从Asm访问用C定义的全局变量 [英] Accessing global variable defined in C from Asm
问题描述
我有一个C文件，其中包含全局变量foo.如何从另一个Assemby程序访问 foo .
我正在使用i586-elf-as(GNU汇编程序)和i586-elf-gcc(gnu编译器)进行构建.
I have a C file which contain a global variable foo. How I can access foo from another assemby program.
I am using i586-elf-as (GNU assembler) and i586-elf-gcc (gnu compiler) for building.
推荐答案
您可以只使用符号名称； as将所有未定义的符号视为外部符号.
You can just use the symbol name; as treats all undefined symbols as external.
检查编译器输出(gcc -S)和/或文档，以了解C变量名称是否以前置_开头. (在许多非ELF平台上，int myglobal变为asm _myglobal，但在Linux/ELF上仍为myglobal.)
Check compiler output (gcc -S) and/or documentation to find out if C variable names get a leading _ prepended or not. (int myglobal becomes asm _myglobal on many non-ELF platforms, but still myglobal on Linux/ELF.)
当然，如果使用C ++编译器(除了extern "C"变量之外)，则会发生C ++名称更改.
And of course C++ name mangling happens if you use a C++ compiler, except for extern "C" variables.
如果要显式声明，则有一个.extern指令，GAS会忽略该指令(与其他Unix汇编程序兼容). GAS手册中的文档
If you want to declare it explicitly, there's a .extern directive which GAS ignores (for compat with some other Unix assemblers). Documentation in the GAS manual
.extern foo # ignored, no extra checking is done because of this
例如在x86-64，lea myglobal(%rip), %rsi或mov $myglobal, %esi上，以AT& T语法将地址保存到寄存器中.
For example on x86-64, lea myglobal(%rip), %rsi or mov $myglobal, %esi to get the address into a register in AT&T syntax.
或mov myglobal(%rip), %eax从中加载.或者mov global, %eax使用32位绝对地址以32位模式从其加载，因为RIP相对寻址仅在64位模式下可用.
Or mov myglobal(%rip), %eax to load from it. Or mov global, %eax to load from it in 32-bit mode, using a 32-bit absolute address because RIP-relative addressing is only available in 64-bit mode.
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