尝试从2D数组中的字符串获取字符时出现分段错误

Question

I'm trying to get a character from a string in a 2D array. I currently have this code:

Board setData(Board b, char c, int i, int j) {
  printf("setData called\n char = %c, i = %d, j = %d\n", c, i, j);
  int index = getCharIndex(c);
  printf("Index is %d\n", index);
  char* data = b.cells[i][j];
  printf("Pre-Data = %s\n", data);
  printf("data[index] = %c\n", data[index]); // <---
  data[index] = c;
  printf("Post-Data = %s\n", data);
  b.cells[i][j] = data;
  printf("setData complete!\n");
  return b;
}

where Board is a typedef struct. All of the prints show the values as expected. However, a segmentation fault occurs when the marked line is executed. We know from the previous line that 'data' is a string, so why when trying to find a character from the string does it fail? This isn't because the value of index is outside the range of the string, it fails when index is 0 when the string consists of multiple characters. Any help is very much appreciated.

EDIT: mcve:

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

typedef struct {
    char *cells[10][10];
} Board;

Board newBoard() {
  Board board;
  for (int i = 0; i < 10; i++) {
    for (int j = 0; j < 10; j++) {
      board.cells[i][j] = "----";
    }
  }
  return board;
}

Board setData(Board b, int index, char c, int i, int j) {
  printf("setData:\nindex = %d\nchar = %c\ni = %d\nj = %d\n", index, c, i, j);
  char* data = b.cells[i][j];
  printf("data = %s\nSetting data[%d] to %c\n", data, index, c);
  data[index] = c;
  printf("Assigning data back to b.cells[%d][%d]", i, j);
  b.cells[i][j] = data;
  return b;
}

int main(int argc, char *argv[argc]) {
  Board b = newBoard();

  int index = atoi(argv[1]);
  char c = argv[2][0];
  int i = atoi(argv[3]);
  int j = atoi(argv[4]);

  setData(b, index, c, i, j);

}

which gives the output:

setData:
index = 0
char = A
i = 2
j = 2
data = ----
Setting data[0] to A
Segmentation fault

The desired result is that the '-' in the string within the cell array at the specified i,j gets changed at the index place to the given character.

Answer 1

The issue is that "----" , being a string literal, is most probably located in read-only (write-protected) memory block. If you're going to modify its contents, do the copy with eg strdup("----") instead, and don't forget to free the memory at some point.