线程 1：EXC_BAD_ACCESS（代码=1，地址=0x0）内存分配

Question

I'm writing code for the addition of two matrics in Xcode Version 11.2 . The display of the initial matrics works, but the addition of them doesn't. The error appears at the line aloc3[i][j]=*(aloc1+i*m+j)+*(aloc2+i*m+j); in "add" function. I really don't know what I have done wrong and I have spent hours to try to fix the problem , but nothing seem to fix it. Can someone help me?

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


void add(int *aloc1,int *aloc2,int **aloc3,int m,int n);
void display(int *aloc,int m,int n);

int main() {
    int *aloc1,*aloc2,*aloc3;
    int m,n,i,j;

    printf("\n Enter the number of rows : ");
    scanf("%d",&m);

    printf("\n Enter the number of colomns : ");
    scanf("%d",&n);

    if ((aloc1=(int*)malloc(n*m*sizeof(int))))
    {
        printf("\n Enter the elements of the first matrix: ");

        for(i=0;i<m;i++)
            for(j=0;j<n;j++)
                scanf("%d",aloc1+i*m+j);
    }

    if ((aloc2=(int*)malloc(n*m*sizeof(int))))
    {
        printf("\n Enter the elements of the second matrix: ");

        for(i=0;i<m;i++)
            for(j=0;j<n;j++)
                scanf("%d",aloc2+i*m+j);
    }

    printf("\n The initial matrics are: \n");
    display(aloc1,m,n);

    printf("\n \n");
    display(aloc2,m,n);

    add(aloc1,aloc2,&aloc3,m,n);

    printf("\n The addition of the matrics is: \n");
    display(aloc3,m,n);

    return 0;
}

void add(int *aloc1,int *aloc2,int **aloc3,int m,int n)
{
    int i,j;

    for(i=0;i<m;i++)
      for(j=0;j<n;j++)
        aloc3[i][j]=*(aloc1+i*m+j)+*(aloc2+i*m+j);
}

void display(int *aloc,int m,int n)
{
    int i,j;
    for(i=0;i<m;i++)
    {
        for(j=0;j<n;j++)
            printf("%d ",*(aloc+m*i+j));

        printf("\n");
    }
}

Answer 1

You are passing a pointer to a single int* to add 's aloc3 parameter. Therefore accessing it with aloc3[i] for i>0 is undefined behavior, as aloc3 is not pointing to an array.

You probably intended to write (*aloc3)[i*m+j] , going by the indexing of the other parameters.

Even then, you never set aloc3 to any value in main , nor do you set any value to *aloc3 in add . Thus (*aloc3)[i*m+j] will be trying to dereference an indeterminate value, causing undefined behavior.

You seem to have forgotten a malloc for alloc3 , either in add or in main .