获取当前进程描述符地址 -- current

在内核代码中当需要访问当前进程的task_struct结构时使用的指针current实际上是个宏定义，它是根据当前进程的堆栈指针ESP计算出来的。


#define current get_current()




static inline struct task_struct * get_current(void)




...{


return current_thread_info()->task;


}


static inline struct thread_info *current_thread_info(void)




...{


struct thread_info *ti;


__asm__("andl %%esp,%0; ":"=r" (ti) : "0" (~(THREAD_SIZE - 1)));


return ti;


}


#define THREAD_SIZE (8192)

注：
(1)THREAD_SIZE(8K)即thread_info结构体的大小
(2)"andl %%esp,%0; ":"=r" (ti) : "0" (~(THREAD_SIZE - 1))
将内核堆栈栈顶ESP指针和(~(THREAD_SIZE - 1)相与：获得的结果为内核堆栈最底端地址(也就是结构体thread_info的地址)




struct thread_info ...{


struct task_struct *task;


struct exec_domain *exec_domain;


unsigned long flags;


unsigned long status;


__u32 cpu;


__s32 preempt_count;


mm_segment_t addr_limit;


struct restart_block restart_block;


unsigned long previous_esp;


__u8 supervisor_stack[0];


};






struct task_struct ...{


volatile long state;


struct thread_info *thread_info;


atomic_t usage;


unsigned long flags;

ESP、thread.info、task_struct、task_struct.thread的关系结构示意图
|------------|<---0x015fbfff
| kernel | |
| stack | |
| | |
| | |
| /|/ | current--+--->|--------------|
| | | | |
| |<---%%esp | |--------------|
| | (ESP) | | *thread_info |
| | | |--------------|
| | | | |
| | | | |
-----------|------------|<---0x015fa030 | | |
/|/ | *task |-----------------+ |--------------|
| |------------| | |
thread_info| | | thread |
| | | | |
/|/ | | |--------------|
-----------|------------|<---0x15fa000 | |
| |
|--------------|
task_struct