Can't understand the output of this code stack storage function call c

Question

I had the following code, however I don't understand what and why it outputs what it does.

int main(){
   int *i;
   int *fun();
   i=fun();
   printf("%d\n",*i);
   printf("%d\n",*i);
}

int *fun(){ 
   int k=12;
   return(&k);
}

The output is 12 and a garbage value. Can somebody explain the output?

Shouldn't it return garbage values both times?

I know that k is local to fun(), so it would be stored on a stack and that it would be destroyed when fun() goes out of scope. What concept am I missing here?

Kiril Kirov · Accepted Answer · 2012-08-12 19:58:23Z

5

Wouldn't it return garbage values both of the time?

After the return of fun, k does not exist anymore, so printing the value, stored in the address of k is undefined behaviour.

That's why you have different/garbage value.

k is local to fun(), so it would be stored on a stack and that activation would be destroyed when the fun ends, or am I missing some concept?

You're not missing anything, except the fact, that the stack isn't immediately "annulled", or something like this. In other words, after the return of fun, the compiler's free to do whatever it wants with this memory.

edited Aug 12, 2012 at 19:58

answered Aug 12, 2012 at 19:52

Kiril Kirov

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3 Comments

ishan soni Over a year ago

but everytime i run i get 12,garbage value.it return 12 for the first printf everytime.i don't get that.i even compiled and ran it on TC,it gives the same answer 12 for the first printf and then a garbage value?

Kiril Kirov Over a year ago

@ishansoni - that's what undefined behaviour is - you really don't know what happens, who else changes this memory, what could happen when you try to change it, or if you dereference the pointer i.

ishan soni Over a year ago

i read somewhere that in TC garbage collector is called after a printf statement so maybe thats why.

shf301 · Accepted Answer · 2012-08-12 19:55:25Z

4

The stack isn't immediately cleared when a function returns, so the 12 will still be on the stack after fun() returns - until something else overwrites it.

You'll see different results in different compilers and different build options (debug vs. release).

answered Aug 12, 2012 at 19:55

shf301

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