如何用c語言初始化寄存器無符號長

Question

unsigned long start, stop; 
register unsigned long sp asm ("sp"); 

start = sp; 

這行代碼聲明變量sp沒有初始值設定項。

Answer 1

不同的體系結構對堆棧指針使用不同的寄存器。這裏是一個示例（linux + elf）_start函數來演示：

//unfortunately gcc has no builtin for stack pointer, so we use assembly 
#ifdef __x86_64__ || defined __i386__ 
    #define STACK_POINTER "esp" 
#elif defined __aarch64__ 
    #define STACK_POINTER "x13" 
#elif defined __arm__ 
    #define STACK_POINTER "r13" 
#endif 
char **environ; 
void exit(int); 
int main(); 
void _start(void){ 
    register long *sp __asm__(STACK_POINTER); 
//if you don't use argc, argv or envp/environ, you can just remove them 
    long argc = *sp; 
    char **argv = (char **)(sp + 1); 
    environ = (char **)(sp + argc + 1); 
    exit(main(argc, argv, environ)); 
    __builtin_unreachable(); //or for(;;); to shut up gcc 
}