What's the meaning of this piece of code? void (*signal(int sig, void (*func)(int)))(int);

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).

Answer 2

This is one of the classical examples of how convoluted C declarations can become.
To understand this declaration, it usually helps to introduce a typedef:

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

The typedef declares a pointer to a function (taking an int parameter and returning nothing). The function signal can now be seen as a function that takes two parameters (an int and a pointer to a function) and returns a pointer to a function.

This can also be derived from the original declaration, but it takes a bit of practice. The usual way is to start at the identifier that names the outermost entity (signal is this case):

signal is a ...

Then you read right until you find an unmatched closing parenthesis or the end of the declaration: void (*signal(int sig, void (*func)(int))(int)

signal is a function taking ... returning ...

Now you can choose between parsing the parameters first, or the return value first. I will do the return value first. For that, you read backwards to find the matching open parenthesis: void (signal( / ... */ ))(int)

`signal is a function taking ... returning a pointer to ...

Reading back and forth this way you get at successive stages:

`signal is a function taking ... returning a pointer to a (function taking ... returning ...)

`signal is a function taking ... returning a pointer to a (function taking ... returning void)

`signal is a function taking ... returning a pointer to a (function taking an int and returning void)

`signal is a function taking two parameters: (an int) and (a pointer to a function taking an int and returning void), and returning a pointer to a (function taking an int and returning void)