Writing to char* array in C throws segfault - c

Question

I'm an infosec student, who has just begun learning C. I've been given an assignment to write a simple translation program using this prototype: char* dictionary[number_of_words][2];.
The output should look something like this:
Enter # of words to add: 3
dictionary[0][0]= plane
dictionary[0][1]= Flugzeug
dictionary[1][0]= house
dictionary[1][1]= Haus
dictionary[2][0]= cat
dictionary[2][1]= Katze
Enter english word to translate: house
---> Haus
My current (work in progress) code looks like this:
void clean_stdin(void);
int main(void)
{
unsigned int i = 0,j = 0,size;
printf("Enter # of words to add: ");
scanf("%u",&size);
clean_stdin(); //clears input buffer
char* dictionary[size][2];
while(i <= size)
{
printf("dictionary[%u][%u]= ",i,j);
fgets(dictionary[i][j],100,stdin);
if(j >= 1)
{
j = 0;
++i;
}
else
j = 1;
}
return 0;
}
The code throws no errors when compiled with gcc -Wall main.c, but this is the behaviour I get:
Enter # of words to add: 3
dictionary[0][0]= plane
dictionary[0][1]= Flugzeug
dictionary[1][0]= house
dictionary[1][1]= Haus
fish: “./a.out” terminated by signal SIGSEGV (Adressbereichsfehler)
*****
Enter # of words to add: 4
dictionary[0][0]= plane
fish: “./a.out” terminated by signal SIGSEGV (Adressbereichsfehler)
There's a fundamental flaw somewhere in my thought process. Any help/heads up is very appreciated. Cheers!

Answer 1

Let's look at this part of your code:
char* dictionary[size][2];
while(i <= size)
{
printf("dictionary[%u][%u]= ",i,j);
fgets(dictionary[i][j],100,stdin);
Each dictionary[i][j] is a pointer to char, but you haven't set them to point anywhere meaningful yet - each array element contains some random bit pattern that may or may not correspond to a writable address. Your first few entries get written somewhere, but eventually you try to write to a memory location you don't own or don't have access to.
You will need to set aside additional memory to store each individual string. You either need create a 3D array of char (not char *):
#define MAX_STR_LEN 100
...
char dictionary[size][2][MAX_STR_LEN+1];
or you will need to dynamically allocate memory for each array entry:
while ( i < size ) // <, not <=
{
dictionary[i][j] = malloc( sizeof *dictionary[i][j] * (MAX_STR_LEN + 1));
if ( !dictionary[i][j] )
{
// memory allocation failed, handle error
}
printf("dictionary[%u][%u]= ",i,j);
fgets(dictionary[i][j],MAX_STR_LEN,stdin);
If you allocate memory dynamically, you will need to explicitly free it when you're done:
for ( i = 0; i < size; i++ )
{
free( dictionary[i][0] );
free( dictionary[i][1] );
}

Answer 2

instead of messing around with arrays of multiple dimensions, I suggest you use structures:
#include <stdio.h>
#include <string.h>
int main()
{
unsigned int size;
printf("Enter # of words to add: ");
if (scanf("%u", &size) != 1)
return 1; // cannot convert input to integer
#define WORD_MAX_SIZE 100
struct {
char word[WORD_MAX_SIZE];
char translation[WORD_MAX_SIZE];
} dictionary[size];
memset(dictionary, 0, sizeof(dictionary));
for (int i = 0; i < size; i++) {
printf("dictionary[%u].word= ", i);
fgets(dictionary[i].word, WORD_MAX_SIZE, stdin);
printf("dictionary[%u].translation= ", i);
fgets(dictionary[i].translation, WORD_MAX_SIZE, stdin);
}
// do something with your stuff
}

Answer 3

fgets(dictionary[i][j],100,stdin); reads from an uninitialized variable (dictionary[i][j] was never given a value), which has undefined behavior.
Also, i is going out of bounds later.