Large arrays, malloc, and strcmp

I have two fairly large 2d arrays to hold strings, and I want to compare each string in the second array to every string in the first to see if any match. The following is how I create the arrays, and how I compare them. It works if x(which is unsigned long b/c max number can be 4000000000) is a small number, but when it is a large number( 500000+ ), the programs seg faults at strcmp. Any ideas?

Code:

unsigned long int x;
int y;

printf( "Enter the number of random words to be produced: " );
	scanf( "%u", &x );

printf( "Enter the maximum length of each word: " );
	scanf( "%d", &y);

char **buffer;
	buffer = malloc( x * sizeof(char *) + 1);
	for( i=0; i<x; i++) {
		buffer[i] = malloc( y * sizeof(char) + 1 );
	}

// Generate random words and store in a 2d buffer array.
// rand()%25 ensures only letters are generated.
    
	for( i=0; i<x; i++ ) {
		for( j=0; j<y-1; j++ ) {
			
			r = rand()%25;
			buffer[i][j] = letters[r];
			
		}
		// Ensure each string is null-terminated.
		buffer[i][j] = '\0';
	}

FILE *dict;
		dict = fopen( "linux.words", "r" );
		
	if( ! dict ) {
		printf( "Error: Failed to open linux.words\n" );
	}
	
		
	char dict_words[SIZE][WORD_SIZE], *z;
	i = 0;
	while( fgets( dict_words[i], 256, dict ) != NULL ) {
		
		i++;
	}
	
	// Remove newline character from each word to avoid comparison errors later.
	int p, len;
	for( p=0; p<i; p++ ) {
		if(( z = strchr( dict_words[p], '\n' ) )) *z = 0;
	}
	
	// Ensures all words in linux.words start with lower case letter.
	for( p=0; p<i; p++ ) {
		dict_words[p][0] = tolower( dict_words[p][0] );
	}


	}
	int k;
	for( j=0; j<i; j++ ) {
		for( k=0; k<x; k++ ) {
			if( strcmp( dict_words[j], buffer[k] ) == 0 ) {
				fputs( buffer[k], words_out);
				fputs( "\n", words_out);
				
			}
		}
	}

You're not checking the return value of your malloc()'s for NULL, plus you didn't say how big y is (the total amount of memory you're allocating is roughly x*y*sizeof(char *)).

Edit: Never mind my second comment, I overlooked the part about strcmp().

Edit: Your code is not self-contained - for example, the values of SIZE and WORD_SIZE, whatever they are, must be large enough. You also haven't given the definition of letters[].