c - data/bss/heap 和 stack 中的内存分配

Question

我有以下代码:

#include <stdio.h>

int global_var;

int global_initialized_var=5;

void function(){
int stack_var;

printf("The function's stack_var is at address 0x%08x\n", &stack_var);
}


int main(){
int stack_var;
static int static_initialized_var = 5;
static int static_var;
int *heap_var_ptr;

heap_var_ptr = (int *) malloc(4);

// Next variables will be at data segment 

printf("global_initialized_var is at address 0x%08x\n", &global_initialized_var);
printf("static_initialized_var is at address 0x%08x\n\n", &static_initialized_var);

// These will be in the bss segment

printf("static_var is at address 0x%08x\n", &static_var);
printf("global_var is at address 0x%08x\n", &global_var);

// This will be in heap segment

printf("heap_var is at address 0x%08x\n\n", heap_var_ptr);

// These will be in stack segment 

printf("stack_var is at address 0x%08x\n", &stack_var);
function();
}

我得到以下信息:

# ./memory_segments 
global_initialized_var is at address 0x0804a018
static_initialized_var is at address 0x0804a01c

static_var is at address 0x0804a028
global_var is at address 0x0804a02c
heap_var is at address 0x09285008

stack_var is at address 0xbf809fbc
The function's stack_var is at address 0xbf809f8c

假设前 2 个变量因为它们被初始化为 static 和 global 应该在 .data 段中，而其他 2 个 static_var 和 global_var 应该在 .bss 段中。我认为我得到的地址暗示它们都在同一个内存区域中。我会盲目猜测，我会说这是 .bss 段。

无论如何，问题如下，我说得对吗？？如果我是，怎么可能找出这些区域(bss、数据等)的“限制”在哪里，或者它们从哪里开始等等。

Answer 1

假设您正在使用类似gcc memaddr.c -g -o memaddr 的东西进行编译，您可以使用objdump -h 来显示您的部分的大小和地址:

$ objdump -h memaddr | grep -e 'Size' -e '\.data' -e '\.bss'
Idx Name          Size      VMA               LMA               File off  Algn
 23 .data         00000018  0000000000601018  0000000000601018  00001018  2**3
 24 .bss          00000018  0000000000601030  0000000000601030  00001030  2**3
$

您还可以使用 objdump -t 来显示您的符号所属的地址和部分:

$ objdump -t memaddr | grep "_var"
000000000060102c l     O .data  0000000000000004              static_initialized_var.2049
0000000000601040 l     O .bss   0000000000000004              static_var.2050
0000000000601044 g     O .bss   0000000000000004              global_var
0000000000601028 g     O .data  0000000000000004              global_initialized_var
$

因此我们可以看到 .data 和 .bss 部分相当小并且恰好彼此相邻，因此 .data 和 .bss 地址如此接近也就不足为奇了。