在C叉搜索 [英] Searching with fork in C

问题描述

我应该越来越舒适叉了，我看到了一个锻炼的说要用叉子调用搜索索引从0到15我们假设每个进程只能做两件事情的数组。 ..（1）是检查，看看是否阵列是长度为1，和（2）比较的阵列的数目的单个元素被搜索。基本上我传递一个数值，它应该做叉的数量有限，并返回该数字的索引。这里是我的code ..

 的#define MAXINDEX 16INT forkSearch（int类型的[]，诠释搜索，诠释开始，诠释完）{
  如果（开始==结束）{
    如果（*（A + END）==搜索）{
      返回结束;
    }
  }
  其他{
    将为pid_t孩子= fork（）的;
    如果（孩子== 0）返回forkSearch（一，搜索，开始，结束/ 2）;
    否则返回forkSearch（一个，搜索，（START +端）/ 2，端）;
  }
}INT主（INT ARGC，CHAR *的argv []）{
  INT searchArray [MAXINDEX] = {1，12，11，5，10，6,4，9，13，2,8，14，如图3所示，\\
                               15，7};
  的printf（应该是1。12 =％d个\\ n个指数，forkSearch（searchArray，
                                                       12，0，MAXINDEX））;
  返回0;
}
 

在此迅速爆炸计划的回归似乎一切都可以是1，10，11，或13为什么没有这方面的工作像它应该。

解决方案

 如果（孩子== 0）返回forkSearch（一，搜索，开始，结束/ 2）;
 

这是错误的结束在那里，它应该是（启动+前端）/ 2 ，并开始索引在右半部分搜索应该是（开始+结束）/ 2 + 1 。否则，如果右半边是（开始+结束）/ 2 ..结束，当末开始== + 1 的启动的递归调用是老启动价值，你有一个无限循环。

您计划未定义的行为，因为

  INT forkSearch（int类型的[]，诠释搜索，诠释开始，诠释完）{
  如果（开始==结束）{
    如果（*（A + END）==搜索）{
      返回结束;
    }
  }
 

不返回值，如果开始==结束，但 *（A + END）！=搜索。添加退出（0）; 如果退出未找到目标进程

  INT searchArray [MAXINDEX] = {...};forkSearch（searchArray，12，0，MAXINDEX）
 

将导致超出边界访问在 searchArray [MAXINDEX] ，也未定义的行为。

I'm supposed to be getting comfortable with fork, and I saw an exercise that said to use a fork call to search an array indexed from 0 to 15. We're to assume that each process can only do two things...(1) is to check to see if an array is length 1, and (2) compare a single element of an array to the number were searching for. Basically i pass it a number, and its supposed to do a finite number of forks and return the index of that number. Here's my code..

#define MAXINDEX 16

int forkSearch(int a[], int search, int start, int end){
  if(start == end){
    if(*(a + end) == search){
      return end;
    }
  } 
  else{
    pid_t child = fork();
    if(child == 0) return forkSearch(a, search, start, end/2);
    else return forkSearch(a, search, (start + end)/2, end);
  }
}

int main(int argc, char* argv[]){
  int searchArray[MAXINDEX] = {1, 12, 11, 5, 10, 6, 4, 9, 13, 2, 8, 14, 3,\
                               15, 7};
  printf("Should be 1. Index of 12 = %d\n", forkSearch(searchArray,
                                                       12, 0, MAXINDEX));
  return 0;
} 

Everything in the return of this quickly exploding program seems to be either 1, 10, 11, or 13. Why isn't this working like it should.

解决方案

if(child == 0) return forkSearch(a, search, start, end/2);

That's the wrong end there, it should be (start+end)/2, and the start index of the search in the right half should be (start+end)/2 + 1. Otherwise, if the right half is (start+end)/2 .. end, when end == start+1, the start for the recursive call is the old start value and you have an infinite loop.

Your programme has undefined behaviour because

int forkSearch(int a[], int search, int start, int end){
  if(start == end){
    if(*(a + end) == search){
      return end;
    }
  }

doesn't return a value if start == end, but *(a+end) != search. Add an exit(0); after the inner if to exit the processes that didn't find the target.

int searchArray[MAXINDEX] = {...};

forkSearch(searchArray, 12, 0, MAXINDEX)

will lead to an out-of-bounds access at searchArray[MAXINDEX], also undefined behaviour.

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