C中的二维数组指针访问分段错误

Question

我正在尝试用 C 语言编写康威生命游戏的代码。我在包含我的宇宙的二维数组方面遇到了一些问题。我将向您展示导致我出现问题的部分代码。实际上有两个问题我无法处理。

#include <stdio.h>
#include <stdlib.h>
#include "file2ppm.h"

typedef enum { false, true } bool;

void *allocateMemoryFor2DArray(int x, int y);                                   //allocate memory for 2D array
void initializeUniverseFromFile(char* path, int x, int y, int array[x][y]);     //set values of cells from a file   
void initializeUniverseRandomly(int x, int y, int array[x][y]);                 //set random values 
int getNumberOfColumns (FILE *file);                                            //get number of columns in file (yDim of Universe)
int getNumberOfLines (FILE *file);                                              //get number of lines in file (xDim of Universe)
void print2DArray (int x, int y, int array[x][y]);                              //print 2D array    
void setDimensionsFromFile(char* path,int* xDim, int* yDim, int* xDimB, int* yDimB);//set dimensions of Universe

int main(int argc, char * argv[])
{
    int i, j, n;        //loop indexes
    int nsteps;         //Number of generations
    int im, ip, jm, jp; //neighbour boundaries
    int yDim, xDim;     //Universe boundarier defined by user (Living area);
    int xDimB, yDimB;   //Universe boundaries expanded to each side by 1
    int nsum, isum;     //sum of neighbours, sum of alive cells after iteration
    //int **old, **new; //Universe 
    int (*old)[xDimB];      //Universe
    int (*new)[xDimB];      //Universe

    float x;            //to randomize first population
    char *inputPath;    //Path to file with ixDimBtial uxDimBverse
    char *outputPath;   //Path to output ppm files


    //temporary copy
    int yDim_copy, xDim_copy;

    printf("argc: %d\n", argc);
    switch (argc){
        case 4:         //Read the initial Universe from file
            //Take arguments
            nsteps = atoi(argv[1]); //Get number of iterations
            outputPath = argv[2];   //Get path for outputimages
            inputPath = argv[3];    //File with initialize uxDimBverse;
            setDimensionsFromFile(inputPath, &xDim, &yDim, &xDimB, &yDimB);
            old = allocateMemoryFor2DArray(xDimB, yDimB);   //Alocate memory for expanded Universes
            new = allocateMemoryFor2DArray(xDimB, yDimB);
            printf("Before initialize: yDim: %d, xDim: %d\n", yDim, xDim);      //Here the values are good
            //tmp copy
            yDim_copy = yDim;
            xDim_copy = xDim;
            initializeUniverseFromFile(inputPath, xDim, yDim, old);
            printf("After initialize: yDim: %d, xDim: %d\n", yDim, xDim);       //And here one dim is changed
            yDim = yDim_copy;       //I took a copy to avoid this problem for now
            xDim = xDim_copy;

            printf("After taking from copy: yDim: %d, xDim: %d\n", yDim, xDim);//Here dimensions are good again
            memcpy (new, old, xDimB*yDimB*sizeof(old[xDimB][yDimB]));           //Copy old to new
            break;


        default:
            printf("Usage: %s iter_number, output_name, yDim, xDim\n", argv[0]);
            printf("or\n");
            printf("Usage: %s iter_number, output_name, ixDimBtial_input_name\n", argv[0]);
            printf("Note: Initial file have to be in ./data/\n");
            return 1;
    }

    print2DArray(xDim, yDim, old);      //this works fine
    printf("In main: yDim: %d, xDim: %d\n", yDim, xDim);
    printf("%d\n", old[0][0]);          //this works fine
    printf("%d\n", old[2][2]);          //Segmentation failure

    return 0;
}

void *allocateMemoryFor2DArray(int x, int y){
    int (*array)[y] = malloc( sizeof(int[x][y]) );      //Allocate memory
    if (array == NULL) {                                /* always check the return of malloc */
        perror("malloc");
        exit(EXIT_FAILURE);
    }
    return array;
}

int getNumberOfColumns (FILE *file){
    int yDim = 0;
    char ch;

    do 
    {
        ch = getc(file);                            //Get 1 char from file

        if (ch == '\n')             //If ch is new line
        {
            rewind(file);           //Move file pointer to beginxDimBng of file
            return yDim;            //Found number of columns       
        }       
        else                
            ++yDim;

    }while (ch != EOF);
}

int getNumberOfLines (FILE *file){
    int xDim = 0;
    char ch;

    do  
    {
        ch = getc(file);                            //Get 1 char from file
        if(ch == '\n')                              //If new line
        {
            ++xDim;                             //Increase xDim (another row)
        }
    }while(ch != EOF);
    rewind(file);                                   //Move file pointer to beginxDimBng of file
    return xDim;
}


void print2DArray (int x, int y, int array[x][y])
{
    int i, j;
    printf("x: %d, y: %d\n", x, y);
    for (i = 1; i <= x; ++i) 
    {
        for (j = 1; j <= y; ++j) 
        {
            printf("%d ", array[i][j]);
        }
        printf("\n");
    }
}

