C 程序在 Windows 中表现出奇怪的值，但在 Ubuntu 中运行良好

Question

我在我的笔记本电脑上使用 Ubuntu 为学校的一项小任务编写了一个程序。我在编写程序时没有遇到任何问题。它工作得很好，并做了它应该做的一切。但是当我将它移到我的 Windows PC 上时，它给了我一些非常奇怪的值，我似乎无法弄清楚它有什么问题。我在 Ubuntu 和 Windows 上都使用相同的 IDE，CodeBlocks 16.01。我已经尝试解决这个问题几个小时，但我卡住了，所以我非常感谢一些帮助。

例如，如果我在每次提示时输入值 50，我将在 while 循环后的两个 printf 方法中得到 200 的结果，就像它应该是因为 50+50+50+50 等于 200。但是在 Windows 中我在 total_weight 中得到 240，在 total_height 中得到 100，这对我来说没有任何意义。这是什么原因？

#include <stdio.h>
#include <stdlib.h>

int main()
{
int weights[3], heights[3], i = 0;
int total_weight, total_height;

while (i <= 3)
{
    printf("Person %d, enter your weight (kg): ",i+1);
    scanf("%d",&weights[i]);
    printf("Person %d, enter your height (cm): ",i+1);
    scanf("%d", &heights[i]);

    //Calculations for total weight and total length of all the people
    total_weight = total_weight + weights[i];
    total_height = total_height + heights[i];

    printf("\n\tPerson %d's weight and height: %dkg, %dcm\n\n", i+1, weights[i], heights[i]);
    i++;
}

//Printing the results of the earlier calculations
printf("\tTotal weight of everyone: %d\n", total_weight);
printf("\tTotal height of everyone: %d\n", total_height);

return 0;

}

Answer 1

你的数组溢出了。您在每次购买中输入 4 个值，仅分配 3 个空间。

Answer 2

您正在使用未初始化的total_height、total_weight：

int total_weight, total_height; 

/* ... */
total_weight = total_weight + weights[i];
total_height = total_height + heights[i];

它们应该用零初始化：

int total_weight = 0, total_height = 0; 

另外，当i == 3 你试图从weights / heights 数组中访问内存时。可以通过两种方式解决：

1. 不允许通过条件更正访问第四个元素：while(i < 3) {
2. 声明更强大的数组int weights[4], heights[4];

Answer 3

在你的代码中，通过说

 while (i <= 3)

你要去off-by-one。这会调用undefined behavior，用于越界内存访问。

详细地说，C 数组具有从 0 开始的索引。因此对于一个维度为 3 的数组，其有效索引范围为 0 到 2，包括 0 到 2。

所以，你应该使用

while (i < 3)

保持在界限内。

也就是说，在您的代码中，total_weight 和 total_height 是自动局部变量，它们在未初始化的情况下使用。因此，它们包含不确定的值，使用它们会再次导致 UB。您必须将它们显式初始化为 0。

Answer 4

在 Linux（基于 Debian，如 Ubuntu）上编译您的代码

gcc-4.9 -O3 -g3  -W -Wall -Wextra  -std=c11 stackover.c -o stackover

导致以下警告：

stackover.c: In function ‘main’:
stackover.c:12:5: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d",&weights[i]);
     ^
stackover.c:14:5: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d", &heights[i]);
     ^
stackover.c:17:42: warning: iteration 3u invokes undefined behavior [-Waggressive-loop-optimizations]
     total_weight = total_weight + weights[i];
                                          ^
stackover.c:9:7: note: containing loop
 while (i <= 3)
       ^
stackover.c:18:18: warning: ‘total_height’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     total_height = total_height + heights[i];
                  ^
stackover.c:17:18: warning: ‘total_weight’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     total_weight = total_weight + weights[i];

考虑到这些，我们可以将代码更改为

#include <stdio.h>
#include <stdlib.h>

int main()
{
  int weights[4], heights[4], i = 0;
  int total_weight = 0, total_height = 0;
  int res;
  // four persons
  while (i <= 3) {
    printf("Person %d, enter your weight (kg): ", i + 1);
    res = scanf("%d", &weights[i]);
    if(res != 1){
      fprintf(stderr,"Only one person, thank you\n");
      exit(EXIT_FAILURE);
    }
    printf("Person %d, enter your height (cm): ", i + 1);
    res = scanf("%d", &heights[i]);
    if(res != 1){
      fprintf(stderr,"Only one person, thank you\n");
      exit(EXIT_FAILURE);
    }
    //Calculations for total weight and total length of all the people
    total_weight = total_weight + weights[i];
    total_height = total_height + heights[i];

    printf("\n\tPerson %d's weight and height: %dkg, %dcm\n\n", i + 1,
           weights[i], heights[i]);
    i++;
  }

  //Printing the results of the earlier calculations
  printf("\tTotal weight of everyone: %d\n", total_weight);
  printf("\tTotal height of everyone: %d\n", total_height);

  exit(EXIT_SUCCESS);   
}