c - 通过函数传递指针

Question

由于一个指针:p_return_index，我卡在了这个程序上。正如它所暗示的那样，它被纳入每个搜索功能，并且应该被赋予找到搜索号码的索引值。如果搜索的数字不在数组中，程序运行正常，但是当我输入数组中的数字时，出现段错误(核心转储)错误。传递指针一定是我做错了什么。因为它被赋予了索引号。我只是不知道如何解决这个问题。请注意，指针是用 NULL 初始化的；我认为这可能与它有关。显然，我不知道。

void show_data(int array[], int max_index, int searched_number);
/* Display the arrays contents on screen */
void search_results(char found, int *p_return_index);
/* Prints if data is found */
int ordered_seq_search(int array[], int max_index, int searched_number, int *p_return_index);
/* Ordered Sequential Search Algorithm */
int probability_search(int array[], int max_index, int searched_number, int *p_return_index);
/* Probability Search Algorithm */
int binary_search( int array[], int max_index, int searched_number, int *p_return_index);
/* Binary Search Algorithm */

/**********************************************************************/
/*                           Main Function                            */
/**********************************************************************/
int main()
{
    int searched_number,
    *p_return_index = NULL,
    max_index = MAX_INDEX,
    seq_data[DATA_SIZE],
    prob_data[DATA_SIZE],
    bin_data[DATA_SIZE];

    char found_status;

    /* Print the program heading and instructions                      */
    printf("\n\n\n\n\n\n\n");
    print_heading();
    print_instructions();

    /* Fill arrays with data                                           */
    fill_array(seq_data, max_index);
    fill_array(prob_data, max_index);
    fill_array(bin_data, max_index);

    while(printf("\n\n\nEnter an integer search target (%d to quit): ",
            QUIT), scanf("%d", &searched_number), searched_number != 0)
    {
        printf("\n");
        printf("\nOrdered Sequential Search:");
        show_data(seq_data, max_index, searched_number);

        if(ordered_seq_search(seq_data, max_index, searched_number, p_return_index) == 1)
        {
            found_status = 'S';
            search_results(found_status, p_return_index);
        }
        else
        {
            found_status = 'U';
            search_results(found_status, p_return_index);
        }

        printf("\n\n");
        printf("\nProbability Search:");
        show_data(prob_data, max_index, searched_number);
        if(probability_search(prob_data, max_index, searched_number, p_return_index) == 1)
        {
            found_status = 'S';
            search_results(found_status, p_return_index);
        }
        else
        {
            found_status = 'U';
            search_results(found_status, p_return_index);
        }

        printf("\n\n");
        printf("\nBinary Search:");
        show_data(bin_data, max_index, searched_number);
        if(binary_search(bin_data, max_index, searched_number, p_return_index) == 1)
        {
            found_status = 'S';
            search_results(found_status, p_return_index);
        }
        else
        {
            found_status = 'U';
            search_results(found_status, p_return_index);
        }
    }

    printf("\nThanks for searching. Have a nice day! ;-)");
    printf("\n\n\n\n\n");

    return 0;
}

/**********************************************************************/
/*                     Print the program heading                      */
/**********************************************************************/
void print_heading()
{
   printf("\n========================================================");
   printf("\n                Program Number: %d",    PROGRAM_NUMBER);
   printf("\n                Programmer: %s",        PROGRAMMER_NAME);
   printf("\n                PCC Course Number: %s", COURSE_NUMBER);
   printf("\n========================================================");
   return;
}

/**********************************************************************/
/*                   Print the program instructions                   */
/**********************************************************************/
void print_instructions()
{
    printf("\nThis program demonstrates various search algorithms.");
    printf("\nYou enter in any whole number, and the program will");
    printf("\nsearch for it in an ORDERED array of whole numbers");
    printf("\nusing each of the following search algorithms:");
    printf("\n     1. Ordered Sequential Search");
    printf("\n     2. Probability Search");
    printf("\n     3. Binary Search");
    printf("\nThe progress of each search is shown so the efficiency");
    printf("\nof the search algorithms can be compared.");
    return;
}

