Token string cuts off when inserting into 2d array in C

Question

I am correctly tokenizing single words from a line of strings; however, inserting them into a 2d array cuts off parts of the token. I also have a problem with NULL, and the code results in a segfault.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>   // strtok

#define MAX_FILE_LENGTH 30
#define MAX_COURSE_LENGTH 30
#define MAX_LINE_LENGTH 1000

void trim(char* str) {
  int l = strlen(str);
  if (str[l - 1] == '\n') {
    str[l - 1] = 0;
  }
}

int main() {
  char filename[MAX_FILE_LENGTH]; 
  char arr[MAX_COURSE_LENGTH][MAX_COURSE_LENGTH];
  const char delim[] = " ";
  char* token;
  int course = 0;
  char c;
  FILE* fp;
  int N = 0;    // number of lines in file

  printf("This program will read, from a file, a list of courses and their prerequisites and will print the list in which to take courses.\n");
  printf("Enter filename: ");
  scanf("%s%c", filename, &c);

  fp = fopen(filename, "r");

  if (fp == NULL) {
    printf("Could not open file %s. Exit\n", filename);
    printf("\nFailed to read from file. Program will terminate.\n");
    return -1;
  }

  while (!feof(fp) && !ferror(fp)) {
    int i = 0;
    if (fgets(arr[N], MAX_LINE_LENGTH, fp)) {
      trim(arr[N]);
      printf("Full line: |%s|\n", arr[N]);
      token = strtok(arr[N], delim);
      arr[N][i] = *token;
      printf("N = %d, i = %d, token = %s arr[%d][%d]: %s\n", N, i, token, N, i, &arr[N][i]);
      while (token != NULL) {
        i++;
        token = strtok(NULL, " \n");
        printf("token at arr[%d][%i]: %s value at arr[%d][%d]: %s\n", N, i, token, N, i, &arr[N][i]);
        arr[N][i] = *token;
        printf("N = %d, i = %d, token = %s arr[%d][%d]: %s\n", N, i, token, N, i, &arr[N][i]);
      }
      N++;
    }
  }
  fclose(fp);
  return 0;
}

The output I'm getting reads:

Full line: |c100 c200|
N = 0, i = 0, token = c100 arr[0][0]: c100
token at arr[0][1]: c200 value at arr[0][1]: 100
N = 0, i = 1, token = c200 arr[0][1]: c00
token at arr[0][2]: (null) value at arr[0][2]: 00
zsh: segmentation fault  ./a.out

My file is a list of courses and I am to build an adjacency matrix with the list of prerequisite courses.

c100 c200
c300 c200 c100
c200 c100

I tried to reset each index to NULL or '\0' before inserting the tokens, but the same result occurred. Inserting the first word in the [N][0]th index of the inner array works, but there is something I'm missing when inserting into other indexes of the inner array.

Answer 1

Much worse than all of my comments above, which could lead to undefined behavior, is the active and always UB you have by using %s to print a single character.

The assignment

arr[N][i] = *token;

assigns the single character in token[0] to the single character element arr[N][i].

You then use %s to print &arr[N][i], but the %s format expects a null-terminated string which you don't have.

To have a string you need an array of characters, and what you maybe should do is something like (but I'm just guessing):

char arr[MAX_COURSE_LENGTH][MAX_COURSE_LENGTH][MAX_COURSE_LENGTH];

and

strcpy(arr[N][i], token);

---

And as mentioned by Allan Wind, you're forgetting that strotk can (and will) return a NULL pointer.

Also note that all upper-case names are usually used for macros and symbolic constants. That makes it kind of confusing to see the name N being used as an index variable.