[SOLVED] Which C specifiers may cause ABI incompatibility between the caller and callee?

Which C specifiers may cause ABI incompatibility between the caller and callee?

Suppose there is a file with a declaration of a function which differs from the actual definition in another file i.e.

FileA.c

short foo();

int main(){
   foo();
}

FileB.c

int foo(){
   //Some code
}

This is obviously wrong code but it illustrates what the topic is.

The problem is that a compiler will not produce a machine code that will reserve enough space for the return value of foo when it's called. I think this can be called ABI incompatibility.

There are multiple different specifiers one may apply to a function or variable i.e. type qualifiers, storage class specifiers, atomics etc. I wonder which of these could potentially cause any kind of incompatibilities between a caller and a callee. Consider the case that these files might be compiled to separate binaries but still call the function via a pointer to an address.

Solution

I wonder which of these could potentially cause any kind of incompatibilities between a caller and a callee"

Simply assume any difference risks incompatibility.

Some difference are OK, yet those are often implementation defined. There is little value in exploiting a difference that is compatibility.