Nested function pointer declarations

Question

I found this declaration in the APUE book in the chapter on signals:

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

I don't fully understand the syntax. The declaration of (*func) is syntactically what I expect for a function parameter/pointer. I don't understand the syntax where "signal" is declared, and the trailing (int). I was wondering if someone could clarify this. -Thanks

`cdecl` can help you. Unfortunately finding the correct way to feed the type into `cdecl` is tricky — phuclv, Jul 30 '22 at 02:21

score 1 · Accepted Answer · edited Jul 30 '22 at 02:30

1

The function signal returns a pointer to a function of type void (int), and has two arguments: an int and a pointer to a function of type void (int).

So something like this should work

void foo(int);

void (*signal(int signo, void (*func)(int)))(int)
{
    return func;
}

int main(int argc, char *argv[])
{
    signal(42, foo);

    return 0;
}

edited Jul 30 '22 at 02:30

Jonathan Leffler

730,956
141
904
1,278

answered Jul 29 '22 at 20:59

beothunder

551
2
14

H.S. · Answer 2 · 2022-07-30T03:22:20.207

Use spiral rule (reading inside out clockwise) to understand the syntax:

Find the identifier

 void (*signal(int signo, void (*func)(int)))(int);
        ^    ^
        |    |
        +----+
          |
       Identifier name

signal is a ....

Move right

 void (*signal(int signo, void (*func)(int)))(int);
              ^                            ^
              |                            |
              +----------------------------+
                             |
                         arguments (so, its a function)

signal is a function which accepts int and void (*) (int) [pointer to a function which accepts a int argument and returns nothing (void)] as arguments ....

Can't move right anymore because of the right parenthesis, so move left.

 void (*signal(int signo, void (*func)(int)))(int);
       ^
       |
    pointer

signal is a function which accepts int and void (*) (int) as arguments and returns a pointer ....

Can't move left anymore because of the left parenthesis, so keep going right.

 void (*signal(int signo, void (*func)(int)))(int);
                                             ^   ^
                                             |   |
                                             +---+
                                               |
                                            argument 
                   (which means signal() is returning a function pointer)

signal is a function which accepts int and void (*) (int) as arguments and returns a pointer to a function which accepts a int argument and ....

Can't move right anymore because we're out of symbols, so go left.

 void (*signal(int signo, void (*func)(int)))(int);
 ^  ^
 |  |
 +--+
   |
 return type of function whose pointer returned by signal() function

signal is a function which accepts int and void (*) (int) as arguments and returns a pointer to function which accepts a int argument and returns nothing (void).

Nested function pointer declarations

2 Answers2