C-关闭一个错误，但没有分段错误？ [英] C - Off by one error, but no segmentation fault?

问题描述

我最近在C语言中编写了此代码：

I recently wrote this code in C:

#include <stdio.h>

#define N_ROWS 100


int main() {

  char *inputFileName = "triangle_data.txt";
  FILE *inputFile = fopen(inputFileName, "r");

  if (inputFile == NULL) {
    printf("ERROR: Failed to open \"%s\".\n", inputFileName);
    return -1;
  }

  int triangle[(N_ROWS*(N_ROWS+1))/2 - 1];
  size_t size = sizeof(triangle)/sizeof(int);
  size_t index;


  for (index = 0; !feof(inputFile); ++index) {
    fscanf(inputFile, "%d", &triangle[index]);
  }

  return 1;
}

，并期望出现分段错误，因为 N_ROWS *（N_ROWS + 1））/ 2 足以容纳文件中的数据，但是如您所见，我将数组做成了一个元素较小。不知何故，这不会触发细分错误。如果我将 for 循环的主体替换为：

and was expecting a Segmentation Fault, since N_ROWS*(N_ROWS+1))/2 is just enough space to hold the data in the file, but as you can see I made the array one element smaller. Somehow this doesn't trigger a segmentation fault. It does if I replace the body of the for-loop with:

int tmp;
fscanf(inputFile, "%d", &tmp);
triangle[index] = tmp;

这里发生了什么。如果将数组中的三个元素缩小，它仍然不会触发分段错误。五个要素可以触发一个。我确定文件中有足够的数据。

What is happening here. If I make the array three elements to small it still doesn't trigger a segmentation fault. Five elements to small trigger one. I'm sure there is enough data in the file.

作为测试，我随后打印了该数组，如果选择较小的数组，则缺少元素。

As a test I printed the array afterwards and if I choose a smaller array there were elements missing.

PS：在OS X上与 clang 一起编译。

PS: Compiled with clang on a OS X.

推荐答案

这是怎么回事？

What is happening here?

当您在数组对象之外进行编写时，您的程序会调用未定义的行为。 C语言中未定义的行为是未定义的，您的程序可以在今天运行，并在所有其他天内崩溃，甚至可以打印莎士比亚的完整著作。

Your program invokes undefined behavior as you are writing outside your array object. Undefined behavior in C is undefined, your program can work today and crash all the other days or even print Shakespeare complete works.

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