elf加载程序如何初始化全局变量 [英] How do global variables get initialized by the elf loader

问题描述

对于C中的全局变量，例如

For global variables in C like

int aglobal = 5;

int aglobal = 5;

5何时转移装入程序将其放入aglobal中，以及如何知道将5放入aglobal中。

When does the 5 get transferred into aglobal by the loader and how does it know to put 5 in aglobal.

在函数中带有静态声明的相同情况。像

Same situation with a static declaration in a function. Like

int afunc（）
{
static int astatic = 8;
返回静态值；
}

int afunc() { static int astatic = 8; return astatic; }

推荐答案

在数据节中形成一个int大小的空间，在其中编码了值5，并且名为 aglobal的全局非功能符号将添加到指向它的符号表中。对aglobal的引用将转换为重定位，在链接时解析该重定位以指向该数据块，因此在完全链接的图像指令中，指令将直接从保存5值的内存中的那个点加载。

An int-sized space is made in a data section, with the value 5 encoded in it and a global non-function symbol named 'aglobal' is added to the symbol table pointing at it. References to aglobal are turned into relocations that are resolved at link-time to point to that data block, so in a fully-linked image instructions will load directly from that spot in memory that holds the 5 value

例如，（x86）程序集可能类似于：

For example, the (x86) assembly might look something like:

.data
.globl aglobal
aglobal: .long 5

.text
main:
    mov eax, aglobal

在目标文件中，mov指令将变为 mov eax，0 ，重定位 R_386_32 aglobal + 0 ，因为目标文件不确定该数据节在内存中的位置。

In an object file, the mov instruction will turn into mov eax, 0 with a relocation R_386_32 aglobal+0, because the object file doesn't know for sure where the data section will be in memory.

完全链接的图像，可能类似于：

In a fully-linked image, it might be something like:

mov eax, 0x804a010

现在已知data节中4个字节的实际地址，因此可以直接指定

Now the actual address of the 4 bytes in the data section is known, so it's specified directly

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