英文:
Global variable became null when jumping to a function during the porting process
问题
我正在尝试将在Linux上运行的程序移植到Windows上。这个程序是一个多线程程序，由于初始设计不够完善，许多全局变量散布在代码中。现在我遇到了一个非常棘手的问题：在跳转到某个函数时，一个未使用的全局变量被设置为0x0。
我在Linux上使用的编译器版本是gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44)，而我在Windows 10上使用的编译器版本是gcc.exe (x86_64-win32-seh-rev2, Built by MinGW-W64 project) 12.2.0。
通过检查内存内容，我发现这个全局变量的地址非常接近于跳转到的函数的地址（如下图所示）：
查看图片描述
通过使用objdump命令检查可执行文件的段，内存布局如下（部分）：
查看图片描述
我想知道这个问题的原因，并希望找到解决方法。
-----补充说明------
对不起，我的描述不是很清楚，这里有一个简化的示例：
// file1.h
extern struct ITEM *item;
void init();
// file1.c
struct ITEM *item;
void init() {
item = (struct ITEM *) malloc(sizeof(struct ITEM));
item->a = 1;
item->b = 2;
...
}
// file2.h
void funcA() {
// 从不使用item
...
}
// main.c
...
#include "file1.h"
#include "file2.h"
...
void main(void) {
init();
funcA();
}
在我调用init()函数之后，全局变量item被初始化并分配了一个值，但在调用funcA()函数时，在汇编中的call指令执行后，item指针变成了0x0。
英文:
I am trying to port a program running on Linux to Windows. This program is a multi-threaded program, and due to the initial design not being perfect enough, many global variables are scattered throughout the code. Now I have encountered a very tricky problem: when jumping to a certain function, an unused global variable is set to 0x0.
The compiler version I'm using on Linux is gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44), while the compiler version I'm using on Windows 10 is gcc.exe (x86_64-win32-seh-rev2, Built by MinGW-W64 project) 12.2.0.
By examining the memory contents, I found that the address of this global variable is very close to the address of the function being jumped to (as shown in the figure below):
enter image description here
By using the objdump command to check the segment of the executable file, the layout of the memory is as follows(a part of):
enter image description here
I want to know the reason for this problem and hope to find a solution.
-----ADDITION------
Sorry, my description is not very clear, and here's a simplified example:
// file1.h
extern struct ITEM *item;
void init();
// file1.c
struct ITEM *item;
void init() {
item = (struct ITEM *) malloc(sizeof(struct ITEM));
item->a = 1;
item->b = 2;
...
}
// file2.h
void funcA() {
// never use item
...
}
// main.c
...
#include "file1.h"
#include "file2.h"
...
void main(void) {
init();
funcA();
}
After I call the init() function, the global variable item is initialized and assigned a value, but when calling the funcA() function, after the call instruction in assembly is executed, the item pointer becomes 0x0
答案1
得分: 2
> 在跳转到某个函数时，一个未使用的全局变量被设置为0x0。
"全局变量" - 具有 文件作用域 的变量，除非程序员明确初始化为其他值，否则始终会初始化为零，因为它们具有 静态存储期。您的程序表现如预期 - 所以似乎出现问题的是您的期望，而不是程序行为。
来源：C17 5.1.2 和 6.7.9 §10。
英文:
> when jumping to a certain function, an unused global variable is set to 0x0.
"Global variables" - variables with file scope, are always initialized to zero unless explicitly initialized to something else by the programmer, since they have static storage duration. Your program behaves as expected - so what's wrong appears to be your expectations, not the program behavior.
Sources: C17 5.1.2 and 6.7.9 §10.
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