How do I read this complex declaration in C? [duplicate]

Answer 1

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

       signal(                  )         // signal is a function
              int, void (*)(int)          // the parameter types of the function:
                                          //    an int and a function pointer (take int, return void)
void (*                          )(int);  // the return type of the function:
                                          //    a function pointer (take int, return void)

// Edit referring to John's answer.

Answer 2

No that's right. signal takes 2 arguments, an int and a pointer to a function and returns a pointer to a function (with the same signature as the func argument.)

It's similar to the (imo) more readable:

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

Answer 3

Using cdecl.org, you get

declare signal as function (int, pointer to function (int) returning void) returning pointer to function (int) returning void

for the input

void (*signal(int, void(*)(int)))(int)

This means signal is a function. The result of of calling signal is pointer to a function void f(int).

Explanation: The signal() call installs a new signal handler and returns the old signal handler (so you can restore it later if you want to).

Answer 4

Start with the leftmost identifier and work your way out, remembering that [] and () bind before *, so *a[] is an array of pointers, (*a)[] is a pointer to an array, *f() is a function returning a pointer, and (*f)() is a pointer to a function:

       signal                                     -- signal
       signal(                          )         -- is a function
       signal(    sig,                  )         -- with a parameter named sig
       signal(int sig,                  )         --   of type int
       signal(int sig,        func      )         -- and a parameter named func
       signal(int sig,      (*func)     )         --   which is a pointer
       signal(int sig,      (*func)(   ))         --   to a function
       signal(int sig,      (*func)(int))         --     taking an int parameter
       signal(int sig, void (*func)(int))         --     and returning void
      *signal(int sig, void (*func)(int))         -- returning a pointer
     (*signal(int sig, void (*func)(int)))(   )   -- to a function
     (*signal(int sig, void (*func)(int)))(int)   --   taking an int parameter
void (*signal(int sig, void (*func)(int)))(int);  --   and returning void

signal associates a signal handler function func with a signal sig, and returns the pointer to the old signal handler function:

void new_interrupt_handler(int sig)
{
  ... // do something interesting with interrupt signal
}

int main(void)
{
  void (*old_interrupt_handler)(int);
  ...
  /**
   * Set up our new interrupt handler
   */
  old_interrupt_handler = signal(SIGINT, new_interrupt_handler);
  ...
  /**
   * Restore original interrupt handler
   */
  signal(SIGINT, old_interrupt_handler);
  ...
}

Answer 5

signal is a function which takes two parameters and returns a pointer to a function which takes an int as the parameter and returns void.

The two parameters that signal takes are an int and a pointer to a function which takes int as a parameter and returns void.

And yes, you got the description and the overall idea right.