I need to write an IEEE single-precision floating point number to a 32-bit hardware register at a particular address. To do that, I need to convert a variable of type float to an unsigned integer. I can get the integer representation like this:
float a = 2.39;
unsigned int *target;
printf("a = %f\n",a);
target = &a;
printf("target = %08X\n",*target);
which returns:
a = 2.390000
target = 4018F5C3
All good. However this causes a compiler warning "cast.c:12: warning: assignment from incompatible pointer type"
Is there any other way to do this which doesn't generate the warning? This is for specific hardware, I don't need to handle different endianness etc and I don't want to loop through each char for performance reasons as some other questions tend to suggest. It seems like you might be able to use reinterpret_cast in C++ but I am using C.
EDIT: The union solution works everywhere I have tried it but somewhere on SO I had been pointed at standards that showed it didnt have to work. See the link below in the comments to find a LOT more info on this (Thank you Daniel!). Supposed to work or not supposed to work I would use it with care, I imagine endianness, etc gets involved as well (doubles broken into bytes, etc).
Another solution is a dummy asm function. For example on arm:
some disassembly is required, needs to be on a system that doesnt have an fpu and/or uses gprs for carrying around floats.
memcpy as already mentioned is the cleanest and most reliable and portable solution (be aware of endianness).
You could creat a
union
type that contains a float and an unsigned int, store a value into the float member, then read it out of the int, like so:If you're trying to simply display the integral value of the
float
as it's stored in memory, then try using a union:Store the float value:
Print both float and integer values:
No compiler warnings. I use GNU compiler (gcc) on Linux. Notice that
target
is not a pointer; this is the beauty (and hideousness) of unions. ;-)You can use type punning with a union,
to get the bits. Or you can use
memcpy
.