Saving code space by altering a function call in C - c

Question

I am calling a function that returns a variable through a pointer parameter. I do not care about the return value of this parameter nor do I want to make a dummy variable to pass to the function. For a simple example's sake, let's say the function is as follows and I don't care about, nor want to make a dummy variable for parameter "d".
void foo(int a, int b, int* c, int* d)
{
*c = a+b;
*d = a+b+*c;
}
I understand that a NULL pointer is in theory a pointer to a location that is not the address of any object or function. Would it be correct to pass NULL into "d" in this function if NULL was defined as the following? Or is this going to change whatever is at the 0'th element in memory?
#define NULL ((void *)0)
The target device is an MSP430 and I am using IAR C. No operating system is used therefore no memory management is implemented
EDIT: Please note that I do not want to create a dummy variable. Also if there was a way to fool the compiler into optimizing the "d" parameter out without altering the function definition, this is preferable.
EDIT#2: I would rather not use the & operator in the function call as it generates inefficient code that I do not want to generate
EDIT#3: For those who don't believe me when I am talking about the & operator... the compiler manual states "Avoid taking the address of local variables using the & operator. This is inefficient
for two main reasons. First, the variable must be placed in memory, and thus cannot be placed in a processor register. This results in larger and slower code. Second, the optimizer can no longer assume that the local variable is unaffected over function calls."

Answer 1

No, it is not correct.
The C standard does not define the behavior when you do this. On many systems, it will cause an error (some sort of memory fault) when foo attempts to store to address 0. If it does not, then you will have written data to address 0, presumably overwriting something else there that may have been needed, so your system may fail at a later time.

Answer 2

You should change your function a bit to allow passing NULL
void foo(int a, int b, int* c, int* d)
{
if(c != NULL)
{
*c = a+b;
if(d != NULL)
{
*d = a+b+*c;
}
}
}
Now you can safely pass NULL. Otherwise, as the other answers already state, you end up dereferencing a NULL pointer which results in undefined behavior.

Answer 3

In your example, if you don't care about the pointer d and you pass NULL as you defined then it'll probably crash due to dereferencing NULL.
You should pass a valid pointer even if you don't care about the result.
Why not just declare a temporary and pass?
int tempd;
foo(a,b,&c, &tempd);

Answer 4

There is no such thing as the 0th element in memory due to virtual memory. However, if you attempt this, your program will crash with a memory exception. I assume you want to ignore d if it's null so simply do this:
if(d != NULL)
{
*d = a+b+*c
}

Answer 5

Since you don't want to create a dummy variable and can't change the function you'll most likely end up scribbling at the memory position 0 on your device whatever that means. Maybe it's a memory mapped hardware register, maybe it's just normal physical memory.
If it's a register, maybe it doesn't have any effect unless you write the magical value 4711 into it which will happen once every three months and the device halts and catches fire. (has happened to me, it's fun to overwrite the boot eeprom on a device)
Or if it's memory maybe you'll send a NULL pointer to a different function later and that function will happily read the value that this function wrote there and you'll end up at 5 in the morning tearing your hair out and yelling "this can't possibly affect that!". (has happened to me on some ancient unix that used to map the NULL page)
Maybe your compiler adds a safety net for you. Maybe it doesn't. Maybe the next version will. Maybe the next hardware revision will come with memory unmapped at address 0 and the device will halt.
I'd create a dummy variable in the calling function and move on to a more interesting problem, but if you're a stress junkie, pass NULL and see what happens (today or in 10 years).

Answer 6

In that specific example code both passed in pointers are dereferenced. Dereferencing NULL is undefined behavior. Just go with the dummy variables.
In general: If a function accepts a pointer it should state if the null pointer is a valid value for the argument. If it doesn't say anything stay on the safe side and just assume it isn't. This will safe you a lot of grief.

Answer 7

Interesting question! Generally speaking, NULL is reserved as an "invalid" address. You shouldn't try to write to it, but I don't think the standard specifies what should happen if you do. On a Windows machine, this will generate an access violation exception. I don't know what will happen on your device.
In any case, passing NULL is the usual way to indicate that you're not interested in the value of an out parameter. But your function must be aware of this and act accordingly:
if( c ) {
*c = a+b;
if( d ) {
*d = a+b+*c;
}
}
Edit
If you can't change the function definition, then you're probably out of luck. You can trick the compiler into not passing d if the calling convention is cdecl. Just declare the function without the d parameter:
extern void foo( int a, int b, int * c );
However, you're definitely into dangerous shenanigans territory here. The function definition will still expect the d paramater, so it will see random garbage.
The only other thing I can think of is passing a fixed address. Since you're writing for a specific device, there might be an address range that's safe to write to. (That is, it won't cause exceptions or corrupt actual memory.)
void * SafeAddress = (void *)0x12345678;
foo( a, b, &c, SafeAddress );
Of course, the easiest thing is to just use the dummy variable. I know you've said more than once that this generates inefficient code, but does that have to be the case? Does it make a difference if the dummy is a local variable versus a global one?

Answer 8

The function tries to store value at the provided address. If the address is invalid, then there will be a malfunction of some sort -- whether you use an operating system or not is irrelevant.
Either you have to give the function some valid address (even if you don't care for the value in a particular case), or you have to change the function so that it does not store the value (and, probably, does not even even compute it), if the address for it is NULL (which may or may not be 0x0 on your platform, BTW).
You keep repeating, that you "don't want" to do the former and can not do the latter. Well, then you have an unsolvable dilemma. Maybe, there already exists some other address, where dummy values like this can be stored in your program (or on the platform) -- you can pass that.
If there is no OS involved, then you must be dealing with some funky programmable device, which means, there ought to be seasoned C-programmers around you. Ask them for confirmation of what you are told here -- clearly, you aren't trusting the answers given to you by several people already.

Answer 9

It will try to assign something at memory location 0x0, so I'd say it will crash

Answer 10

I was able to save code space without increasing memory usage on the stack by declaring a dummy variable as well as a pointer to the dummy variable.
int Dummy;
int* Dummy_ptr = &Dummy;
This allowed the compiler to make optimizations on the function call as the & operator was not used in the function call.
The call is now
foo(a, b, c_ptr, Dummy_ptr);
EDIT: For those of you who don't believe me.
I took a look at the assembler. The Dummy variable exists on the stack, though because it is not used later on, and because it is only a return from the function the address is never passed to the function and any use of that variable in the function is optimized out.