customization of putchar function

"So I can not switch between output function (e.g. lcd_putchar and putchar) which I can do in PIC-C(C compiler for PIC Microcontroller) very flexible as:
- printf("......",.....);//default serial port
- printf(lcd_putc,"......",.....);//redirect to LCD"

char (* putchar_fp)(char c);

char lcd_putchar(char c)
{
    /* Your LCD output handler goes here */
}

char ser_putchar(char c)
{
    /* Body of Keil's putchar goes here */
}

char putchar(char c)
{
    /* Body of Keil's putchar moved to ser_putchar()
     * and replaced with...
     */
    return putchar_fp(c);
}

int my_printf(char (* which_putchar_fp)(char), const char *format, ...)
{
    int     ret_val;
    va_list ap;

    putchar_fp = which_putchar_fp;

    va_start(ap, format);
    rtn = vprintf(format, ap);
    va_end(ap);

    return ret_val;
}

int main(void)
{
    my_printf(lcd_putchar, "Hello world on %s\n", "LCD");
    my_printf(ser_putchar, "Hello world on %s\n", "SER");
}

"So I can not switch between output function (e.g. lcd_putchar and putchar) which I can do in PIC-C(C compiler for PIC Microcontroller) very flexible as:
- printf("......",.....);//default serial port
- printf(lcd_putc,"......",.....);//redirect to LCD"

char (* putchar_fp)(char c);

char lcd_putchar(char c)
{
    /* Your LCD output handler goes here */
}

char ser_putchar(char c)
{
    /* Body of Keil's putchar goes here */
}

char putchar(char c)
{
    /* Body of Keil's putchar moved to ser_putchar()
     * and replaced with...
     */
    return putchar_fp(c);
}

int my_printf(char (* which_putchar_fp)(char), const char *format, ...)
{
    int     ret_val;
    va_list ap;

    putchar_fp = which_putchar_fp;

    va_start(ap, format);
    rtn = vprintf(format, ap);
    va_end(ap);

    return ret_val;
}

int main(void)
{
    my_printf(lcd_putchar, "Hello world on %s\n", "LCD");
    my_printf(ser_putchar, "Hello world on %s\n", "SER");
}

And for good measure, you'd probably want to 'reset' putchar_fp back to putchar so that any subsequent calls to normal printf() used ser_putchar() as originally indended. So make that...

int my_printf(char (* which_putchar_fp)(char), const char *format, ...)
{
    int     ret_val;
    va_list ap;

    putchar_fp = which_putchar_fp;

    va_start(ap, format);
    ret_val = vprintf(format, ap);
    va_end(ap);

    putchar_fp = ser_putchar;
    return ret_val;
}

One more comment, should you choose to use function pointers, would be to have you take a look at Keil's function pointer appnote.

http://www.keil.com/appnotes/docs/apnt_129.asp

Dear Dan Henry, Andy Neil
Thank you very much for your guide and C51 module. After receiving your replies, I tested on two way successfully. The first one I simply use a global variable to switch functions and on the other one I write codes like your guide which used function pointer.
Again thank you so much, see you next post.