.

For example, should code like this never be used? Or would it be good practice to use an unsigned char in this case, since it allocates less memory than a short int?

#include <stdio.h>

typedef unsigned char short_loop;

int main(int argc, char *argv[])
short_loop i;

for (i = 0; i < 138; i++)
printf ("StackOverflow Example Code\n");

return 0;

I'm asking for future reference, and only using the code above to help illustrate.

Answer Source

I wouldn't do it.

If your program is working with large arrays of values, then there is a benefit in terms of memory usage of using an array of char rather than an array of int.

But, for the control variable of a single loop, there is unlikely to be much benefit, and potentially a performance hit, for several reasons.

Comparing i < 138 will promote i to int before doing the comparison, since 138 is of type int. Promotions (and down-conversions) also potentially occur with initialising and incrementing i as well.

int is typically the "native type" on the host machine - which normally means it is a type that is preferred on the host (e.g. hardware provides registers that are optimised for performance when operating on that type). Which, even if some technique is used to prevent promotion of unsigned char to int before doing the comparisons in the loop, operations on int may be more effective anyway.

So, in the end, your approach might or might not give a performance benefit. The only way to be sure would be to profile the code .... and the benefits (or otherwise) would be compiler dependent (e.g. affected by optimisation approaches) and host dependent (e.g. how efficient are operations on unsigned char in comparison with operations on int)

Your approach also makes the code harder to understand, and therefore harder to get right. And, if some other programmer (or you) modify the code in future, any benefit may be negated .... for example, by accidentally reintroducing unintended type conversions.

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