如何评估gcc格式溢出检查大小72 [英] How to evaluate gcc format-overflow checking size 72

问题描述

我的示例代码(t.c)

My sample code (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(gcc 7.1.1)的警告消息

Warning message by gcc -Wall -Wformat=2 -m64 t.c (gcc 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'). 我很纳闷，为什么会有72号呢?如何评估.在此样本中，无需将wtime增大为72.

The number 20 in the message is 19 and one terminator ('\0'). I am wonder, why there is a number 72? How to evaluate it. It is not necessary to increase wtime as size 72 in this sample.

推荐答案

"%d"格式用于int，通常是32位带符号整数类型.这意味着打印时最多可以包含11个字符(10位数字加上负号可能的负号).您有六个含义，最多可以包含66个字符.

The "%d" format is for int, which is usually a 32-bit signed integer type. That means it can have up to 11 characters when printed (10 digits plus a possible minus for negative numbers). You have six of those meaning that's up to 66 character.

然后您还有五个其他字符(每个'/'和':'两个，以及一个空格).

Then you have five other characters (two each of '/' and ':', and a space).

加终止符.

那是72.

请记住，"%04d"的"04"部分表示它将打印四个数字，并在左边填充零，但是如果还有更多数字，它将打印所有这些数字.这不是绝对限制.

And remember that the "04" part of "%04d" means it will print four digits left-padded with zeros, but if there's more digits then it will print all of them. It's not an absolute limit.

因此，至少20个字符，但最多72个字符.

Therefore at least 20 characters, but up to 72.

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