C language from BaseBand Processor Eng perspective Part-6 Function Pointers

Function Pointers

In modem code, depending on the parameters different function needs to be called.

Ex

For SMS , CALL , USSD , INTERNET , LOCATION  different function needs to be called.

We can't have one to one mapping from up to down for these functionality s.

So when at UI your press any one of the options.  There will a common function whose work will be to post or call respective functions.

We use function pointers mostly in those cases.


Ex , Check the below two codes

typedef enum{
call =0,
sms,
ussd,
location
}request_type;



switch(request_type)

case (call) :
   call_fucntion();
case(sms):
   sms_function();
case(ussd)
  ussd_function();


The other way is

typedef void (*func_dummy)(void);

func_dummy functions[4];

functions[call] = call_function;
fucntions[sms] = sms_function;

Now rather than having switch statement with lot of cases, we can have simply functions pointer , in which index will be request.

so  functions(request_type)  will call the correct functions. These also helps in debugging, since rather than checking
1) enums
2) switch statement, also break after each case
3) whether correct function is mapped or not
4) having user readable names of cases an function names



In case of function pointers we have to worry only about 3 part.


There are mostly two ways of defining  function pointer

typedef void(*func_dummy)(void);

  func_dummy functions[8];

typedef void(func_dummy)(void)

   func_dummy* functions[8]; // Note we have use pointer here

Since
  func_dummy functions[8];  // This will throw error saying function array can't be declared.

Not sure why , but I think this is due to  function is always accessible by address.

Ex :

int a;

here  a  and &a  are different, one is value and other is  address of a

but in case of function

void hello()
{
}

hello  or &hello    both are same ,  implicitly hello is same as &hello.

So when you declare array of functions, they is no way of assigning values to it . So we have to always define  array of function pointers.

.