Double and triple pointers in C

问题

I have a small program as shown below.This program is an attempt to better understand pointers in 'C' how variables are arranged 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 since

cp contains the address of c
cpp contains the address of cp
cppp contains the address of cpp

Now I do an 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 have repasted the relevant lines of intrest.

0000000000601020 D c
0000000000601028 D cp
0000000000601030 D cpp
0000000000601038 D cppp

To the best of my understanding the interpretation is as follows.

'c' is a part of the data segment (denoted by 'D') and is at address 0000000000601020 According to my program that is the address of 'cp'.This is the case with all the variables.

Am I missing something vital here.It is a beginners attempt to understand C pointers well.

回答1:

'c' is a part of the data segment (denoted by 'D') and is at address 0000000000601020 According to my program that is the address of 'cp'.

Actually, when you printed with:

printf("PTR (cp)      : %p \n",cp);

It's the value of cp, not the address of cp. The value of cp is exactly the address of c, since cp is a pointer that points to c.

来源：https://stackoverflow.com/questions/22002248/double-and-triple-pointers-in-c

标签

pointers

memory-management

pointer-to-pointer