What is guaranteed about the size of a function pointer?

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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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