如何评估gcc格式溢出检查大小72

我的示例代码(t.c)

#include <stdio.h>
#include <time.h>
int main() {
        /* 12345678901234567890
         * 2017/09/06 12:28:30  */
        char wtime[19];
        time_t t=time(NULL);
        struct tm *pt = localtime(&t);
        sprintf(wtime, "%04d/%02d/%02d %02d:%02d:%02d", pt->tm_year+1900,
            pt->tm_mon+1, pt->tm_mday, pt->tm_hour, pt->tm_min, pt->tm_sec);
        printf("%s %ld\n", wtime, sizeof(int));
        return 0;
}

gcc -Wall -Wformat=2 -m64 t.c (通用条款7.1.1)

t.c:9:47: warning: ‘sprintf’ writing a terminating nul past the end of the destination [-Wformat-overflow=]
  sprintf(wtime, "%04d/%02d/%02d %02d:%02d:%02d", pt->tm_year+1900,
                                               ^
t.c:9:2: note: ‘sprintf’ output between 20 and 72 bytes into a destination of size 19
  sprintf(wtime, "%04d/%02d/%02d %02d:%02d:%02d", pt->tm_year+1900,
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      pt->tm_mon+1, pt->tm_mday, pt->tm_hour, pt->tm_min, pt->tm_sec);
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

消息中的数字20是19和一个终止符(“\0”)。