/**********************************************************************/
/* Fill the array */
/**********************************************************************/
void fill_array(int array[], int max_index)
{
   int index_value;

    for(index_value = 0; index_value <= max_index; index_value++)
        array[index_value] = (index_value * 5) + 10;
    return;
}

/**********************************************************************/
/* Show the data */
/**********************************************************************/
void show_data(int array[], int max_index, int searched_number)
{
   int count;

    printf("\n   Array Index: ");
    for (count = 0; count <= MAX_INDEX ; count++)
        printf("[%2d]", count);
    printf("\n    Array Data: ");
    for (count = 0; count <= MAX_INDEX; count++)
        printf(" %2d ", array[count]);
    printf("\n   User Target:  %2d", searched_number);
    return;
}


/*****************************************************************/
/* Found Stuff */
/************/
void search_results(char found, int *p_return_index)
{
    printf("\nSearch Outcome: ");
    if(found == 'S')
        printf("Successful - target found at index [%2d]", *p_return_index);
            //The run-time error happens here. So, it definitely has to be the pointer.
    if(found == 'U')
        printf("Unsuccessful - target not found");
    if(found != 'S' && found != 'U')
        printf("Undetermined");
    return;
}

/**********************************************************************/
/* Ordered Sequential Search */
/**********************************************************************/
int ordered_seq_search(int array[], int max_index, int searched_number, int *p_return_index)
{
    int index = 0;

    printf("\n   Search Path: ");
    while (printf("[%2d]", index),
             index < max_index && searched_number != array[index - 1] &&
             searched_number > array[index])
    {

        index++;
    }

    if(searched_number == array[index])
    {
        p_return_index = &index;

        printf("\n%d", *p_return_index);
            //This is just to show that `p_return_index` does get the right value.
        return 1;
    }
    else
        return 0;
}

/**********************************************************************/
/* Probability Search */
/**********************************************************************/
int probability_search(int array[], int max_index, int searched_number, int *p_return_index)
{
    int index = 0;
    int temp;

    printf("\n   Search Path: ");
    while (printf("[%2d]", index),
             index < max_index && searched_number != array[index])
    {

        index++;
    }


    if(searched_number == array[index])
    {
        p_return_index = &index;
        if(index > 1)
        {
            temp = array[index - 1];
            array[index - 1] = array[index];
            array[index] = temp;
            array = array - 1;
        }
        return 1;
    }
    else{
        return 0;
    }
}

/**********************************************************************/
/* Binary Search */
/**********************************************************************/
int binary_search( int array[], int max_index, int searched_number, int *p_return_index)
{
    int begin, end, middle;

    begin = 0;
    end = max_index;

    printf("\n   Search Path: ");

    while( begin <= end )
    {
        middle = (begin + end) / 2;
        if(searched_number > array[middle])
        {
            printf("[%2d]", middle);
            begin = middle + 1;

        }
        else if(searched_number < array[middle])
        {
            printf("[%2d]", middle);
            end = middle - 1;
        }
        else
        {
            printf("[%2d]", middle);
            begin = end + 1;
        }
    }

    if(searched_number == array[middle])
    {
        p_return_index = &middle;
        return 1;
    }
    else
        return 0;
}

帮助将不胜感激。

Answer 1

你正在做的是:

1. 您声明一个指针并为其分配 NULL。

2. 您将此 NULL 值传递给函数。

3. 该函数尝试读取指针后面的内存。由于指针仍存储地址 NULL，因此该函数尝试读取地址 NULL 处的内存，该地址不存在。

---

使用指针参数作为返回值的习惯用法是这样的:

1. 您可以像以前那样定义一个指针参数。

   void foo(int* myReturnArgument);
   

2. 在调用代码中，您定义了一个不是指针的变量。然后您获取该变量的地址并将其传递给函数。

   int bar;
   foo(&bar);
   

3. 在函数中，您操作指针指向的值。

   void foo(int* myReturnArgument) {
       *myReturnArgument = 42;
   }