sizeof(struct foo) not constant?

Question

Im developing with Microchips XC8 C compiler 1.12.

I have the following code snippet but the compiler is not happy with it.

struct _foo {
    int a;
};

enum BAR {
    CONST1 = sizeof(struct _foo)
};

The error I get is: main.c:6: error: integer expression required Why does it generate that error? Isn't the size of any struct in C constant? If I use sizeof(int) everything works fine.

Note: the XC8 compiler v1.12 follows the C90 standard, not the modern C99 standard.

PS This does compile:

char abc[sizeof(struct _foo)];

And this expression also needs to be a compile-time constant leading me to think that this actually isn't what's going wrong in the enum declaration.

Answer 1

This looks like a limitation / bug of the Microchip XC8 compiler.

In a enumeration constant definition, the compiler does not consider sizeof (X) as constant expression (even if actually it is in c90 or c99) when X is either an aggregate or union type, or an object of an aggregate type or union type.