如何在C中实现stdarg [英] How to implement stdarg in C

问题描述

出于好奇，我希望为标准C库中的某些功能编写最少的替代品.到目前为止，我已经完成了printf()，strlen()，strcpy()，memcpy()，memset()等...，但是当我尝试使用printf函数时，我不知道如何实现！我该怎么做呢?

For curiosity, I'm looking to write minimal replacements for some of the functions in the standard C library. So far, I have finished printf(), strlen(), strcpy(), memcpy(), memset(), etc... but when I try to use the printf function, I don't know how to implement stdarg.h! What is a way that I could do that?

它使用宏还是实际函数?

Does it use macros or actual functions?

我正在32位x86上使用gcc或clang，如果这有助于使其更易于回答.

I am using gcc OR clang on 32-bit x86, if it helps to make this any easier to answer.

推荐答案

在具有cdecl调用约定的32位x86上，参数在堆栈上传递:

On 32-bit x86 with the cdecl calling convention, parameters are passed on the stack:

^ higher addresses (lower on the stack)
|
| caller local variables
| ...
| argument 3
| argument 2
| argument 1
| return address
| saved EBP (usually)
| callee local variables
|
v lower addresses (higher on the stack)

您可以将va_list实现为指针. va_start可以获取传递给它的参数的地址，并添加该参数的大小以移至下一个参数. va_arg可以访问指针并将其碰到下一个参数. va_copy可以只复制指针值. va_end无需执行任何操作.

You can implement va_list as a pointer. va_start can take the address of the argument passed to it and add the size of that argument to move to the next argument. va_arg can access the pointer and bump it to the next argument. va_copy can just copy the pointer value. va_end doesn’t need to do anything.

另一方面，如果您没有使用cdecl(也许正在使用fastcall)，不是32位，不是x86，则此操作无效；您可能需要处理寄存器，而不仅仅是指针值.甚至仍然不能保证它能正常工作，因为您依赖的是不确定的行为；仅作为一个潜在问题的示例，内联会破坏所有内容.这就是为什么标头文件仅将其typedef保存到内置的编译器中的原因-在C语言中实现该方法是没有希望的，您需要编译器支持.而且甚至不让我开始实施setjmp和longjmp…

If, on the other hand, you’re not using cdecl (maybe you’re using fastcall), you’re not 32-bit, you’re not x86, this won’t work; you might need to muck with registers rather than just pointer values. And even still it’s not guaranteed to work, since you’re relying on undefined behavior; as as example of only one potential problem, inlining could ruin everything. And that’s why the header file just typedefs it to a compiler built-in—implementing this in C is hopeless, you need compiler support. And don’t even get me started on implementing setjmp and longjmp…

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