C++ 获取ms级的计时

#include <time.h>
#include <sys/timeb.h>

struct timeb tb;
struct tm *now;
char sec1[16];
char mss1[4];
char sec2[16];
char mss2[4];
int min1, s1, ms1, min2, s2, ms2, timespan;

//第一次计时

ftime(&tb);
now = localtime(&tb.time);
sprintf(sec1, "%02d-%02d", now->tm_min, now->tm_sec);
sprintf(mss1, "%03d", tb.millitm);
min1 = now->tm_min;
s1 = now->tm_sec;
ms1 = tb.millitm;
printf("%s.%s \n", sec1, mss1);

//第二次计时

//计算时间间隔

ftime(&tb);
now = localtime(&tb.time);
sprintf(sec2, "%02d-%02d", now->tm_min, now->tm_sec);
sprintf(mss2, "%03d", tb.millitm);
printf("%s.%s \n", sec2, mss2);
min2 = now->tm_min;
s2 = now->tm_sec;
ms2 = tb.millitm;
//int timespan = (min2 * 60 + s2 - min1 * 60 - s1) * 1000 + ms2 - ms1;
timespan = ((min2 - min1) * 60 + s2 - s1) * 1000 + ms2 - ms1;
printf("中间间隔%d ms\n", timespan );