Return address of local variable in C

Question

Say I have the following two functions:

1

int * foo()
{
  int b=8;
  int * temp=&b;
  return temp;
}

2

int * foo()
{
   int b=8;
   return &b;
}

I don't get any warning for the first one (e.g. function returns address of a local variable) but I know this is illegal since b disappears from the stack and we are left with a pointer to undefined memory.

So when do I need to be careful about returning the address of a temporary value?

Answer 1

The reason you are not getting a warning in the first snippet is because you aren't (from the compiler's perspective) returning an address to a local variable.

You are returning the value of int * temp. Even though this variable might be (and in this example is) containing a value which is an address of a local variable, the compiler will not go up the code execution stack to see whether this is the case.

^{Note: Both of the snippets are equally bad, even though your compiler doesn't warn you about the former. Do not use this approach.}

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You should always be careful when returning addresses to local variables; as a rule, you could say that you never should.

static variables are a whole different case though, which is being discussed in this thread.