c - C语言中如何初始化寄存器unsigned long

Question

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

start = sp;

这行代码声明 - 声明变量 sp 而不使用初始值设定项。

Answer 1

不同的体系结构使用不同的寄存器作为堆栈指针。下面用一个例子(linux+elf)_start函数来演示:

//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 
}