void setDimensionsFromFile(char* path, int* xDim, int* yDim, int* xDimB, int* yDimB)
{
    FILE *file;
    int i,j;
    file = fopen(path, "r");        //open file 
    if (file)
    {
        *yDim = getNumberOfColumns (file);                  
        *xDim = getNumberOfLines (file);
        *xDimB = *xDim + 2;  // add 2 for left and right torus topology
        *yDimB = *yDim + 2;
        if (*xDim > 0 && *yDim > 0)
        {   
            printf("UxDimBverse dimension %d x %d\n", *xDim , *yDim);
        }           
        else
        { 
            perror("Wrong initialize file");
            exit(EXIT_FAILURE); 
        }       

        fclose(file);                       //Close file
    }
    else
    {
        perror("Open initialize file error");   
        exit(EXIT_FAILURE);
    }
}


void initializeUniverseFromFile(char* path, int x, int y, int array[x][y])
{ 
    FILE *file;
    int i,j;
    file = fopen(path, "r");        //open file 
    if (file)
    {
        //IxDimBtialize array
        char *ch;
        for (i = 1; i <= x; i++)
        {
            //printf("i: %d ", i);
            for (j = 1; j <= y; j++)
            {
                //printf("j: %d ", j);
                if (fscanf(file, "%c", ch) != 1)
                {
                    perror("Read error\n");
                }
                else if (isdigit((unsigned char)*ch))
                {
                    printf("%d", atoi(ch));
                    array[i][j] = atoi(ch);
                } 
                else 
                {
                    //printf("Numer znaku: %d ", ch);
                    j--;
                }
            }
            printf("\n");
        }
        printf("Done reading\n");
        fclose(file);           //Close file 
    } 
    else 
    {               //File open error
        perror("Open initialize file error");   
        exit (EXIT_FAILURE);
    }
    printf("In initialize yDim: %d, xDim: %d\n", x, y);
}

void initializeUniverseRandomly(int x, int y, int array[x][y])
{
    int i, j;
    float r;
    for (i = 1; i <= x; i++) {
        for (j = 1; j <= y; j++) {
            r = rand() / ((float)RAND_MAX + 1);
            if (r<0.5) {
                array[i][j] = i*x+j;
            }
            else {
                array[i][j] = i*x+j;
            }
        }
    }
}

问题 1： 我设置了宇宙的维度（变量 xDim 和 yDim）。我在第 45 行打印它们（检查注释//这里的值很好）。然后我打电话给initializeUniverseFromFile(inputPath, xDim, yDim, old); 并且其中一个维度值发生了变化。我不知道为什么。这就是我临时复制这个变量的原因，因为我花了 2 个小时来寻找那个错误。我什至在initializeUniverseFromFile 函数的末尾打印出这些变量，它们很好。但是当我再次回到 main 时（第 50 行），值发生了变化。这是小问题。更糟糕的是：

问题 2 我有一个指针 int (*old)[xDimB];

xDimB 是 xDim+2 所以它总是大于 xDim

我用函数初始化数组 allocateMemoryFor2DArray(xDimB, yDimB)

然后我这样做：

print2DArray(xDim, yDim, old);      //this works fine
printf("In main: yDim: %d, xDim: %d\n", yDim, xDim); //Dimensions are fine
printf("%d\n", old[0][0]);          //this works fine
for (i=0; i<=xDim; ++i)
     printf("%d ", old[0][i];       //this works fine too
printf("%d\n", old[2][2]);          //Segmentation failure

用函数打印整个数组很好 从第一行打印值很好 从数组中间打印值会产生错误。

Answer 1

我设置了宇宙的维度（变量 xDim 和 yDim）

您需要在声明 VLA int (*old)[xDimB]; 和 int (*new)[xDimB]; 之前包含有效数字。当 xDim 和 yDim 具有已知的、经过验证的值时，将这些 VLA 声明移到程序中的后一点。

Answer 2

有两件事我无法在代码中解释，这可能是导致分段错误的原因。

1)

void *allocateMemoryFor2DArray(int x, int y);

当你清楚地返回一个整数数组时为什么它是无效的。

2) 你如何在单个指针上返回一个二维数组？ 要定义一个二维数组，你需要一个双指针：

int** array = new int*[x];

然后你遍历 x 来定义第二个维度：

for (int i=0;i<x;i++)
    array[i] = new int[y];

你最终返回你的双指针，它是对你的数组的引用。

你的函数会变成这样：

int** allocateMemoryFor2DArray(int x, int y)
{

    int** array = new int*[x];
    for (int i=0;i<x;i++)
       array[i] = new int[y];
    return array;
}

类似这些功能：

void initializeUniverseFromFile(char* path, int x, int y, int array[x][y])
void initializeUniverseRandomly(int x, int y, int array[x][y])
void print2DArray (int x, int y, int array[x][y])

应该变成：

void initializeUniverseFromFile(char* path, int x, int y, int** array)
void initializeUniverseRandomly(int x, int y, int** array)
void print2DArray (int x, int y, int** array)

最后你在 main 中的指针也应该变成双指针。

第一行打印而其他行不打印的原因是您没有循环抛出第一个维度来初始化第二个维度中的数组。