C language pointer reference learning

For example

 1 #include<stdio.h>
 2 int test_num;
 3 void func(int *p)
 4 {
 5     p = &test_num;
 6 }
 7 int main(void)
 8 {
 9     int *p;
10     func(p);
11     *p = 1000;
12     return 0;
13 }

Run out of core

Disassembly:

gcc -S t3.c -o t3.s

    .file    "t3.c"
    .comm    test_num,4,4
    .text
    .globl    func
    .type    func, @function
func:
.LFB0:
    .cfi_startproc
    pushq    %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    movq    %rdi, -24(%rbp)
    movq    $test_num, -8(%rbp)
    popq    %rbp
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
.LFE0:
    .size    func, .-func
    .globl    main
    .type    main, @function
main:
.LFB1:
    .cfi_startproc
    pushq    %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    subq    $16, %rsp
    movq    -8(%rbp), %rax
    movq    %rax, %rdi
    call    func
    movq    -8(%rbp), %rax
    movl    $1000, (%rax)
    movl    $0, %eax
    leave
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
.LFE1:
    .size    main, .-main
    .ident    "GCC: (Ubuntu 4.8.4-2ubuntu1~14.04.4) 4.8.4"
    .section    .note.GNU-stack,"",@progbits

Before the main function call func:

    movq - 8 (RBP%),% rax 
    movl $ 1000 (% rax)

An obvious mistake, the contents of ebp 4 bytes of content moving to the currently esp pointed, in fact, made a copy of * p, the use for the func function;
after calling the function func:
　　movq $ test_num, - 8 (% rbp)
so, func function operation only * p copy, the copy store the first address of a.
After ret back to the main function, a variable is assigned the value 100, main function in the * p, is not known, because only a copy of it to know.

Change Source:

 1 #include<stdio.h>
 2 int test_num;
 3 void func(int **p)
 4 {
 5     *p = &test_num;
 6 }
 7 int main(void)
 8 {
 9     int *p;
10     func(&p);
11     *p = 1000;
12     return 0;
13 }

Disassembly process:

 1     .file    "t3.c"
 2     .comm    test_num,4,4
 3     .text
 4     .globl    func
 5     .type    func, @function
 6 func:
 7 .LFB0:
 8     .cfi_startproc
 9     pushq    %rbp
10     .cfi_def_cfa_offset 16
11     .cfi_offset 6, -16
12     movq    %rsp, %rbp
13     .cfi_def_cfa_register 6
14     movq    %rdi, -8(%rbp)
15     movq    -8(%rbp), %rax
16     movq    $test_num, (%rax)
17     popq    %rbp
18     .cfi_def_cfa 7, 8
19     ret
20     .cfi_endproc
21 .LFE0:
22     .size    func, .-func
23     .globl    main
24     .type    main, @function
25 main:
26 .LFB1:
27     .cfi_startproc
28     pushq    %rbp
29     .cfi_def_cfa_offset 16 
30      .cfi_offset 6 - 16 
31      movq% RSP,% RBP
 32      .cfi_def_cfa_register 6 
33      subq $ 16 ,% RSP
 34      leaq - 8 (RBP%),% see a
 35      movq% seen fit,% RDI
 36      call func
 37      movq - 8 (RBP%),% see a
 38      movl $ 1000 , (% see a)
 39      movl $ 0 ,% eax
40     leave
41     .cfi_def_cfa 7, 8
42     ret
43     .cfi_endproc
44 .LFE1:
45     .size    main, .-main
46     .ident    "GCC: (Ubuntu 4.8.4-2ubuntu1~14.04.4) 4.8.4"
47     .section    .note.GNU-stack,"",@progbits

We can see there is a change

In other words, as a copy function func for the use of the contents of ebp is not the next four bytes, but the content in the address pointed to (ie mian function * p address).

C language pointer reference learning

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