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


        

Answer 1

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)