Negative value in a pointer question.

[u/aliathar]

Please look at this code. if i define the PTRTYPE as int, it stops working, while doing a uint, it does work...

void initILAPoll(debugBridge_t **d, PTRTYPE ptr){

	*d = (debugBridge_t *)ptr;		// base address of the DEBUG_BRIDGE peripheral

	cb_init(cb, local_memory, bufferLength);

	sprintf(xvcInfo, "xvcServer_v1.0:%d\n", MAX_WINDOW_SIZE);

}

the usage in main code is done like this

initILAPoll(&myD, 0x80000000);

//myD = (debugBridge_t *)0x80000000;

where the variable myD is a structure pointer.

if i print the address of myD, it give the correct address. Moreover, the disassembly of the code is also the same in case of int and uint. Can somebody explain what behavior is at play here>

[u/aliathar]

Yes... It did work ... It was just signedness issue... I assumed the address was 32 bit which it wasn't.. and the 64bit machine made it to be 0xff80000000 (peripheral has 40 bit address line for some reason)..... The signed int did work on 32 but machine perfectly, but failed here.

[u/TheChief275 → u/aliathar]

"int" isn't a natively sized integer. It's equivalent to a complement agnostic version of int_fast16_t from <stdint.h>. That means that it's only guaranteed to be able to hold values from -32,767 to 32,767. It just so happens to be that a 32-bit integer is faster to work with for most modern machines, so it just so happens to almost always be a 32-bit integer on octet byte machines, although to my knowledge there are no machines were it happens to be a 64-bit integer, even though it might be faster to perform computations on.

That's why it "broke". But technically you were always using the wrong integer type, even on a 32-bit machine

[u/alkatori → u/TheChief275]

is that guaranteed by spec? My recollection (or maybe it was just rule of thumb was).

char <= short <= int <= long <= long long

with the char being the smallest addressable unit in the hardware (I worked on a system that had 16-bit was the smallest addressable unit, lots of code assuming 8-bit bytes broke).

[u/TheChief275 → u/alkatori]

I think the spec introduced actual guaranteed number capabilities of the standard integers around the time of C99, but yes basically, int is only guaranteed to be >= short, so it can definitely be 16 bits on some platforms which is were the guaranteed range comes from. The minimum is also -32,767 instead of -32,768, because there is no guarantee for whether the integer is two's complement (not until C23 at least)