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In C, I need to know the size of a struct, which has function pointers in it. Can I be guaranteed that on all platforms and architectures:
I assume the answer is yes to all of these, but I want to be sure. For context, I'm calling sizeof(struct mystruct) and nothing more.
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Usually it depends upon the word size of underlying processor for example for a 32 bit computer the pointer size can be 4 bytes for a 64 bit computer the pointer size can be 8 bytes. So for a specific architecture pointer size will be fixed. It is common to all data types like int *, float * etc.
A pointer is a variable whose value is the address of another variable or memory block, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable or block address.
Function pointers can be useful when you want to create callback mechanism, and need to pass address of a function to another function. They can also be useful when you want to store an array of functions, to call dynamically for example.
Function Pointer Syntax void (*foo)( int ); In this example, foo is a pointer to a function taking one argument, an integer, and that returns void. It's as if you're declaring a function called "*foo", which takes an int and returns void; now, if *foo is a function, then foo must be a pointer to a function.
From C99 spec, section 6.2.5, paragraph 27:
A pointer to void shall have the same representation and alignment requirements as a pointer to a character type. Similarly, pointers to qualified or unqualified versions of compatible types shall have the same representation and alignment requirements. All pointers to structure types shall have the same representation and alignment requirements as each other. All pointers to union types shall have the same representation and alignment requirements as each other. Pointers to other types need not have the same representation or alignment requirements.
So no; no guarantee that a void * can hold a function pointer.
And section 6.3.2.3, paragraph 8:
A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer.
implying that one function pointer type can hold any other function pointer value. Technically, that's not the same as guaranteeing that function-pointer types can't vary in size, merely that their values occupy the same range as each other.
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No, no, no.
C doesn't favour Harvard architectures with different code and data pointer sizes, because ideally when programming for such an architecture you want to store data in program memory (string literals and the like), and to do that you'd need object pointers into the code space. But it doesn't forbid them, so as far as the standard is concerned function pointers can refer to an address space which has a different size from the data address space.
However, any function pointer can be cast to another function pointer type[*] and back without trashing the value, in the same way that any object pointer can be cast to void* and back. So it would be rather surprising for function pointers to vary in size according to their signature. There's no obvious "use" for the extra space, if you have to be able to somehow store the same value in less space and then retrieve it when cast back.
[*] Thanks, schot.
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