Why modifying two-dimensional array with function doesn't work?

Question

It is my code for working with two dimensional arrays.

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>

void print_array(double ** m, int n)
{
    int i, j;
    for (i = 0; i < n; i++) {
        for (j = 0; j < n; j++)
            printf("%.2f ", m[i][j]);

        printf("\n");
    }
    return;
}

double ** allocate_array(int n)
{
    double ** m = (double **)calloc(sizeof(double *), n);
    int i;
    for (i = 0; i < n; i++)
        m[i] = (double *)calloc(sizeof(double), n);

    return m;
}

void free_array(double ** m, int n)
{
    int i;
    for (i = 0; i < n; i++)
        free(m[i]);
    free(m);

    return;
}

// array_sum(result, sum1, sum2, dimensions)
void array_sum(double ** sum, double ** x1, double ** x2, int n)
{
    double **s;
    s = allocate_array(n);

    int i, j;
    for (i = 0; i < n; i++)
        for (j = 0; j < n; j++)
            s[i][j] = x1[i][j] + x2[i][j];

    memcpy(sum, s, sizeof(**s)*n);
    free_array(s, n);

    printf("hhh\n");
    print_array(sum, n);
    return;

}

int main()
{
    int n;
    FILE* f;
    f = fopen("array.txt", "r");
    fscanf(f, "%d", &n);

    double **m;
    m = allocate_array(n);

    int i; int j;
    for (i = 0; i < n; i++)
        for (j = 0; j < n; j++)
            fscanf(f, "%lf", &m[i][j]);

    //print_array(m, n);
    double **k;
    k = allocate_array(n);
    array_sum(k, m, m, n);
    //print_array(k, n);




    free_array(m, n);
    return 0;
}

Why freeing of s results that sum also freed and as result function doesn't return correct result? What I am should use for modifying objects like arrays os structs in function through pointer?

Answer 1

对于初学者这个陈述

memcpy(sum, s, sizeof(**s)*n);
               ^^^^^^^^^^^     

无效。

应该看起来像

memcpy(sum, s, sizeof( *s ) * n);
               ^^^^^^^^^^^^

此语句将指向s的数组中的指针复制到sum指向的数组。

然而在此声明之后

free_array(s, n);

这些指针变得无效，因为这些指针所指向的内存已被释放。

考虑到您分配了占用不同内存范围的n + 1个数组，并且无法使用memcpy复制所有这些数组。

只需写下

void array_sum(double ** sum, double ** x1, double ** x2, int n)
{
    int i, j;
    for (i = 0; i < n; i++)
        for (j = 0; j < n; j++)
            sum[i][j] = x1[i][j] + x2[i][j];

    printf("hhh\n");
    print_array(sum, n);
}

另外你应该添加声明

free_array(k, n);

主要是由k ..指向的空闲分配数组。