How to understand this define

Question

Nowadays , i was reading the APUE.and i found the function defined as below:

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

i was confused, i know signal is pointer to a function and the last (int) is his parameter. i did not know what is (int signo,void (*func)(int)).

Answer 1

The general procedure: find the leftmost identifier and work your way out. Absent an explicit grouping with parentheses, postfix operators such as () and [] bind before unary operators like *; thus, the following are all true:

T *x[N]             -- x is an N-element array of pointer to T
T (*x)[N]           -- x is a pointer to an N-element array of T
T *f()              -- f is a function returning a pointer to T
T (*f)()            -- f is a pointer to a function returning T

Applying these rules to the declaration, it breaks down as

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

In short, signal returns a pointer to a function returning void. signal takes two parameters: an integer and a pointer to another function returning void.

You could use typedefs to make this easier to read (and the man page for signal on Ubuntu linux does just that); however, I think it's valuable to show the non-typedef'd version to demonstrate exactly how the syntax works. The typedef facility is wonderful, but you really need to understand how the underlying types work in order to use it effectively.

The signal function sets up a signal handler; the second argument is the function that is to be executed if a signal is received. A pointer to the current signal handler (if any) is returned.

For example, if you want your program to handle interrupt signals (such as from Ctrl-C):

static int g_interruptFlag = 0;

void interruptHandler(int sig)
{
  g_interruptFlag = 1;
}

int main(void)
{
  ...
  /**
   * Install the interrupt handler, saving the previous interrupt handler
   */
  void (*oldInterruptHandler)(int) = signal(SIGINT, interruptHandler);

  while (!g_interruptFlag)
  {
    // do something interesting until someone hits Ctrl-C
  }

  /**
   * Restore the previous interrupt handler (not necessary for this particular
   * example, but there may be cases where you want to swap out signal handlers
   * after handling a specific condition)
   */
  signal(SIGINT, oldInterruptHandler);
  return 0;
}

EDIT I extended the example code for signal to something that's hopefully more illustrative.

Answer 2

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

signal is function that takes int and a pointer to function taking int and returning void and returns a function pointer taking int and returning void. That is,

typedef void(*funcPtr)(int)

then we have

funcPtr signal(int signo, funcPtr func); //equivalent to the above

The syntax is indeed strange, and such things better be done with a typedef. As an example, if you want to declare a function that takes an int and returns a pointer to a function taking char and returning double will be

double (*f(int))(char);

Edit: after a comment that reads "Wooooooow", I am providing another example which is more "woooow" :)

Let's declare a function that takes
1. a pointer to array of 5 pointers to functions each taking float and returning double.
2. a pointer to array of 3 ponters to arrays of 4 ints
and returns a pointer to function that takes a pointer to function taking int and returning a pointer to function taking float and returning void and returns unsigned int.

The typedef solution would be this:

typedef double (*f1ptr) (float);
typedef f1ptr (*arr1ptr)[5];
typedef int (*arr2ptr)[4];
typedef arr2ptr (*arr3ptr)[3];
typedef void(*f2Ptr)(float);
typedef f2ptr (*f3ptr)(int);
typedef unsigned int (*f4ptr) (f3ptr);
f4ptr TheFunction(arr1ptr arg1, arr3ptr arg2);

Now, the funny part :) Without typedefs this will be:

 unsigned int (*TheFunction( double (*(*)[5])(float), int(*(*)[3])[4]))( void(*(*)(int))(float))

My god, did I just write that? :)