When to use casting in C

Question

I have a statement in C code which I suspect may be giving me periodic errors, so want to make sure I am doing the right thing as it mixes types. Objective is to change timebase from 1/32768 seconds to 1/1024, with all times 32 bit integers.

What I have is this:

ts_sys = latest_timestamp * VELO_TICKS_FROM_RTC;

Where ts_sys and latest_timestamp are both unsigned 32 bit integers. VELO_TICKS_FROM_RTC is a define as follows:

#define VELO_TICKS_PER_SECOND         1024   
#define VELO_TICKS_FROM_RTC        (VELO_TICKS_PER_SECOND / 32768.0f)

Should I be using a cast here to make sure the division doesn't return an integer (which would be zero) and therefore return the wrong thing? For example would this be better:

ts_sys = (uint32_t) ((float)latest_timestamp * VELO_TICKS_FROM_RTC);

but that seems like overkill..

Answer 1

"Should I be using a cast here to make sure the division doesn't return an integer?"

No, 1024 is of type int and latest_timestamp is of type uint32_t. Both get converted to float in the arithmetic expressions before the respective calculation is done:

Otherwise, if the corresponding real type of either operand is float, the other operand is converted, without change of type domain, to a type whose corresponding real type is float.

C18, §6.3.1.8/1; "Usual arithmetic conversions"

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"but that seems like an overkill..."

It is.

Answer 2

Should I be using a cast here to make sure the division doesn't return an integer (which would be zero) and therefore return the wrong thing?

no, you are doing A/B, and B is a float, so the compiler promotes A to float and the result is a float!