Question

I came across this piece of code and completely got lost interpreting its meaning.

#include <signal.h>
void (*signal(int sig, void (*func)(int)))(int);

What is a detailed explanation for the code at line 2?

I know that void and int are types, the *func is a pointer for a function, and the brackets are for priority. But I still don’t get the (*signal …), the (int), and the whole thing combined together. The more detailed, the better.

Probably I’ve known the meaning/effect of this declaration. But I had to make some more trials to help me understand what’s going on, as below:

  1 #include <signal.h>
  2 void (*signal)(int sig, void (*func)(int));
  3 void (*signal)(int);  // then void (signal)(int) again.
  4 //void (*signal(int sig, void (*func)(int)))(int); //break this line into two lines above
  5
  6 int main(){}

In the above code, I broke void (*signal(int sig, void (*func)(int)))(int) into two lines. For line 3, I tried both void (*signal)(int) and void (signal)(int), with the same error result that indicated that I was trying to redeclare signal:

TestDeclaration.c:2: error: ‘signal’ redeclared as different kind of symbol
/usr/include/signal.h:93: error: previous declaration of ‘signal’ was here
TestDeclaration.c:3: error: ‘signal’ redeclared as different kind of symbol
/usr/include/signal.h:93: error: previous declaration of ‘signal’ was here

Now I know both the trials are incorrect ways of declaration, but why are they incorrect? Why is the original way of declaration NOT a redeclaration?

Answer 1

It’s the declaration of a function taking an int and a pointer to a function (taking int returning void) and returning a pointer to a function (taking int and returning void).

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Explanation, or guide to interpretation

You can interpret by treating everything in parentheses as a single entity and then working inwards using the "declaration follows usage" rule.

void (*signal(int sig, void (*func)(int)))(int);

The entity in the brackets looks like a function taking int and returning void.

Stripping away the outer part:

*signal(int sig, void (*func)(int))

So, signal takes some parameters and returns something that can be dereferenced (due to the leading *) to form a function taking int and returning void.

This means signal is a function returning a pointer to a function (taking int and returning void).

Looking at the parameters it takes an int (i.e. sig) and void (*func)(int) which is a pointer to a function (taking int and returning void).