分段错误双指针重新分配

Question

I've got a small part of simple code that keeps giving me segmentation fault each time I go through the realloc for loop in the second if statement.

  *x = (int**)realloc(*x,nNew*sizeof(int*));

   for(int i=0;i<n;i++) 
      x[i] = (int*) realloc(x[i],nNew*sizeof(int));  // seg fault

I think that the memory allocation has been done correctly (apparently not) but I cannot find the error.In the first if statement (where I de-allocate memory) it works correctly.

Sorry for the bad format.

void update(int **x,int n,int nNew)
{
   int  temp;    

   if(n>nNew)
   {
     *x = (int **) realloc(*x,nNew*sizeof(int*));

     for(int i=0;i<n;i++)
       *(x+i) =(int *) realloc(x[i],nNew*sizeof(int));

     printf("temp %d",x[nNew-1][nNew-1]);
   }

   else if(n<nNew)
   {
      temp = x[n-1][n-1];

     *x = (int**)realloc(*x,nNew*sizeof(int*));

     for(int i=0;i<n;i++) 
       x[i] = (int*) realloc(x[i],nNew*sizeof(int));   // seg fault

     printf("temp %d",**temp);
     for(int i=n;i<(nNew);i++)
     {
        for(int j=n;j<(nNew);j++)
          x[i][j] = temp;
     }
   }
} 

int main()
{
  int nNew = 9;
  int n = 8;

  int **x = (int **) malloc(n*sizeof(int*));

  for (int i=0; i<n; i++) 
    x[i] = (int *)malloc(n * sizeof(int)); 


  int count=0 ;
  for(int i=0;i<n;i++)
  {
     count ++;
     for(int j=0;j<n;j++)
       x[i][j] = count;
  } 

  update(x,n,nNew);

  for(int i=0;i<nNew;i++)
    free(x[i]);
  free(x);

  return 0; 
}

Answer 1

You're not reallocating what you think you are.

When you do this:

*x = (int**)realloc(*x,nNew*sizeof(int*));

for(int i=0;i

Since x has type int ** , *x has type int * . Also, *x is the same as x[0] . So you're not reallocating the array of pointers on the first line but the first array of int . You need to call realloc on the original pointer, and you need to pass the address of that pointer so that the change is visible in the calling program.

Then for the reallocation of the individual arrays, you need to call realloc on the original n to grow or shrink them, then either free the extra rows if growing or malloc the newly added rows. In the growing case you want to realloc the array of pointers fisrt, and in the shrinking case you want to do that last.

So after those changes your update function should look like this:

void update(int ***x, int n, int nNew)
{
    if(n>nNew) {
        // shrink each row to be kept
        for(int i=0;i<nNew;i++)
           (*x)[i] = realloc((*x)[i],nNew*sizeof(int));

        // free the extra rows
        for(int i=nNew;i<n;i++)
            free((*x)[i]);

        // shrink the list of rows
        *x = realloc(*x,nNew*sizeof(int*));

    } else if(n<nNew) {
        // grow the list of rows
        *x = realloc(*x,nNew*sizeof(int*));

        // grow the existing rows
        for(int i=0;i<n;i++)
            (*x)[i] = realloc((*x)[i],nNew*sizeof(int));

        // create the new rows
        for(int i=n;i<nNew;i++)
            (*x)[i] = malloc(nNew*sizeof(int));
    }
}

And you call it like this:

update(&x,n,nNew);