Linux0.11内核--指针的指针 任务结构体

在linux内核进程调度函数schedule()中定义了struct task_struct **p,它是指向指针的指针，由于linux内核中task数组定义如下：

struct task_struct *task[NR_TASKS] = { &(init_task.task), }; 

/*
 *  'schedule()' is the scheduler function. This is GOOD CODE! There
 * probably won't be any reason to change this, as it should work well
 * in all circumstances (ie gives IO-bound processes good response etc).
 * The one thing you might take a look at is the signal-handler code here.
 *
 *   NOTE!!  Task 0 is the 'idle' task, which gets called when no other
 * tasks can run. It can not be killed, and it cannot sleep. The 'state'
 * information in task[0] is never used.
 */
void schedule(void)
{
	int i,next,c;
	struct task_struct ** p;

/* check alarm, wake up any interruptible tasks that have got a signal */

	for(p = &LAST_TASK ; p > &FIRST_TASK ; --p)
		if (*p) {
			if ((*p)->alarm && (*p)->alarm < jiffies) {
					(*p)->signal |= (1<<(SIGALRM-1));
					(*p)->alarm = 0;
				}
			if (((*p)->signal & ~(_BLOCKABLE & (*p)->blocked)) &&
			(*p)->state==TASK_INTERRUPTIBLE)
				(*p)->state=TASK_RUNNING;
		}

/* this is the scheduler proper: */

	while (1) {
		c = -1;
		next = 0;
		i = NR_TASKS;
		p = &task[NR_TASKS];
		while (--i) {
			if (!*--p)
				continue;
			if ((*p)->state == TASK_RUNNING && (*p)->counter > c)
				c = (*p)->counter, next = i;
		}
		if (c) break;
		for(p = &LAST_TASK ; p > &FIRST_TASK ; --p)
			if (*p)
				(*p)->counter = ((*p)->counter >> 1) +
						(*p)->priority;
	}
	switch_to(next);
}

其中LAST_TASK 定义为  task[NR_TASK], FIRST_TASK 定义为 task[0]，task[]数组是一组指针，它指向的是每个task在内存中的存储

地址。*p即取p的内容，它内容的含义就是指向实际任务控制块在内存的位置，也就是task[i]的内容（指针的值）

下面是自己为了更好理解指针的指针写的一个小实例

pointer.h

typedef unsigned int sigset_t; 
typedef struct desc_struct {
	unsigned long a,b;
} desc_table[256];
struct i387_struct {
	long	cwd;
	long	swd;
	long	twd;
	long	fip;
	long	fcs;
	long	foo;
	long	fos;
	long	st_space[20];	/* 8*10 bytes for each FP-reg = 80 bytes */
};
struct sigaction {
	void (*sa_handler)(int);
	sigset_t sa_mask;
	int sa_flags;
	void (*sa_restorer)(void);
};
struct tss_struct {
	long	back_link;	/* 16 high bits zero */
	long	esp0;
	long	ss0;		/* 16 high bits zero */
	long	esp1;
	long	ss1;		/* 16 high bits zero */
	long	esp2;
	long	ss2;		/* 16 high bits zero */
	long	cr3;
	long	eip;
	long	eflags;
	long	eax,ecx,edx,ebx;
	long	esp;
	long	ebp;
	long	esi;
	long	edi;
	long	es;		/* 16 high bits zero */
	long	cs;		/* 16 high bits zero */
	long	ss;		/* 16 high bits zero */
	long	ds;		/* 16 high bits zero */
	long	fs;		/* 16 high bits zero */
	long	gs;		/* 16 high bits zero */
	long	ldt;		/* 16 high bits zero */
	long	trace_bitmap;	/* bits: trace 0, bitmap 16-31 */
	struct i387_struct i387;
};
struct task_struct {
/* these are hardcoded - don't touch */
	long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
	long counter;
	long priority;
	long signal;
	struct sigaction sigaction[32];
	long blocked;	/* bitmap of masked signals */
/* various fields */
	int exit_code;
	unsigned long start_code,end_code,end_data,brk,start_stack;
	long pid,father,pgrp,session,leader;
	unsigned short uid,euid,suid;
	unsigned short gid,egid,sgid;
	long alarm;
	long utime,stime,cutime,cstime,start_time;
	unsigned short used_math;
/* file system info */
	int tty;		/* -1 if no tty, so it must be signed */
	unsigned short umask;
	struct m_inode * pwd;
	struct m_inode * root;
	struct m_inode * executable;
	unsigned long close_on_exec;
	struct file * filp[20];
/* ldt for this task 0 - zero 1 - cs 2 - ds&ss */
	struct desc_struct ldt[3];
/* tss for this task */
	struct tss_struct tss;
};
struct task_struct *task[64];
struct task_struct _task[64];

