矩阵复制导致分段错误

Question

I must write a function in C that takes in a matrix (src) and 2 integer values(x,y), then gives out a matrix which contains src x by y times. For example

3 5
2 1

with (2,3) is going to be

3 5 3 5    
2 1 2 1    
3 5 3 5    
2 1 2 1    
3 5 3 5    
2 1 2 1

I am given the structure

struct Mat {
  int cols; // number of columns
  int rows; // number of rows
  int** row_ptrs; // pointer to rows (the actual matrix)
} Mat;

and wrote this function:

#include "exercise_1.h"
#include <stdlib.h>

Mat* matrixRepeat(Mat* src, int num_row_blocks, int num_col_blocks) 
{
  Mat *newMat = malloc(sizeof(Mat));
  newMat->rows = src->rows * num_row_blocks;
  newMat->cols = src->cols * num_col_blocks;

  newMat->row_ptrs = calloc(newMat->rows, sizeof(int*));

  for(int i = 0; i < newMat->cols; i++)
    newMat->row_ptrs[i] = calloc(newMat->cols, sizeof(int));


  for(int i = 0; i < newMat->rows; i++)
    for(int j = 0; j< newMat->cols; j++)
      newMat->row_ptrs[i][j] = src->row_ptrs[i%src->rows][j%src->cols];

  return newMat;
}

Then I am given some test programs: half of them works just fine, the other tough gives me segfault. I know for sure that the tests are correct, so there must be a problem in my program. Can you help me find it?

Answer 1

The condition in the loop

  for(int i = 0; i < newMat->cols; i++)
                     ^^^^^^^^^^^
    newMat->row_ptrs[i] = calloc(newMat->cols, sizeof(int));

is wrong. There must be

  for(int i = 0; i < newMat->rows; i++)
                     ^^^^^^^^^^^
    newMat->row_ptrs[i] = calloc(newMat->cols, sizeof(int));

Note: I think you mean

typedef struct Mat {
^^^^^^^
  int cols; // number of columns
  int rows; // number of rows
  int** row_ptrs; // pointer to rows (the actual matrix)
} Mat;