the function is supposed to return the length of a line specified by the 2nd argument
but the result is either 0 or the one in the 2nd line whatever n may be:

int line_length(const char *filename, int n)
{
    FILE *fp;
    int c;
    int len = 0;

    if ((fp = fopen(filename, "r")) == NULL)
        return -1;
    while (n > 1)
    {
        while ((c = fgetc(fp)) != 'n')
           /*  if (c = EOF)  // I removed these couple of lines, otherwise it 
                return 0; */ // yields always 0 since I THINK EOF always pops 
        n--;                 // once the line is finished
    }
    while ((c = fgetc(fp)) != 'n' && c != EOF)
        len++;
    return len;
}

I removed these couple of lines, otherwise it
yields always 0 since I THINK EOF always pops
once the line is finished