代码：
extern inline int strncmp(const char * cs, const char * ct, int count)
{
register int __res;
__asm__("cld\n"
"1:\tdecl %3\n\t"
"js 2f\n\t"
"lodsb\n\t"
"scasb\n\t"
"jne 3f\n\t"
"testb %%al, %%al\n\t"
"jne 1b\n"
"2:\txorl %%eax,%%eax\n\t"
"jmp 4f\n"
"3:\tmovl $1,%%eax\n\t"
"j1 4f\n\t"
"negl %%eax\n"
"4:"
:"=a" (__res):"D" (cs), "S" (ct), "c" (count):"si","di","cx");
return __res;
}
我不明白“ js 2f\n\t ”中的f和“ jne 1b\n ”中的b，如何理解？我应该看哪本书？谢谢你。
在这种情况下f，意味着向前和b向后。Sojs 2f表示向前跳转到标签 2，如果 sign set。
你会想要调查的gcc inline assembly。我似乎在网上找不到任何参考来包含这一点，但我知道你可以在Professional Assembly Language中找到它。
为什么我们不能使用命名标签？引用书中的一段话：
If you have another asm section in your C code, you cannot use the same labels again, or an error message will result due to duplicate use of labels.
So what can we do ?
The solution is to use local labels. Both conditional and unconditional branches allow you to specify a number as a label, along with a directional flag to indicate which way the processor should look for the numerical label. The first occurrence of the label found will be taken.
About modifiers:
Use the f modifier to indicate the label is forward from the jump instruction. To move backward, you must use the b modifier.
This is documented in the manual for the assembler.