C. 函数修改动态分配的二维数组时的段错误

Question

我需要的是一个像这样修改给定的二维矩阵指针的函数:

void intMatrixAll(int row, int col, int **matrix);

现在，函数应该分配内存并且可以使用矩阵。行和列在运行时给出。

#include <stdio.h>
#include <stdlib.h>
#define PRINTINT(X) printf("%d\n", X);
void intMatrixAll(int row, int col, int **matrix);

int main(void) {
   int testArrRow = 4;
   int testArrCol = 6;
   int **testMatrix = NULL;
   intMatrixAll(testArrRow, testArrCol, testMatrix);
   testMatrix[2][2] = 112; //sementation fault here :(
   PRINTINT(testMatrix[2][2]);
   system("PAUSE");
   return 0;
}

void intMatrixAll(int row, int col, int **matrix) {
   printf("intMatrixAll\n");
   //allocate pointers:
   matrix = malloc(row * sizeof(int *));
   if(matrix == NULL) printf("Failed to allocate memmory.\n");
   for(int i=0; i<row; i++) {
      //allocate space for cols: 
      matrix[i] = malloc(col * sizeof(int));
      if(matrix[i] == NULL) {
         printf("Failed to allocate memmory for arr[%d].\n", i);
         exit(0);
      }
   }
}

为什么会出现错误？

Answer 1

测试矩阵仍然是 NULL。您需要从 intMatrixAll() 返回新分配的指针。要么从函数返回值，要么传入 testMatrix 的地址以便对其进行设置。

#include <stdio.h>
#include <stdlib.h>
#define PRINTINT(X) printf("%d\n", X);
void intMatrixAll(int row, int col, int **matrix);

int main(void) {
   int testArrRow = 4;
   int testArrCol = 6;
   int **testMatrix = NULL;
   intMatrixAll(testArrRow, testArrCol, &testMatrix);
   testMatrix[2][2] = 112; //sementation fault here :(
   PRINTINT(testMatrix[2][2]);
   system("PAUSE");
   return 0;
}

void intMatrixAll(int row, int col, int ***matrix) {
   printf("intMatrixAll\n");
   //allocate pointers:
   *matrix = malloc(row * sizeof(int *));
   if(*matrix == NULL) printf("Failed to allocate memmory.\n");
   for(int i=0; i<row; i++) {
      //allocate space for cols: 
      *matrix[i] = malloc(col * sizeof(int));
      if(*matrix[i] == NULL) {
         printf("Failed to allocate memmory for arr[%d].\n", i);
         exit(0);
      }
   }
}