gcc中的汇编

C中引用汇编格式如下：

asm( code-string [ : output-list [ : input-list [ : overwrite-list ] ] ] );

#define __get_user_asm(x, addr, err, itype, rtype, ltype, errno)        \
        __asm__ __volatile__(                                   \
                "1:     mov"itype" %2,%"rtype"1\n"              \
                "2:\n"                                          \
                ".section .fixup,\"ax\"\n"                      \
                "3:     mov %3,%0\n"                            \
                "       xor"itype" %"rtype"1,%"rtype"1\n"       \
                "       jmp 2b\n"                               \
                ".previous\n"                                   \
                ".section __ex_table,\"a\"\n"                   \
                "       .align 8\n"                             \
                "       .quad 1b,3b\n"                          \
                ".previous"                                     \
                : "=r"(err), ltype (x)                          \
                : "m"(__m(addr)), "i"(errno), "0"(err))


其中最后两行分别是output-list和input-list，第一行是把addr移到x中去。

"=r"(err)表示输出为整数。
"m"(__m(addr))应该表示引用内存，但是内存地址还是内存中的内容？还需进一步核实。

调用函数中根据size不同指明了ltype的具体类型，
        switch (size) {
        case 1:__get_user_asm(*(u8*)dst,(u8 *)src,ret,"b","b","=q",1);
                return ret;
        case 2:__get_user_asm(*(u16*)dst,(u16*)src,ret,"w","w","=r",2);
                return ret;
        case 4:__get_user_asm(*(u32*)dst,(u32*)src,ret,"l","k","=r",4);
                return ret;
        case 8:__get_user_asm(*(u64*)dst,(u64*)src,ret,"q","","=r",8);
                return ret;