Use select interface to write high-precision delay

Use select interface to write high precision delay.

select interface

int select(int maxfdp, fd_set *readset, fd_set *writeset, fd_set *exceptset,struct timeval *timeout);

principle

• Use the timeout parameter of select to implement a timer;
• Set the value of timeval, and set other parameters to NULL, select will exit when the internal time runs out.

Example

#include <stdio.h>  
#include <sys/time.h>  
 
int main()  
{  
	struct timeval tv;  
	
	while(1)
	{
		tv.tv_sec = 1;  // 定时1秒
		tv.tv_usec = 0;  
		
		switch(select(0, NULL, NULL, NULL, &tv)) 
		{
		case -1:  // 错误
			printf("Error!\n");
			break;
		case 0: //超时
			printf("timeout expires.\n");
			break;
		default:
			printf("default\n");
			break;
		}
	}
 
	return 0;  
}

void usleep(unsigned long usec)
{
    struct timeval tv;
    tv.tv_sec = usec / 1000000;
    tv.tv_usec = usec % 1000000;

    int err;
    do {
        err = select(0, NULL, NULL, NULL, &tv);
    } while(err < 0 && errno == EINTR);
}

note

• The delay resolution supported by the kernel generally cannot reach the microsecond level;
• Due to the delay of kernel scheduling;
• The kernel resolution is generally a multiple of 10ms.