Typedef function and is it useful? [duplicate]

Question

I'm studying in-depths of languages, so I can understand what's happening in code, rather than print some things and watch and see what happens.

Recently, in search for better implementation for class function table I found myself stumbled upon this C language standard: http://www.iso-9899.info/wiki/Typedef_Function_Type

I've tried it out and it seems working feature:

typedef void fnr(int x);

main()
{
    fnr t;
}

This seemed a glorious day for me searching the way to pack up functions into my structure, until I realized, that fnr t; is not as useful as I had intended. It can neither be assigned, nor used the proper way I wished it to be (probably lambda for C-users). It does not even exist in disassembly!

What does this language feature do? What can it be used for besides simplifying function pointers?

Answer 1

You've declared fnr as a function type. While a function type cannot be assigned to, a pointer to a function type can. For example:

typedef void fnr(int x);

void f(int x)
{
    printf("x=%d\n", x);
}

int main()
{
    fnr *t = f;
    t(1);
}

You could also define the typedef as a function pointer:

typedef void (*fnr)(int x);
...
fnr t = f;

Using a typedef for a function pointer is most useful when a function pointer is either passed to or returned from a function. As an example, let's look at the signal function which does both:

  typedef void (*sighandler_t)(int);

  sighandler_t signal(int signum, sighandler_t handler);

The second parameter to this function is a pointer to a signal handling function, and it also returns a pointer to a signal handling function. Without the typedef, it would look like this:

void (*signal(int signum, void (*handler)(int)))(int)