I have a small program as shown below. This program is an attempt to better understand pointers in 'C', how variables are located in memory.
#include <stdio.h> const char *c = "hello"; const char **cp = &c; const char ***cpp = &cp; const char ****cppp = &cpp; int main() { printf("PTR (c) : %p \n",c); printf("PTR (cp) : %p \n",cp); printf("PTR (cpp) : %p \n",cpp); printf("PTR (cppp) : %p \n",cppp); printf("CONTENT (c) : %c \n",*c); printf("CONTENT (cp) : 0x%x \n",*(unsigned int*)cp); printf("CONTENT (cpp) : 0x%x \n",*(unsigned int*)cpp); printf("CONTENT (cppp): 0x%x \n",*(unsigned int*)cppp); return 0; }
The output that I get on my PC (Ubuntu 12.04) x86_64 is as follows
PTR (c) : 0x4006dc PTR (cp) : 0x601020 PTR (cpp) : 0x601028 PTR (cppp) : 0x601030 CONTENT (c) : h CONTENT (cp) : 0x4006dc CONTENT (cpp) : 0x601020 CONTENT (cppp): 0x601028
The output looks good because
- cp contains the address c
- cpp contains the address cp
- cppp contains the cpp address
Now I am running nm on the binary executable of the above program.
0000000000600e50 d _DYNAMIC 0000000000600fe8 d _GLOBAL_OFFSET_TABLE_ 00000000004006d8 R _IO_stdin_used w _Jv_RegisterClasses 0000000000600e30 d __CTOR_END__ 0000000000600e28 d __CTOR_LIST__ 0000000000600e40 D __DTOR_END__ 0000000000600e38 d __DTOR_LIST__ 0000000000400860 r __FRAME_END__ 0000000000600e48 d __JCR_END__ 0000000000600e48 d __JCR_LIST__ 0000000000601040 A __bss_start 0000000000601010 D __data_start 0000000000400690 t __do_global_ctors_aux 0000000000400460 t __do_global_dtors_aux 0000000000601018 D __dso_handle w __gmon_start__ 0000000000600e24 d __init_array_end 0000000000600e24 d __init_array_start 0000000000400680 T __libc_csu_fini 00000000004005f0 T __libc_csu_init U __libc_start_main@ @GLIBC_2.2.5 0000000000601040 A _edata 0000000000601050 A _end 00000000004006c8 T _fini 00000000004003c8 T _init 0000000000400410 T _start 0000000000601020 D c 000000000040043c t call_gmon_start 0000000000601040 b completed.6531 0000000000601028 D cp 0000000000601030 D cpp 0000000000601038 D cppp 0000000000601010 W data_start 0000000000601048 b dtor_idx.6533 00000000004004d0 t frame_dummy 00000000004004f4 T main U printf@ @GLIBC_2.2.5
Below I tried the corresponding lines.
0000000000601020 D c 0000000000601028 D cp 0000000000601030 D cpp 0000000000601038 D cppp
As far as I understand, the interpretation is as follows.
'c' is part of the data segment (denoted by "D") and is located at 0000000000601020. According to my program, this is the address of "cp". This applies to all variables.
I missed something important here. This is an attempt by beginners to understand C-pointers well.