using printf for serial & LCD together

1) Yes, you can write code that sends the data to multiple destinations. It's just up to you to implement the back-end code that takes care of the resulting characters and distributes them where you want them.

2) Alas, I don't think Keil have fcvt() or ecvt() which would have been alternatives to printf()/sprintf(). But it is often possible to work with fixed-point arithmetic instead of floating point with very good results.

"in C51, the default printf() is used for serial comm"

No: you need to be clear that printf() is entirely unaware of what physical interface is used to send the output that it generates!

It is entirely a matter of the putchar() implementation.

"the question is, can putchar() be used on both, serial comm. and 8-bit LCD, at same time"

Why would that not be possible??

char putchar( char c)
{
   put_to_serial( c );
   put_to_lcd( c );
   put_to_whatever_else_you_like( c );
   return c;
}

it works, thanks,..

Hello,

This method sends data to the two peripherals simultaneously.
What if we want to send different data to LCD, than to the UART ?

Best regards.
Wilu

You can implement own printf() functions by using sprintf() to a temporary buffer and have:

int lcd_printf(const char fmt,...) {
    ...
}

int com0_printf(const char *fmt,...) {
    ...
}

int com1_printf(const char *fmt,...) {
}

or use fprintf() to direct the output:

fprintf(lcd,"Hello world!\n");
fprintf(com0,"This is com0.\n");
fprintf(com1,"This is com1.\n");

Not in C51, you can't:

http://www.keil.com/support/man/docs/c51/c51_xa_librarydif.htm

You would have to create your own...

"You would have to create your own..."

Which isn't very hard to do. Either by use of our friend "sprintf()" or by setting a global variable to inform where the data is expected to be sent and have a putchar() that looks at that global variable. Just as long as the fprintf() implementation resets that global variable after the call so direct use of printf() doesn't get the output rerouted.

A large part of the C runtime library is standard C code, and potentially using other C RTL functions. And the parts that isn't standard C is normally assembler for extra speed or size gains - not because of limitations the C language.

In the end, the user doesn't need any presupplied file system implementation to be able to play with fprintf(), fputs(), fgets(), fputchar(), ...