C : Passing auto variable by reference

user_2705907 0
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I came across a piece of code which is working fine as of now,but in my opinion its undefined behavior and might introduce a bug in future.

Pseudo code :

void OpertateLoad(int load_id)
{
   int value = 0;

   /* code to calculate value */

   SetLoadRequest(load_id,&value);

   /*some processing  not involving value**/

}

void SetLoadRequest(int load_id, int* value)
{
   /**some processing**/
   LoadsArray[load_id] = *value; 
   /**some processing**/ 
}

In my understanding C compiler will not guarantee where Auto variables will be stored. It could be stack/register(if available and suitable for processing).

I am suspecting that if compiler decides to store value onto general purpose register then, SetLoadRequest function might refer to wrong data.

Am I getting it right?or I am overthinking it?

I am using IARARM compiler for ARM CORTEX M-4 processor.

----------:EDIT:----------

Answers Summarize that " Compiler will ensure that data is persisted between the calls, no matter where the variable is stored ".

Just want to confirm : Is this behavior also true 'if a function is returning the address of local auto variable and caller is de-referencing it?'.

If NO then is there anything in C standard which guarantees the behavior in both cases? Or As I stated earlier its undefined behavior?

cpu-registers c
4 Answers

You are overthinking it. The compiler knows that, if value is in a register, that it must be stored to memory before passing a pointer to that memory to SetLoadRequest.

More generally, don't think about stack and registers at all. The language says there's a variable (without saying how it's implemented), and that you can take its address and use that in another function to refer to the variable. So you can!

The language also says that local variables cease to exist when leaving a block, so this permission does not extend to returning pointers to local variables (which causes undefined behavior if the caller does anything at all with the pointer).

user_85862270

I am overthinking it?

Yes!

The compiler will take care of this. If it stores it in a register, it will know how to handle it (with a memory load).

user_24113200

To my understanding, the scope of value is the body of OpertateLoad. However, SetLoadRequest assigns the value pointed to, so the actual value of value is copied. No undefined behaviour is involved.

user_33554760

C11 draft standard n1570:

6.2.4 Storage durations of objects

6 For such an object that does not have a variable length array type, its lifetime extends from entry into the block with which it is associated until execution of that block ends in any way. (Entering an enclosed block or calling a function suspends, but does not end, execution of the current block.) If the block is entered recursively, a new instance of the object is created each time. The initial value of the object is indeterminate. If an initialization is specified for the object, it is performed each time the declaration or compound literal is reached in the execution of the block; otherwise, the value becomes indeterminate each time the declaration is reached.

user_31859680
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