linux线程的优先级设置

linux并不是实时操作系统，把下面的代码运行一遍就能够理解了，代码有很详细的注释。
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <signal.h>
#include <string.h>
void * thr_fun(void *arg)
{
int policy, ret;
struct sched_param param;
//获取线程调度参数
ret = pthread_getschedparam(pthread_self(), &policy, &param);
if(ret!=0)
{
printf("pthread_getschedparam %s\n", strerror(ret) );
exit(1);
}
if (policy == SCHED_FIFO)
{
printf("policy:SCHED_FIFO\n");
}
else if (policy == SCHED_OTHER)
{
printf("policy:SCHED_OTHER\n");
}
else if (policy == SCHED_RR)
{
printf("policy:SCHED_RR\n");
}
printf("param:%d\n", param.sched_priority);
long long i;
while (1) {
i++;
i *= 2;
}
pthread_exit(NULL);
}
int main(void)
{
int ret;
pthread_t tid;
pthread_attr_t attr;
int policy, inher;
struct sched_param param;

//初始化线程属性
pthread_attr_init(&attr);
//获取继承的调度策略
ret = pthread_attr_getinheritsched(&attr, &inher);
if (ret!=0)
{
printf("pthread_attr_getinheritsched\n%s\n", strerror(ret));
exit(1);
}
//
if (inher == PTHREAD_EXPLICIT_SCHED)
{
printf("PTHREAD_EXPLICIT_SCHED\n");
}
else if (inher == PTHREAD_INHERIT_SCHED)
{
printf("PTHREAD_INHERIT_SCHED\n");
inher = PTHREAD_EXPLICIT_SCHED;
}
//设置继承的调度策略
//必需设置inher的属性为 PTHREAD_EXPLICIT_SCHED，否则设置线程的优先级会被忽略
ret = pthread_attr_setinheritsched(&attr, inher);
if (ret!=0)
{
printf("pthread_attr_setinheritsched\n%s\n", strerror(ret));
exit(1);
}

policy = SCHED_FIFO;//在Ubuntu9.10上需要root权限
//设置线程调度策略
ret = pthread_attr_setschedpolicy(&attr, policy);
if (ret!=0)
{
printf(" pthread_attr_setschedpolicy\n%s\n", strerror(ret));
exit(1);
}
param.sched_priority = 3;
//设置调度参数
ret = pthread_attr_setschedparam(&attr,