{"id":4172,"date":"2022-05-23T08:30:00","date_gmt":"2022-05-23T06:30:00","guid":{"rendered":"https:\/\/msalamon.pl\/?p=4172"},"modified":"2025-12-27T15:46:02","modified_gmt":"2025-12-27T14:46:02","slug":"how-to-properly-construct-a-function-pointer-callback","status":"publish","type":"post","link":"https:\/\/msalamon.pl\/en\/how-to-properly-construct-a-function-pointer-callback\/","title":{"rendered":"How to Properly Construct a Function Pointer (Callback)?"},"content":{"rendered":"\n

I\u2019m coming today with an interesting topic. It will be a pointer to a function. Unlike \u201cregular\u201d pointers, they are a bit harder. Because functions define more things than just a regular variable.<\/strong><\/p>\n\n\n\n

Pointer to a function<\/h2>\n\n\n\n

Pointers can point to any memory cell<\/strong>, and our program code is nothing more than successive instructions in Flash memory. So you can point to it.<\/p>\n\n\n\n

The compiler in cooperation with the linker handles determining a function\u2019s address very well.<\/p>\n\n\n\n

A function has certain fixed elements. These are arguments and the returned result.<\/strong> We provide these things from the outside, but inside the function is always the same. After all, it resides in non-volatile memory \u2013 Flash.<\/p>\n\n\n\n

In reality, the function code always uses the arguments in the same way, in the same order, and always returns the result the same way. Only the data \u201cunderneath\u201d changes. The operations are always the same \ud83d\ude42<\/p>\n\n\n\n

So if we provide in code the address of a function together with the list of arguments it requires, then in fact we can point to any function. Having two functions with the same arguments means that we can call them \u201cinterchangeably\u201d.<\/p>\n\n\n\n

Function pointer structure<\/h2>\n\n\n\n

How to build a function pointer? I already mentioned to you that the compiler must know the entire required list of arguments and the \u201creturn value\u201d.<\/strong> This will have to be attached to the pointer.<\/p>\n\n\n\n

Another important element is informing the compiler that this is a pointer to a function.<\/strong> How? By adding the function call operator,<\/strong> that is, parentheses on the right side of the identifier.<\/p>\n\n\n\n

Let\u2019s take an example, the simplest function:<\/p>\n\n\n\n

void MyFunction(void) { \u2026 }<\/strong><\/p>\n\n\n\n

Never mind what it does.<\/p>\n\n\n\n

How to build a pointer to it? First, the name <\/strong>of the pointer and indicating that it will be a pointer<\/strong>.<\/p>\n\n\n\n

(*FunPtr)<\/strong><\/p>\n\n\n\n

Let\u2019s put it in parentheses right away so that operator binding order doesn\u2019t win out. We have a pointer named FunPtr. We still don\u2019t know what type or what it points to.<\/p>\n\n\n\n

It is to be a pointer to a function, so we must add the function call operator. In the same way we wrote the function.<\/p>\n\n\n\n

(*FunPtr)()<\/strong><\/p>\n\n\n\n

This is already a function pointer. Now the entire litany of arguments and the return type.<\/strong> The compiler MUST <\/strong>know this. It must know what EXACTLY <\/strong>the pointer points to. After all, we need to pass those arguments from the outside somehow.<\/p>\n\n\n\n

Our function took nothing and returned nothing, so it\u2019s simple.<\/p>\n\n\n\n

void (*FunPtr)(void)<\/strong><\/p>\n\n\n\n

In the arguments, you provide only their types. In the order that the function accepts them. Now we can assign the function to the pointer.<\/p>\n\n\n\n

The function name itself is converted to its address. That\u2019s what is used here. The pointer name is also an address. So we use just the names. The correctness of the remaining elements is checked by the compiler \u201cin the background\u201d.<\/p>\n\n\n\n

FunPtr = MyFunction;<\/strong><\/p>\n\n\n\n

From now on the FunPtr pointer points to our function. How to call the function via the pointer? <\/strong>With the function call operator \ud83d\ude42<\/p>\n\n\n\n

FunPtr();<\/strong><\/p>\n\n\n\n

If FunPtr is an address in memory, then the parentheses tell the compiler \u201ccall the function at this address\u201d.<\/p>\n\n\n\n

A more difficult pointer<\/h2>\n\n\n\n

Ok, that was trivial because we had no arguments. Let\u2019s do something crazy. Let\u2019s point to an example transfer function:<\/p>\n\n\n\n

uint8_t ReadFromI2C(uint8_t Address, uint8_t Register, uint8_t *Data, uint16_t Size){\u2026}<\/strong><\/p>\n\n\n\n

The function accepts in its arguments:<\/p>\n\n\n\n