尝试将数组从stdin复制到2d数组时，为什么会出现分段错误？

Question

This is in relation to the HackerRank Restaurant problem, which I have solved in other languages but am trying, now, to solve in C. https://www.hackerrank.com/challenges/restaurant

I first tried to store the result if read_slice_dimension() directly into the multidimensional array, but because I cannot pass an array back from a function without making it static, this does not work because then every nested array points to the same static 2 integer array in memory, which, as a result of it being overwritten every time read_slice_dimension() is called, means I would have an array containing num_slices pointers to the last array read in from stdin.

Thus my decision to try memcpy , so that I could copy the arry from read_slice_dimension() to a new block of memory so that it persists and is not lost when I read in the next slice. It appears, however, that memcpy is not the way to do this. What is?

// Gets the number of slices for this test according to the first input value from stdin.
int read_num_slices() {
  int num_slices = 0;

  scanf("%i", &num_slices);

  if (num_slices == 0) {
    goto error;
  }

  return num_slices;

error:
  flag_error("ERROR: Could not parse the number of entries from first input line.");
}

// Gets a single line from stdin and attempts to parse it into a 2D int array representing the dimensions of a slice.
int* read_slice_dimension() {
  static int slice_dimension[2] = {0};

  scanf("%i %i", &slice_dimension[0], &slice_dimension[1]);

  if (slice_dimension[0] + slice_dimension[1] == 0) {
    goto error;
  }

  return slice_dimension;

error:
  flag_error("ERROR: Could not parse line entered into a 2 integer array representing the slice's dimensions.");
}

// Gets all of the bread slices to be processed.
//
// This function reads from stdin.  The first line should be a single integer that specifies the number of slices to be
// processed by this current test.  The subsequent lines should be two integers separated by a space which represent
// the 2D dimensions of each slice.
int** get_slices() {
  int num_slices = read_num_slices();
  static int** slices;
  slices = (int**)malloc(num_slices * sizeof(int*));

  int i = 0;
  for (i; i < num_slices; i++) {
    int* slice = slices[i];
    slice = (int*)malloc(2 * sizeof(int));
    memcpy(slice, read_slice_dimension(), 2 * sizeof(int));
    printf("%i %i\n", slices[i][0], slices[i][1]); // CAUSES SEGMENTATION FAULT
  }

  return slices;
}

Answer 1

You're setting slice = slices[i] , then calling malloc . That's backwards. Call malloc , then set slices[i] . – user3386109