I know this isn't a Keil specific question, but I know you guys are smart and that I'll just get a direct non-mocking answer.
Is it legal to take the address of a function argument?
void Bar(unsigned char *Pointer) { // Access data byte via the pointer } int Foo(unsigned char Arg) { Bar(&Foo); // <<<<<< Is this legal? }
I know you cannot take the address of a variable of type register and I think that knowledge is clouding my memory.
The first function takes a pointer - so you already got the address of the buffer where you are expected to store the answer.
The second function takes an int - that is a local copy so you may take the address of this local variable Arg but can't return back a change.
But note that your code didn't take the address of an argument but instead tried to get the address of the function itself - and you shouldn't use any & to get the address of a function. The function name itself represents the pointer to that function.
The following will use a function pointer:
void Bar(int (*fnk)(unsigned char)) { fnk('A'); } int Foo(unsigned char Arg){ return Arg + 1; } void main() { Bar(Foo); while (1) ; }
And this code actually uses the address of a parameter:
void Bar(unsigned char *Pointer) { // Access data byte via the pointer *Pointer = *Pointer + 1; } int Foo(unsigned char Arg) { Bar(&Arg); // Let Bar() play with our Arg and modify it. return Arg; }
OMG. I feel even more like a noobie now.
I intended to write:
void Bar(unsigned char *Pointer) { // Access data byte via the pointer } int Foo(unsigned char Arg) { Bar(&Arg); // <<<<<< Is this legal? }
Fortunately you saw my stupidity and answered it with:
Thanks.
This is what a couple of years away from serious programming has done to me :(
Understand me correctly.
The function Bar() can modify the parameter Arg that the function Foo() did send.
But such a change fill not migrate back to whatever code that did call Foo(). Arg is a local variable inside Foo() and any changes to Foo() (inside Foo() or by the call to Bar()) will just happen to this local Arg. As soon as Foo() ends, the local parameter Arg will be thrown away.
So yes - you can send the address of parameters to other functions and have these functions change the value of your parameter. The function that takes the pointer doesn't care about if the pointer points to an argument, a local variable or a global variable or even a member of a struct.
So yes - you can send the address of parameters to other functions and have these functions change the value of your parameter.
The emboldened part is the key part for me.
Lifetime of variables is not something I'd forgotten about :)
Thanks for taking the time to answer. I always did like your clear complete responses.
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