17th July 2017, 1 min read

C Pointer Surprises

An article by Krister Walfridsson on C pointers are not hardware pointers demonstrated that even adjacent integer variables having the same hardware address may compare unequal regarding C pointers.

See the following C program:

#include <stdio.h>

int main(int argc, char *argv[]) {
        int x, y;
        int *p = &x + 1;
        int *q = &y;
        printf("%p %p %d\n", (void*)p, (void*)q, p == q);
        return 0;
}

You have to compile with optimization enabled, e.g., cc -O3. Otherwise gcc adds some stuff between variables. On AMD/Intel/ARM CPUs the output looks something like this:

0xbe849afc 0xbe849afc 0

I.e., the pointers point to the same address, but the pointer comparison gives "false".

Added 06-Aug-2017: As hinted by the comment given by Ashwin Nanjappa below, the compiler actually does not generate compare instructions, but rather just adds 0=false.

Disassembling

$ cc -Wall -O3 -c ptrcomp.c
$ objdump -d ptrcomp.o

gives

ptrcomp.o:     file format elf64-x86-64

Disassembly of section .text.startup:

0000000000000000 <main>:
   0:   48 83 ec 18             sub    $0x18,%rsp
   4:   48 8d 3d 00 00 00 00    lea    0x0(%rip),%rdi        # b <main+0xb>
   b:   31 c9                   xor    %ecx,%ecx
   d:   48 8d 54 24 04          lea    0x4(%rsp),%rdx
  12:   64 48 8b 04 25 28 00    mov    %fs:0x28,%rax
  19:   00 00
  1b:   48 89 44 24 08          mov    %rax,0x8(%rsp)
  20:   31 c0                   xor    %eax,%eax
  22:   48 89 d6                mov    %rdx,%rsi
  25:   e8 00 00 00 00          callq  2a <main+0x2a>
  2a:   48 8b 4c 24 08          mov    0x8(%rsp),%rcx
  2f:   64 48 33 0c 25 28 00    xor    %fs:0x28,%rcx
  36:   00 00
  38:   75 07                   jne    41 <main+0x41>
  3a:   31 c0                   xor    %eax,%eax
  3c:   48 83 c4 18             add    $0x18,%rsp
  40:   c3                      retq
  41:   e8 00 00 00 00          callq  46 <main+0x46&gt;

Xoring oneself gives zero.