CODEWITHSUNDEEP
cxcodepointers
pcbtt
pcbtt

Reputation: 53

XCode C compiler gives different result about integer pointer to const integer value, why?

In XCode I've tried to manipulate const int value by using pointer. Here is the code:

const int con = 5;

int *p;
p = &con;

(*p) +=1;

printf("Add of constant:%p\n",&con);
printf("Add of pointer:%p\n",p);

printf("%d - %d",con,*p);

Result is like that on XCode:

Add of constant:0x7fff5fbff79c
Add of pointer:0x7fff5fbff79c
5 - 6

but on linux virtual machine values of con and *p is same 6.

Why there is a difference between values on XCode?

Upvotes: 1

Views: 83

Answers (1)

nnovich-OK
nnovich-OK

Reputation: 3183

Tried this with VisualStudio, get the same result as with XCode. Assembly listing proved @Gerhardh point:

  • (*p) +=1; successfully increases value of con in RAM
  • printf("Add of constant:%p\n",&con); prints address of this memory correctly
  • printf("%d - %d",con,*p); doesn't read con value from RAM, but passes 5 directly into printf. Optimization threw away unnecessary read for value known at compile time. Here is related assembly listing
printf("%d - %d",con,*p);

mov         eax,dword ptr [p]                   //get p
mov         ecx,dword ptr [eax]                 //get *p
push        ecx                                 //push *p (3rd param)
push        5                                   //push 5 (2nd param). No read of con
push        offset string "%d - %d" (415800h)   //push addr of format string (1st param)
call        dword ptr [__imp__printf (4182BCh)] //call printf()

Obviously, compiler on your VM didn't perform the same optimization.

Upvotes: 1

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