pointer.c

#include "pointer.h"
#include<stdio.h>
#define FIRST_TASK task[0]
#define LAST_TASK task[63]
int main(void)
{
	int i;
	for(i=0;i<64;++i)
	{
		_task[i].counter=(long)i;
		task[i]=&_task[i];
	}
	struct task_struct *p;
	struct task_struct **q;
	printf("sizeof task_struct is %d/n",sizeof(p));
	long tmp;
	//printf("%d/n",(&LAST_TASK)->counter);
	//*p=&LAST_TASK;
	//printf("%d/n",(*p)->counter);
	for(p=LAST_TASK;p>FIRST_TASK;--p)
	{
		
		printf("task[%d] : 0x%x  &task[%d] : 0x%x/n",p->counter,p,p->counter,&p);
		//printf("After SUB %d/n",tmp-(long)p);
		//tmp=p;
		//if(p)
			//printf("process %d is runing/n",(p)->counter);
	}
	for(q=&LAST_TASK;q>&FIRST_TASK;--q)
	{
		
		printf("task[%d] : 0x%x  &task[%d] : 0x%x/n",(*q)->counter,*q,(*q)->counter,q);
		//printf("After SUB %d/n",tmp-(long)p);
		//tmp=p;
		//if(*q)
			//printf("process %d is runing/n",(*q)->counter);
	}
	return 0;
}

makefile

pointer:pointer.o
	gcc -o pointer pointer.o
pointer.o:pointer.c
	gcc -g -c pointer.c
clean:
	-rm -f pointer pointer.o

