C string pointer initializes itself then de-initializes itself? - c

Question

This is an excerpt from a Conway's Game of Life-program that I'm writing. In this part I'm trying to get the program to read a file that specifies what cells are to be populated at the start of the game (i.e. the seed).
I get a weird bug. In the read_line function, the program crashes online[i++] = ch statement. When I debug the program, I see that the line-pointer is NULL when it crashes. Fair enough, I think, I should initialize line. But here is the (for me) strange part:
The read_line function has already successfully execute twice and got me the first two lines (4\n and 3 6\n) from the seed file. And when I look at the execution in the debugger, I see that line is indeed holding a value in those first two executions of read_line. How is this possible? How can line be initialized without me initializing it and then suddenly not be initialized anymore?
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#define MAX_COORDINATE_SIZE 50
#define MAX_FILENAME_SIZE 20
#define MAX_GENERATIONS 10
#define MAX_REPETITION_PERIOD 4
struct coord{ //Holds coordinates to a cell
int x;
int y;
};
struct cell{
int pop; //Populated
int age;
};
struct coord *read_init(FILE *fp, int *i);
static int read_line(FILE *fp, char *line, int max_length);
struct coord read_coords(char *line);
struct cell **create_board(int x, int y);
struct cell **start_game(FILE *fp, int nrows, int ncols);
struct cell new_cell(int x, int y, int pop, int age);
void print_board(struct cell **board, int nrows, int ncols);
void populate_board(struct coord *coords, struct cell ***board, int *n);
int main(int argc, const char * argv[]) {
int gens;
char gens_string[MAX_GENERATIONS];
if(argc != 3){
fprintf(stderr, "Usage: %s <seed-file> <generations>\n<seed-file> can me up to %d characters long\n", argv[0], MAX_FILENAME_SIZE);
exit(1);
}
FILE *fp = fopen(argv[1], "r");
strncat(gens_string, argv[2], MAX_GENERATIONS);
gens = atoi(gens_string);
int nrows = 10;
int ncols = 10;
struct cell **board= start_game(fp, nrows, ncols);
print_board(board, nrows, ncols);
return 0;
}
struct coord *read_init(FILE *fp, int *n){ //Takes in filename and returns list of coordinates to be populated
char raw_n[100];
struct coord *coords;
char *line;
read_line(fp, raw_n, 100); // get the first line of the file (number of popuated cells)
*n = atoi(raw_n);//make an int out of raw_n
coords = malloc(sizeof(struct coord)*(*n)); //Allocate memory for each coord
for(int i = 0; i<(*n); i++){ // for each line in the file (each populated cell)
read_line(fp, line, MAX_COORDINATE_SIZE);
coords[i] = read_coords(line); //Put coordinates in coords
line = '\0';
}
return coords; // return coordinates
}
static int read_line ( FILE *fp, char *line, int max_length)
{
int i;
char ch;
/* initialize index to string character */
i = 0;
/* read to end of line, filling in characters in string up to its
maximum length, and ignoring the rest, if any */
for(;;)
{
/* read next character */
ch = fgetc(fp);
/* check for end of file error */
if ( ch == EOF )
return -1;
/* check for end of line */
if ( ch == '\n' )
{
/* terminate string and return */
line[i] = '\0';
return 0;
}
/* fill character in string if it is not already full*/
if ( i < max_length )
line[i++] = ch;
}
/* the program should never reach here */
return -1;
}
struct coord read_coords(char *line){ // Returns coordinates read from char *line
struct coord c;
char *x;
char *y;
x = malloc(sizeof(char)*MAX_COORDINATE_SIZE);
y = malloc(sizeof(char)*MAX_COORDINATE_SIZE);
int i = 0;
do{
x[i] = line[i]; //Get the x coordinate
i++;
}while(line[i] != ' ');
i++;
do{
y[i-2] = line[i];
i++;
}while(line[i] != '\0');
c.x = atoi(x)-1;
c.y = atoi(y)-1;
return c;
}
void init_board(int nrows, int ncols, struct cell ***board){
*board = malloc(nrows * sizeof(*board) + nrows * ncols * sizeof(**board));
//Now set the address of each row or whatever stackoverflow says
struct cell * const firstrow = *board + nrows;
for(int i = 0; i < nrows; i++)
{
(*board)[i] = firstrow + i * ncols;
}
for(int i = 0; i < nrows; i++){ //fill the entire board with pieces
for(int j = 0; j < ncols; j++){
(*board)[i][j] = new_cell(i, j, 0, 0);
}
}
}
void print_board(struct cell **board, int nrows, int ncols){
printf("--------------------\n");
for(int i = 0; i<nrows; i++){
for(int j = 0; j<ncols; j++){
if(board[i][j].pop == 1){
printf("%d ", board[i][j].age);
}else if(board[i][j].pop == 0){
printf(" ");
}else{
printf("\n\nERROR!");
exit(0);
}
}
printf("\n");
}
printf("--------------------");
printf("\n");
}
struct cell **start_game(FILE *fp, int nrows, int ncols){ //x,y are no of rows/columns, fn is filename
int n; // n is the number of populated cells specified in the seed
struct coord *coords = read_init(fp, &n); // get the list of coords to populate board with
struct cell **board;
init_board(nrows, ncols, &board); // Set up the board
populate_board(coords, &board, &n); //populate the cells specified in the seed
return board;
}
void populate_board(struct coord *coords, struct cell ***board, int *n){
for(int i = 0; i < *n; i++){
(*board)[coords[i].x][coords[i].y].pop = 1; //populate the cell
}
}
struct cell new_cell(int x, int y, int pop, int age){ //Return new populated or non-populated cell with specified coordinates
struct cell c;
c.pop = pop;
c.age = age;
return c;
}
The seed file:
4
3 6
4 6
5 6
5 7
EDIT:
The error message: Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)
I shall add that if I add a line line = malloc(sizeof(char)*MAX_COORDINATE_SIZE+1) after the declaration of line in read_init, I still get the same error.

