c - 对于每个静态和全局(未初始化/初始化变量)，数据段的内存分配未正确发生

Question

我试图了解内存分配如何适用于执行或编译的任何 C 代码。

我编写了以下 5 个独立的小代码，它们将帮助我理解内存分配过程。

1. segment_size1.c

   int main(){
       printf("hellow world");
       return 0;
   }
   

2. segment_size2.c

   //Adding uninitialized static int variable This would increase the size of BSS by 4 bytes.
   int main()
   {
       static int mystatic;
       printf("Hellow world");
       return 0;
   }
   

3. segment_size3.c

   // Adding initialized static int variable, this would increase the size of initialized data
   // segment by 4 bytes.
   
   int main()
   {
           static int mystatic;
           static int mystatic1 = 100;
           printf("Hellow world");
           return 0;
   }
   

4. segment_size4.c

   // Adding un-initialized global int variable, this would increase the size of BSS segment by 4 bytes.
   
   int myglobal;
   int main()
   {
           static int mystatic;
           static int mystatic1 = 100;
           printf("Hellow world");
           return 0;
   }
   

5. segment_size5.c

   // Adding initialized global int variable, this would increase the size of data segment by 4 bytes.
   
   int myglobal;
   int myglobal2 = 500;
   int main()
   {
       static int mystatic;
       static int mystatic1 = 100;
       printf("Hellow world");
       return 0;
   }
   

根据我的理解，如果我们编译上述文件，那么在编译segment_size2.c之后，BSS大小应该增加4个字节，在编译segment_size3.c之后，数据大小应该增加4个字节。但是当我点击 size 命令时，我得到了以下结果，

    size segment_size1 segment_size2 segment_size3 segment_size4 segment_size5
       text    data     bss     dec     hex filename
       1217     560       8    1785     6f9 segment_size1
       1217     560       8    1785     6f9 segment_size2
       1217     564      12    1793     701 segment_size3
       1217     564      12    1793     701 segment_size4
       1217     568      16    1801     709 segment_size5

这里我们可以清楚地看到，编译segment_size3后，bss和data段同时更新。这怎么可能？

Answer 1

We can clearly see that bss and data segment is simultaneously getting updated

这是保存初始值 != 0 的位置与静态/全局变量的保存位置之间的区别

在main执行之前，初始值!=0被复制到全局/静态变量中，初始值为0的全局/静态变量当然也被设置为0