c - 一个程序为静态对象、自动对象和动态分配的对象使用不同的内存区域

Question

我正在阅读“C Primer Plus”这本书，遇到了理解内存区域的问题。书中写道:

Typically, a program uses different regions of memory for static objects, automatic objects, and dynamically allocated objects. Listing 12.15 illustrates this point.

// where.c -- where's the memory?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int static_store = 30;
const char * pcg = "String Literal";
int main(void)
{
    int auto_store = 40;
    char auto_string[] = "Auto char Array";
    int *pi;
    char *pcl;

    pi = (int *) malloc(sizeof(int));
    *pi = 35;
    pcl = (char *) malloc(strlen("Dynamic String") + 1);
    strcpy(pcl, "Dynamic String");

    printf("static_store: %d at %p\n", static_store, &static_store);
    printf("  auto_store: %d at %p\n", auto_store, &auto_store);
    printf("         *pi: %d at %p\n", *pi, pi);
    printf("  %s at %p\n", pcg, pcg);
    printf(" %s at %p\n", auto_string, auto_string);
    printf("  %s at %p\n", pcl, pcl);
    printf("   %s at %p\n", "Quoted String", "Quoted String");
    free(pi);
    free(pcl);

    return 0;
}

运行代码得到:

static_store: 30 at 0x10a621040
  auto_store: 40 at 0x7ffee55df768
         *pi: 35 at 0x7fbf1d402ac0
  String Literal at 0x10a620f00
 Auto char Array at 0x7ffee55df770
  Dynamic String at 0x7fbf1d402ad0
   Quoted String at 0x10a620f9b

本书的结论:

As you can see, static data, including string literals occupies one region, automatic data a second region, and dynamically allocated data a third region (often called a memory heap or free store).

我可以弄清楚它们的地址不同。我怎么能保证他们是不同地区的呢？

Answer 1

不同的地区有非常不同的地址。如果他们在同一个地区，他们就会有相似的地址。更好的例子，我们在每个区域分配 2 个对象:

#include <stdio.h>
#include <stdlib.h>

int main (void)
{
  int stack1;
  int stack2;
  static int bss1;
  static int bss2;
  static int data1=1;
  static int data2=1;
  int* heap1 = malloc(1);
  int* heap2 = malloc(1);  
  char* rodata1 = "hello";
  char* rodata2 = "world";

  printf(".stack\t%p %p\n",  &stack1,  &stack2);
  printf(".bss\t%p %p\n",    &bss1,    &bss2);
  printf(".data\t%p %p\n",   &data1,   &data2);
  printf(".heap\t%p %p\n",   heap1,    heap2);
  printf(".rodata\t%p %p\n", rodata1,  rodata2);

  free(heap1);
  free(heap2);
}

输出(例如):

.stack  000000000022FE2C 000000000022FE28
.bss    0000000000407030 0000000000407034
.data   0000000000403010 0000000000403014
.heap   0000000000477C50 0000000000477C70
.rodata 0000000000404000 0000000000404006

如您所见，同一段中的两个变量具有几乎相同的地址，唯一的区别是对象的大小(可能还有一些对齐空间)。虽然与其他段中的变量相比，它们的地址非常不同。