size_t strftime(char *str, size_t maxsize, const char *format, const struct tm *timeptr)

struct tm * localtime(const time_t *clock);
struct tm * localtime_r(const time_t *clock, struct tm *result);

localtime   返回静态变量
localtime_r 返回值需要外部传入
     

#include <stdio.h>
#include <time.h>
#include <unistd.h>

// YYYY-MM-DD hh:mm:ss
// 2021-09-01 19:33:07
// char tmpbuf[20];
int main() {
    time_t nowtime;
 
    time(&nowtime);
    
    struct tm *timeinfo;
    char tmpbuf[20];
    
    // localtime 返回静态变量
    timeinfo = localtime(&nowtime);
    // strftime(tmpbuf,20,"%Y-%m-%d %H:%M:%S",timeinfo);
    strftime(tmpbuf,sizeof(tmpbuf),"%Y-%m-%d %H:%M:%S",timeinfo);
    printf("%s\n",tmpbuf);
    
    sleep(1);
    time(&nowtime);
    struct tm timeinfo2 = { 0 };
    // localtime_r 可重入版本 
    localtime_r(&nowtime,&timeinfo2);
    strftime(tmpbuf,sizeof(tmpbuf),"%Y-%m-%d %H:%M:%S",&timeinfo2);
    printf("%s\n",tmpbuf);
    
  
    return 0;
}