宏在GAS中替换为常数 [英] Macro substituting a constant number in GAS

问题描述

X86 GNU Assembly上的那个宏怎么了?表示链接期间未定义符号S.

What't wrong with that macro on X86 GNU Assembly? It says the symbol S is undefined during linking.

.macro S size=40
\size
.endm

我正在使用它

mov %eax, S

推荐答案

宏用于为您经常使用的代码创建模板，而不是输入常数.因此，我不相信汇编程序会在表达式中进行宏扩展.由于只需要一个数字，因此可以使用.set定义一个常量.

Macros are used to create templates for code you frequently use, not to input a constant number. As such, I do not believe the assembler does macro expansion within an expression. Since you simply want a number, you could use .set to define a constant.

.set S, 40
mov %eax, S

此外，如果您通常使用intel语法，请确保您意识到这段代码在做什么:当前，它将eax的值存储在内存中的地址0x28处.如果要将数字40放在eax中，则需要反转操作数，并在S前面使用美元符号.

Also, in case you usually use intel syntax, make sure you realize what this code is doing: It currently stores the value of eax in memory at the address 0x28. If you want to put the number 40 in eax, you need to reverse the operands and use a dollar sign in front of S.

mov $S, %eax

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