How is LLVM heap value preserved after freeing?

Question

I am calling a C function from LLVM code, which allocates some space for a string like this: (char *) malloc(..len..);

This mallocated value in %0 then stored in stack variable %something.

define i8* @someFun() {
entry:
  %something = alloca i8*, align 8
  
  ; mallocing some space for a string
  %0 = call i8* @<function-using-malloc>()
  store i8* %0, i8** %something, align 8
  
  ; freeing heap space
  %1 = call i8* @free(i8* %0)

  %2 = load i8*, i8** %something, align 8
  ret i8* %2
}

After the store instruction, I am freeing %0 and then returning %something from the function.

Then after receiving the returned value from someFun(), the value is still available in main(), however I'm guessing it should have been freed?

define i32 @main() {
entry:
    ; printing value here correctly after free
    %0 = call i8* @someFun()
    ..printing %0..
    
    ret i32 0
}

I'm probably missing some memory management knowledge, so my question is what happens before and after freeing %0 in this case? Is this value gets copied from heap to stack?

Freed memory doesn't just 'go away'. It's still there, waiting to be reused. Accessing it, however, is undefined behaviour and the results are therefore unpredictable at best. — Paul Sanders, Dec 24 '20 at 12:10
@PaulSanders Oh wow, then it explains this behaviour, and it's good to know. Thank you! — Adam, Dec 24 '20 at 12:11

CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP · Answer 1 · 2020-12-24T17:46:41.210

1

I think you just need to read documentation about malloc/free. This question was asked million times in C threads I guess. Here is an example: C - What Happens To Memory After free()?

edited Dec 24 '20 at 17:46

answered Dec 24 '20 at 14:37

CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP

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Thanks, I wasn't thorough then! – Adam Dec 24 '20 at 14:45

How is LLVM heap value preserved after freeing?

1 Answers1