function names do function arguments

void (*FUNC) (int) know what it means to define this function?

First explain void (*FUNC) (int):
void func ()->void func (int)->void *func (int)->void (*func) (int)
Can you see the diagram above? Func is a function pointer whose return type is void, and the function it points to receives an int type parameter. If it is written as void *func (int) It becomes: Func is a function whose return type is a pointer of type void and accepts an int parameter.

So void (*signal (int sinno,void (*FUNC) (int)))) (int) means:
Signal is a function pointer whose return type is void, which receives an argument of type int, but this pointer is the return value of another function, which receives 2 parameters, the first is int, the second one has been explained.

function names do function arguments