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

我最近用 C 写了这段代码:

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".
", 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;
}

并且期待 Segmentation Fault，因为 N_ROWS*(N_ROWS+1))/2 只是足够的空间来保存文件中的数据，但是当你可以看到我将数组缩小了一个元素.不知何故，这不会触发分段错误.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.如果我使数组三个元素变小，它仍然不会触发分段错误.五要素小触发一.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，我后来打印了数组，如果我选择了一个较小的数组，则会丢失元素.

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

这里发生了什么?

PS:在 OS X 上使用 clang 编译.

PS: Compiled with clang on a OS X.

推荐答案

这里发生了什么?

What is happening here?

当您在数组对象之外写入时，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其他日子崩溃，甚至可以打印莎士比亚全集.

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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