Crossword puzzle generator in C

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
int main()
{
    printf("Crossword generator:\n");
    FILE *f = fopen("d:\\temp\\words\\words.txt", "r");
    if (f==NULL) {
        puts("Error opening the file!");
        return 0;
    }
    fseek(f, 0, SEEK_END);
    long size = ftell(f);
    fseek(f, 0, SEEK_SET);

    char * array=malloc(size);
    if (array == NULL) {
        fputs("Memory allocation error!", stderr);
        return EXIT_FAILURE;
    }
    int c, i = 0, lines=0;
    while ((c=fgetc(f))!=EOF) {
        if (c=='\n') {
            lines++;
            c=0;       //replace \n by 0 (end of string)
        }
        array[i++] = c;
    }
    int * word_index = malloc(lines*sizeof(int));
    if (word_index == NULL) {
        fputs("Memory allocation error!", stderr);
        return EXIT_FAILURE;
    }
    word_index[0]=0;
    for (int j=0, line_nr=1; j<i; j++) {
        if (array[j]==0) word_index[line_nr++]=j+1;
    }
//    printf("The file is stored in RAM, its size = %d bytes, nr. of lines=%d\n", i, lines);
    srand(time(NULL));
    int solution_index=(rand()*rand()) % lines;  //0..lines-1
    char * solution = array+word_index[solution_index];
    printf("Puzzle solution=%s\n", solution);

    for (unsigned i=0; i<strlen(solution); i++) {
       char letter=solution[i];
       char *ptr;
       char * word;
       do {
           int index=(rand()*rand()) % lines;  //0..lines-1
           word = array+word_index[index];
           ptr=strchr(word, letter);
       } while(ptr==NULL);
       for (int j=0; j<40-(ptr-word); j++) putchar(' ');
       *ptr = 0;
       printf("%s", word);
       *ptr = letter;
       printf("|%c|%s\n", letter, ptr+1);
    }
}