Why is this C code giving me a bus error?

Question

I have, as usual, been reading quite a few posts on here. I found a particular useful posts on bus errors in general, see here. My problem is that I cannot understand why my particular code is giving me an error.

My code is an attempt to teach myself C. It's a modification of a game I made when I learned Java. The goal in my game is to take a huge 5049 x 1 text file of words. Randomly pick a word, jumble it and try to guess it. I know how to do all of that. So anyway, each line of the text file contains a word like:

   5049
   must
   lean 
   better 
   program 
   now
   ...

So, I created an string array in C, tried to read this string array and put it into C. I didn't do anything else. Once I get the file into C, the rest should be easy. Weirder yet is that it complies. My problem comes when I run it with ./blah command.

The error I get is simple. It says:

zsh: bus error ./blah

My code is below. I suspect it might have to do with memory or overflowing the buffer, but that's completely unscientific and a gut feeling. So my question is simple, why is this C code giving me this bus error msg?

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

//Preprocessed Functions 
void jumblegame();
void readFile(char* [], int);


int main(int argc, char* argv[])
{
    jumblegame();

}

void jumblegame()
{
    //Load File 
        int x = 5049; //Rows
        int y = 256; //Colums
        char* words[x]; 
        readFile(words,x);

    //Define score variables 
        int totalScore = 0;
        int currentScore = 0; 

   //Repeatedly pick a random work, randomly jumble it, and let the user guess what it is

}

void readFile(char* array[5049], int x) 
{
    char line[256]; //This is to to grab each string in the file and put it in a line. 
    FILE *file;
    file = fopen("words.txt","r");

    //Check to make sure file can open 
    if(file == NULL)
    {
        printf("Error: File does not open.");
        exit(1);
    }
    //Otherwise, read file into array  
    else
    {
        while(!feof(file))//The file will loop until end of file
        {
           if((fgets(line,256,file))!= NULL)//If the line isn't empty
           {
               array[x] = fgets(line,256,file);//store string in line x of array 
               x++; //Increment to the next line 
           }    
        }
    }

}

Solution 1

This line has a few problems:

array[x] = fgets(line,256,file);//store string in line x of array 

• You've already read the line in the condition of the immediately preceding if statement: the current line that you want to operate on is already in the buffer and now you use fgets to get the next line.

• You're trying to assign to the same array slot each time: instead you'll want to keep a separate variable for the array index that increments each time through the loop.

• Finally, you're trying to copy the strings using =. This will only copy references, it won't make a new copy of the string. So each element of the array will point to the same buffer: line, which will go out of scope and become invalid when your function exits. To populate your array with the strings, you need to make a copy of each one for the array: allocate space for each new string using malloc, then use strncpy to copy each line into your new string. Alternately, if you can use strdup, it will take care of allocating the space for you.

But I suspect that this is the cause of your bus error: you're passing in the array size as x, and in your loop, you're assigning to array[x]. The problem with this is that array[x] doesn't belong to the array, the array only has useable indices of 0 to (x - 1).

Solution 2

You are passing the value 5049 for x. The first time that the line

array[x] = ... 

executes, it's accessing an array location that does not exist.

It looks like you are learning C. Great! A skill you need to master early is basic debugger use. In this case, if you compile your program with

gcc -g myprogram.c -o myprogram

and then run it with

gdb ./myprogram

(I am assuming Linux), you will get a stack dump that shows the line where bus error occurred. This should be enough to help you figure out the error yourself, which in the long run is much better than asking others.

There are many other ways a debugger is useful, but this is high on the list. It gives you a window into your running program.