程序运行输出：

sizeof task_struct is 4

task[63] : 0x8058204 &task[63] : 0xbfb8fc90

task[62] : 0x8057e48 &task[62] : 0xbfb8fc90

task[61] : 0x8057a8c &task[61] : 0xbfb8fc90

task[60] : 0x80576d0 &task[60] : 0xbfb8fc90

task[59] : 0x8057314 &task[59] : 0xbfb8fc90

task[58] : 0x8056f58 &task[58] : 0xbfb8fc90

task[57] : 0x8056b9c &task[57] : 0xbfb8fc90

task[56] : 0x80567e0 &task[56] : 0xbfb8fc90

task[55] : 0x8056424 &task[55] : 0xbfb8fc90

task[54] : 0x8056068 &task[54] : 0xbfb8fc90

task[53] : 0x8055cac &task[53] : 0xbfb8fc90

task[52] : 0x80558f0 &task[52] : 0xbfb8fc90

task[51] : 0x8055534 &task[51] : 0xbfb8fc90

task[50] : 0x8055178 &task[50] : 0xbfb8fc90

task[49] : 0x8054dbc &task[49] : 0xbfb8fc90

task[48] : 0x8054a00 &task[48] : 0xbfb8fc90

task[47] : 0x8054644 &task[47] : 0xbfb8fc90

task[46] : 0x8054288 &task[46] : 0xbfb8fc90

task[45] : 0x8053ecc &task[45] : 0xbfb8fc90

task[44] : 0x8053b10 &task[44] : 0xbfb8fc90

task[43] : 0x8053754 &task[43] : 0xbfb8fc90

task[42] : 0x8053398 &task[42] : 0xbfb8fc90

task[41] : 0x8052fdc &task[41] : 0xbfb8fc90

task[40] : 0x8052c20 &task[40] : 0xbfb8fc90

task[39] : 0x8052864 &task[39] : 0xbfb8fc90

task[38] : 0x80524a8 &task[38] : 0xbfb8fc90

task[37] : 0x80520ec &task[37] : 0xbfb8fc90

task[36] : 0x8051d30 &task[36] : 0xbfb8fc90

task[35] : 0x8051974 &task[35] : 0xbfb8fc90

task[34] : 0x80515b8 &task[34] : 0xbfb8fc90

task[33] : 0x80511fc &task[33] : 0xbfb8fc90

task[32] : 0x8050e40 &task[32] : 0xbfb8fc90

task[31] : 0x8050a84 &task[31] : 0xbfb8fc90

task[30] : 0x80506c8 &task[30] : 0xbfb8fc90

task[29] : 0x805030c &task[29] : 0xbfb8fc90

task[28] : 0x804ff50 &task[28] : 0xbfb8fc90

task[27] : 0x804fb94 &task[27] : 0xbfb8fc90

task[26] : 0x804f7d8 &task[26] : 0xbfb8fc90

task[25] : 0x804f41c &task[25] : 0xbfb8fc90

task[24] : 0x804f060 &task[24] : 0xbfb8fc90

task[23] : 0x804eca4 &task[23] : 0xbfb8fc90

task[22] : 0x804e8e8 &task[22] : 0xbfb8fc90

task[21] : 0x804e52c &task[21] : 0xbfb8fc90

task[20] : 0x804e170 &task[20] : 0xbfb8fc90

task[19] : 0x804ddb4 &task[19] : 0xbfb8fc90

task[18] : 0x804d9f8 &task[18] : 0xbfb8fc90

task[17] : 0x804d63c &task[17] : 0xbfb8fc90

task[16] : 0x804d280 &task[16] : 0xbfb8fc90

task[15] : 0x804cec4 &task[15] : 0xbfb8fc90

task[14] : 0x804cb08 &task[14] : 0xbfb8fc90

task[13] : 0x804c74c &task[13] : 0xbfb8fc90

task[12] : 0x804c390 &task[12] : 0xbfb8fc90

task[11] : 0x804bfd4 &task[11] : 0xbfb8fc90

task[10] : 0x804bc18 &task[10] : 0xbfb8fc90

task[9] : 0x804b85c &task[9] : 0xbfb8fc90

task[8] : 0x804b4a0 &task[8] : 0xbfb8fc90

task[7] : 0x804b0e4 &task[7] : 0xbfb8fc90

task[6] : 0x804ad28 &task[6] : 0xbfb8fc90

task[5] : 0x804a96c &task[5] : 0xbfb8fc90

task[4] : 0x804a5b0 &task[4] : 0xbfb8fc90

task[3] : 0x804a1f4 &task[3] : 0xbfb8fc90

task[2] : 0x8049e38 &task[2] : 0xbfb8fc90

task[1] : 0x8049a7c &task[1] : 0xbfb8fc90

task[63] : 0x8058204 &task[63] : 0x80586bc

task[62] : 0x8057e48 &task[62] : 0x80586b8

task[61] : 0x8057a8c &task[61] : 0x80586b4

task[60] : 0x80576d0 &task[60] : 0x80586b0

task[59] : 0x8057314 &task[59] : 0x80586ac

task[58] : 0x8056f58 &task[58] : 0x80586a8

task[57] : 0x8056b9c &task[57] : 0x80586a4

task[56] : 0x80567e0 &task[56] : 0x80586a0

task[55] : 0x8056424 &task[55] : 0x805869c

task[54] : 0x8056068 &task[54] : 0x8058698

task[53] : 0x8055cac &task[53] : 0x8058694

task[52] : 0x80558f0 &task[52] : 0x8058690

task[51] : 0x8055534 &task[51] : 0x805868c

task[50] : 0x8055178 &task[50] : 0x8058688

task[49] : 0x8054dbc &task[49] : 0x8058684

task[48] : 0x8054a00 &task[48] : 0x8058680

task[47] : 0x8054644 &task[47] : 0x805867c

task[46] : 0x8054288 &task[46] : 0x8058678

task[45] : 0x8053ecc &task[45] : 0x8058674

task[44] : 0x8053b10 &task[44] : 0x8058670

task[43] : 0x8053754 &task[43] : 0x805866c

task[42] : 0x8053398 &task[42] : 0x8058668

task[41] : 0x8052fdc &task[41] : 0x8058664

task[40] : 0x8052c20 &task[40] : 0x8058660

task[39] : 0x8052864 &task[39] : 0x805865c

task[38] : 0x80524a8 &task[38] : 0x8058658

task[37] : 0x80520ec &task[37] : 0x8058654

task[36] : 0x8051d30 &task[36] : 0x8058650

task[35] : 0x8051974 &task[35] : 0x805864c

task[34] : 0x80515b8 &task[34] : 0x8058648

task[33] : 0x80511fc &task[33] : 0x8058644

task[32] : 0x8050e40 &task[32] : 0x8058640

task[31] : 0x8050a84 &task[31] : 0x805863c

task[30] : 0x80506c8 &task[30] : 0x8058638

task[29] : 0x805030c &task[29] : 0x8058634

task[28] : 0x804ff50 &task[28] : 0x8058630

task[27] : 0x804fb94 &task[27] : 0x805862c

task[26] : 0x804f7d8 &task[26] : 0x8058628

task[25] : 0x804f41c &task[25] : 0x8058624

task[24] : 0x804f060 &task[24] : 0x8058620

task[23] : 0x804eca4 &task[23] : 0x805861c

task[22] : 0x804e8e8 &task[22] : 0x8058618

task[21] : 0x804e52c &task[21] : 0x8058614

task[20] : 0x804e170 &task[20] : 0x8058610

task[19] : 0x804ddb4 &task[19] : 0x805860c

task[18] : 0x804d9f8 &task[18] : 0x8058608

task[17] : 0x804d63c &task[17] : 0x8058604

task[16] : 0x804d280 &task[16] : 0x8058600

task[15] : 0x804cec4 &task[15] : 0x80585fc

task[14] : 0x804cb08 &task[14] : 0x80585f8

task[13] : 0x804c74c &task[13] : 0x80585f4

task[12] : 0x804c390 &task[12] : 0x80585f0

task[11] : 0x804bfd4 &task[11] : 0x80585ec

task[10] : 0x804bc18 &task[10] : 0x80585e8

task[9] : 0x804b85c &task[9] : 0x80585e4

task[8] : 0x804b4a0 &task[8] : 0x80585e0

task[7] : 0x804b0e4 &task[7] : 0x80585dc

task[6] : 0x804ad28 &task[6] : 0x80585d8

task[5] : 0x804a96c &task[5] : 0x80585d4

task[4] : 0x804a5b0 &task[4] : 0x80585d0

task[3] : 0x804a1f4 &task[3] : 0x80585cc

task[2] : 0x8049e38 &task[2] : 0x80585c8

task[1] : 0x8049a7c &task[1] : 0x80585c4