How to do lexical analysis for c/c++ source file?

void main()
{
    clrscr();
    char mstr[50];
    printf("\n\n\tEnter String: ");// Here i want file as input
    gets(mstr);
    Lex_Ana oblex(mstr);//Here how to pass all lines to analysis
    oblex.Find_Lex();
    oblex.Rem_Dup();
    oblex.Sep_Val();
    oblex.Display();
    getch();
}

void main()
{
    clrscr();
    ifstream indata;
    char file_name[40], *mainStr,*ch;
    printf("\n\n\tEnter File name: ");
    gets(file_name);
    indata.open(file_name);
    if(indata==NULL)
    {
        perror("Error while opening the file.\n");
        exit(EXIT_FAILURE);
    }
    else
    {
        while(!indata.eof())
        {
            getline(indata,ch);
            strcat(mainStr,ch);
        }
        Lex_Ana oblex(mainStr);
        oblex.Find_Lex();
        oblex.Rem_Dup();
        oblex.Sep_Val();
        oblex.Display();
        indata.close();
    }
    getch();
}