使用makecontext实现用户线程 现代Unix系统都在ucontext.h中提供用于上下文切换的函数，这些函数有getcontext, setcontext，swapcontext 和makecontext。其中，getcontext用于保存当前上下文，setcontext用于切换上下文，swapcontext会保存当前上下文并切换到另一个上下文，makecontext创建一个新的上下文。实现用户线程的过程是：我们首先调用getcontext获得当前上下文，然后修改ucontext_t指定新的上下文。同样的，我们需要开辟栈空间，但是这次实现的线程库要涉及栈生长的方向。然后我们调用makecontext切换上下文，并指定用户线程中要执行的函数。
在这种实现中还有一个挑战，即一个线程必须可以主动让出CPU给其它线程。swapcontext函数可以完成这个任务，图4展示了一个这种实现的样例程序，child线程和parent线程不断切换以达到多线程的效果。在libfiber-uc.c文件中可以看到完整的实现。
#include
#include
#include
// 64kB stack
#define FIBER_STACK 1024*64
ucontext_t child, parent;
// The child thread will execute this function
void threadFunction()
{
printf( "Child fiber yielding to parent" );
swapcontext( &child, &parent );
printf( "Child thread exiting\n" );
swapcontext( &child, &parent );
}
int main()
{
// Get the current execution context
getcontext( &child );
// Modify the context to a new stack
child.uc_link = 0;
child.uc_stack.ss_sp = malloc( FIBER_STACK );
child.uc_stack.ss_size = FIBER_STACK;
child.uc_stack.ss_flags = 0;
if ( child.uc_stack.ss_sp == 0 )
{
perror( "malloc: Could not allocate stack" );
exit( 1 );
}
// Create the new context
printf( "Creating child fiber\n" );
makecontext( &child, &threadFunction, 0 );
// Execute the child context
printf( "Switching to child fiber\n" );
swapcontext( &parent, &child );
printf( "Switching to child fiber again\n" );
swapcontext( &parent, &child );
// Free the stack
free( child.uc_stack.ss_sp );
printf( "Child fiber returned and stack freed\n" );
return 0;
}
图4用makecontext实现线程
抢占实现的一个最重要的因素就是定时触发的计时器中断，它的存在使得我们能够中断当前程序的执行，异步对进程的时间片消耗情况进行统计，并在必要的时候（可能是时间片耗尽，也可能是一个高优先级的程序就绪）从当前进程调度到其它进程去执行。
对于用户空间程序来说，与内核空间的中断相对应的就是信号，它和中断一样都是异步触发，并能引起执行流的跳转。所以要想实现用户级线程的抢占，我们可以借助定时器信号(SIGALRM)，必要时在信号处理程序内部进行上下文的切换。
为了验证自己的想法，我在上篇文章提到的协同多线程的基础上加上了相关抢占代码，具体实现如下：
#include <stdlib.h>
#include <stdio.h>
#include <ucontext.h>
#include <sys/time.h>
#define STACK_SIZE 4096
#define UTHREAD_MAX_NUM 256
#define INIT_TICKS 10
typedef int uthread_t;
typedef void uthread_attr_t;
uthread_t current = 0;
#define uthread_self() current
struct uthread_struct
{
int used;
ucontext_t context;
char stack[STACK_SIZE];
void* (*func)(void *arg);
void *arg;
void *exit_status;
int ticks;
};
static struct uthread_struct uthread_slots[UTHREAD_MAX_NUM];
void panic(void)
{
fprintf(stderr, "Panic, bala bala...\n");
exit(EXIT_FAILURE);
}
void idle_thread(void)
{
int i;
for (i = 1; i < UTHREAD_MAX_NUM; i ++)
if (uthread_slots[i].used)
break;
if (i == UTHREAD_MAX_NUM)
panic();
if (current != 0)
uthread_slots[current].used = 0;
current = i;
swapcontext(&uthread_slots[0].context,&uthread_slots[current].context);
}
void uthread_context_init(int tid)
{
getcontext(&uthread_slots[tid].context);
uthread_slots[tid].context.uc_stack.ss_sp = uthread_slots[tid].stack;
uthread_slots[tid].context.uc_stack.ss_size =sizeof(uthread_slots[tid].stack);
uthread_slots[tid].context.uc_link = &uthread_slots[0].context;
}
void uthread_init(void)
{
uthread_context_init(0);
uthread_slots[0].used = 1;
makecontext(&uthread_slots[0].context, idle_thread, 0);
}
void uthread_schedule(void);
void uthread_exit(void *exit_status)
{
uthread_slots[current].exit_status = exit_status;
uthread_slots[current].used = 0;
uthread_schedule();
}
void uthread_helper(void)
{
uthread_exit(uthread_slots[current].func(uthread_slots[current].arg));
}
int uthread_create(uthread_t *thread, const uthread_attr_t *attr,
void* (*start_routine)(void*), void *arg)
{
static int last_used = 0;
int i;
for (i = (last_used + 1) % UTHREAD_MAX_NUM; i != last_used;
i = (i + 1) % UTHREAD_MAX_NUM)
if (!uthread_slots[i].used)
break;
if (i == last_used)
return -1;
last_used = i;
if (thread != NULL)
*thread = i;
uthread_context_init(i);
uthread_slots[i].used = 1;
uthread_slots[i].func = start_routine;
uthread_slots[i].arg = arg;
uthread_slots[i].exit_status = 0;
uthread_slots[i].ticks = uthread_slots[current].ticks / 2;
uthread_slots[current].ticks -= uthread_slots[i].ticks;
makecontext(&uthread_slots[i].context, uthread_helper, 0);
return 0;
}
void uthread_schedule(void)
{
int i, prev;
for (i = (current + 1) % UTHREAD_MAX_NUM; i != current;
i = (i + 1) % UTHREAD_MAX_NUM)
if (uthread_slots[i].used)
break;
if (i == current)
panic();
prev = current;
current = i;
swapcontext(&uthread_slots[prev].context,&uthread_slots[current].context);
}
void* thread(void *arg)
{
int i;
for (i = 0; 1; i ++) {
if (i % 1000 == 0)
printf("thread/%d(%s): i = %d\n", current, (char*)arg,i);
uthread_create(NULL, NULL, thread, arg);
if (i % 1000000 == 0)
uthread_schedule();
}
}
void sig_ticks_timer(int signo)
{
if (--uthread_slots[current].ticks <= 0) {
uthread_slots[current].ticks = INIT_TICKS;
uthread_schedule();
}
}
int main(int argc, char *argv[])
{
uthread_t tid;
struct itimerval ticks_timer;
uthread_init();
uthread_create(&tid, NULL, thread, "hw1");
printf("tid is %d\n", tid);
uthread_create(&tid, NULL, thread, "hw2");
printf("tid is %d\n", tid);
signal(SIGALRM, sig_ticks_timer);
ticks_timer.it_interval.tv_sec = 0;
ticks_timer.it_interval.tv_usec = 10000;
ticks_timer.it_value.tv_sec = 0;
ticks_timer.it_value.tv_usec = 10000;
setitimer(ITIMER_REAL, &ticks_timer, NULL);
while (1)
idle_thread();
return 0;
}
似乎该有的都有了，也许真的可以在用户空间实现一个虚拟的操作系统环境玩呢...