stack smashing detected and and with the flag -Wall gives me the Warning -Wchar-subscripts

Question

it s my first program ever i dont know how to solve this problems i'm searching for tutorials but i don't totally understand what i am doing wrong.

void railFenceCipher(char *stringa){
 
  char railFence[3][56];
  char output[56];
  char k;

  for (char i = 0; i < 3; i++) {
    for (char j = 0; j < 56; j++) {
      railFence[i][j] = ' ';
    }
  }

  for (i = 0; i < 3; i++) {
    for (j = i; j < 56; j += 4) {
      railFence[i][j] = stringa[k++];
    }
  }

  k = 0;
  for (i = 0; i < 3; i++) {
    for (j = 0; j < 56; j++) {
      if (railFence[j][i] != ' ') {
        output[k++] = railFence[j][i];
      }
    }
  }

  output[k] = '\0';
  printf("Encrypted message: %s\n", output);

}

this should be a function that take a string from the main function and encrypt it with the rail fence cipher. but i don't know how to solve the errors stack smashing detected and the warning -Wchar-subscripts

Answer 1

This is a major problem:

char railFence[3][56];

// ...

for (i = 0; i < 3; i++) {
  for (j = 0; j < 56; j++) {
    if (railFence[j][i] != ' ') {
      output[k++] = railFence[j][i];
    }
  }
}

The indexes i and j are switched in these loops.