应用层： #include <unistd.h>
1、unsigned int sleep(unsigned int seconds); 秒级
2、int usleep(useconds_t usec); 微秒级：1/10^-6
#define _POSIX_C_SOURCE 199309
#include <time.h>
3、int nanosleep(const struct timespec *req, struct timespec *rem); struct timespec { time_t tv_sec; /* seconds */ long tv_nsec; /* nanoseconds */ }; // The value of the nanoseconds field must be in the range 0 to 999999999.
内核层： include <linux/delay.h> 1、void ndelay(unsigned long nsecs); 纳秒级：1/10^-10 2、void udelay(unsigned long usecs); 微秒级: 1/10^-6 3、void mdelay(unsigned long msecs); 毫秒级：1/10^-3
