从GCC内联汇编内的系统调用 [英] syscall from within GCC inline assembly

问题描述

是有可能写出使用系统调用从内联汇编块中的单个字符？如果是这样，怎么样？它应该看东西是这样的：

is it possible to write a single character using a syscall from within an inline assembly block? if so, how? it should look "something" like this:

__asm__ __volatile__
                    (
                     " movl $1,  %%edx \n\t"
                     " movl $80, %%ecx \n\t"
                     " movl $0,  %%ebx \n\t"
                     " movl $4,  %%eax \n\t"
                     " int $0x80       \n\t"
                     ::: "%eax", "%ebx", "%ecx", "%edx"
                    );

$ 80在ASCII'P'，但不返回任何内容。

$80 is 'P' in ascii, but that returns nothing.

任何建议，多AP preciated！

any suggestions much appreciated!

推荐答案

类似

char p = 'P';

int main()
{
__asm__ __volatile__
                    (
                     " movl $1,  %%edx \n\t"
                     " leal p , %%ecx \n\t"
                     " movl $0,  %%ebx \n\t"
                     " movl $4,  %%eax \n\t"
                     " int $0x80       \n\t"
                     ::: "%eax", "%ebx", "%ecx", "%edx"
                    );
}

添加：请注意，我用 LEA 来加载炭的有效地址为 ECX 注册;为 EBX 的价值我想$ 0 $ 1和它似乎反正工作...

Add: note that I've used lea to Load the Effective Address of the char into ecx register; for the value of ebx I tried $0 and $1 and it seems to work anyway ...

避免使用外部焦炭

int main()
{
__asm__ __volatile__
                    (
                     " movl $1,  %%edx \n\t"
                     " subl $4, %%esp \n\t"
                     " movl $80, (%%esp)\n\t"
                     " movl %%esp, %%ecx \n\t"
                     " movl $1,  %%ebx \n\t"
                     " movl $4,  %%eax \n\t"
                     " int $0x80       \n\t"
                     " addl $4, %%esp\n\t"
                     ::: "%eax", "%ebx", "%ecx", "%edx"
                    );
}

N.B：它的工作原理，因为英特尔处理器的字节序的！ ：D

N.B.: it works because of the endianness of intel processors! :D

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