Answer 1

In read_init() :
struct coord *read_init(FILE *fp, int *n){
//...
char *line;
//...
for(int i = 0; i<(*n); i++) {
read_line(fp, line, MAX_COORDINATE_SIZE);
coords[i] = read_coords(line); //Put coordinates in coords
line = '\0'; // <<--- you set line to NULL here.
*line = 0; // this is what you wanted to do, is not necessary...
}
// ....
}

Answer 2

I get a weird bug.
I suggest asking some questions about the compiler output. We should never blindly ignore warnings, after all. Speaking of reading things, I think you're spending too long reading StackOverflow and not long enough reading K&R2e and doing the exercises. We'll come back to that.
In the read_line function, the program crashes on line[i++] = ch statement ... But here is the (for me) strange part: ... The read_line function has already successfully execute twice and got me the first two lines (4\n and 3 6\n) from the seed file
The C and C++ standards rationalise the concept of "undefined behaviour" for this class of errors that are computationally difficult to diagnose. In other words, because you made an error, the behaviour of your program is undefined. It isn't required that your malfunctioning code crash every time, as that would be defining the undefined; instead they leave this "undefined" and the first two times your erroneously code works (whatever that means), purely by coincidence that the uninitialised variable points somewhere accessible. Later on you assign line = '\0';, which changes line to be a null pointer, and then you try to assign into whatever that null pointer points at. That's more undefined behaviour.
How is this possible? How can line be initialized without me initializing it and then suddenly not be initialized anymore?
line isn't initialised; you're using it uninitialised, which happens to coincidentally work (but needs fixing), then you assign it to be a null pointer and dereference a null pointer (more UB that needs fixing). Such is the nature of undefined behaviour. Such is the nature of learning C by guessing. You need a book!
I shall add that if I add a line line = malloc(sizeof(char)*MAX_COORDINATE_SIZE+1) after the declaration of line in read_init, I still get the same error.
You need to fix all of the errors, not just the one. For assistance you could see the warnings/errors your compiler emits. I see more uninitialised access here:
char gens_string[MAX_GENERATIONS]; // <--- where's the initialisation??
// Snip
strncat(gens_string, argv[2], MAX_GENERATIONS); // Boom
There's some really sus code around this comment: //Now set the address of each row or whatever stackoverflow says ... and on that note I want to point out that there are some subtly toxic know-it-alls who answer questions despite having as many misunderstandings as you, a humble person, and so you shouldn't hope to get the same quality of education from StackOverflow as you would from K&R2e... but apparently I'd be toxic for pointing out the egomaniacs and suggesting decent resources to learn from, so that's none of my business 🙄🤷‍♂️ let's just let the sociopaths sabotage the education of everyone huh?
(*board)[i] = firstrow + i * ncols;
Look, there is no guarantee that this even compiles let alone that the address on the right has a suitable alignment to store the type of value on the left. Misaligned access causes more undefined behaviour, which may also work coincidentally rather than logically. Just as you've never seen alignment violations before, so too has the person who suggested you use this code. Assuming the alignment requirements for your implementation are satisfied by this code, we then have the same questions to raise here:
(*board)[i][j] = new_cell(i, j, 0, 0);
Your whole program needs remodelling around the declaration of board changing from struct cell **board to struct cell (*board)[ncols];, for example. It'll become much simpler, and a whole class of bugs related to alignment requirements will disappear. To see the extent of the simplification, here's what your init_board ought to look like:
void init_board(int nrows, int ncols, struct cell (**board)[ncols]){
*board = malloc(nrows * sizeof(*board));
// NOTE: I snipped the erroneous StackOverflow logic around the comment mentioned above; you don't need that crud because of the more suitable choice of type
for(int i = 0; i < nrows; i++){ //fill the entire board with pieces
for(int j = 0; j < ncols; j++){
(*board)[i][j] = (struct cell){ 0, 0 };
}
}
}