Conditional operator with a constant (true) value?

Question

I was looking at some preprocessor macros used in OpenSSL, and I came across the following from crypto/stack/safestack.h:

#define CHECKED_STACK_OF(t         


        

Answer 1

It's a static assertion about the function type before the cast, providing a type-safe cast.

From C11 (n1570) 6.5.15 (from the constraints section)

Conditional operator

(3) One of the following shall hold for the second and third operands:

• [omitted non-pointer stuff]
• both operands are pointers to qualified or unqualified versions of compatible types;
• one operand is a pointer and the other is a null pointer constant; or
• one operand is a pointer to an object type and the other is a pointer to a qualified or unqualified version of void.

The third operand is a pointer to a function (so the last bullet never applies), so this compiles (without a warning) only if either p is a null pointer constant or of a type compatible with void (*)(type) for the last macro (after conversion to a function pointer, if p is a function designator).