Question in structures (C langauge) ?

[SphinX]

Salam,

While i am reading about Text Interfaces i faces this structure

typedef  struct    _selection
{
       char   prompt[MAX_PROMPT_LEN];
       int      (*function)(int);
       int      fn_arg;
} SELECTION

What should this line do ?

int      (*function)(int);

Thanks

 

[Bingo600]

Thats a pointer to a function that takes an int as argument.

I would suppose it points to the routine that should handle that specific selection entry.

/Bingo

 

[eltonjohn]

having pointers to functions is very useful in OS ..where you can run code dynamicaly youd create an array of pointer of functions and call them all with a loop ..Elegant and very small code

 

[boeysue]

you can do many function for that function pointer for many options.
Just like C++ function overloading.

 

[mik27]

Is a pointer to a function ... look this example:

//Your typedef struct
typedef  struct    _selection 
{ 
       char   prompt[MAX_PROMPT_LEN]; 
       int      (*function)(int); 
       int      fn_arg; 
} SELECTION;

SELECTION mySel;


//various func with same return value and passing parameters 
// like "pointer to a function"

int foo1(int a)
{
    ...
    ...
}

int foo2(int a)
{
    ...
    ...
}

int foo3(int a)
{
    ...
    ...
}


// two functions that make the same thing

void Func(int val)
{

    for(int i=0; i<1000; i++) {
        switch(val) {
            case 1:
                foo1(val);
                break;
            case 2:
                foo2(val);
                break;
            case 3:
                foo3(val);
                break;
         }
    }
}

void BetterFunc(int val)
{
    switch(val) {
        case 1:
            mySel.function = foo1;
            break;
        case 2:
            mySel.function = foo2;
            break;
        case 3:
            mySel.function = foo3;
            break;
     }
     
    for(int i=0; i<1000; i++) 
        mySel.function(val);
}

"Func" e "BetterFunc" functions make the same thing.
In a "Func" function I repeat for 1000 times an "conditional jump" (switch), and then I call my appropriate function (foo1, foo2, foo3) ...
In a "BetterFunc" function I use the "pointer to a function" ... thus I can transport the "conditional jump" (switch) outside the loop, executing it only one time ... and I repeat for 1000 times ONLY the call to my appropriate function (foo1, foo2, foo3) via "pointer to a funcion" (mySel.function)

Mik