connection between variables in different files

too vague a question

1) C or assembler?
2) show examples of both where it is defined and where it is accessed.

Erik

This is bog-standard 'C'; nothing specific to Keil at all - so see any 'C' textbook.

Basically you must have exactly one definition for each & every identifier;
You may have as many 'extern' declarations as you like.

file.c
// definitions
char tom = 0; // definition with initialisation
int  ***;
long harry;

file.h
// declarations
extern char tom; // NO initialisation!!
extern int  ***;
extern long harry;

i will clarify my problem, i wrote with c51 and i attached my program. my project based on example code of Atmel Co. and it include few files. i added program at the main.c and i try to use with variable also at the main and the usb_task.c file

the code of the main is:

#include "config.h"
#include "modules/scheduler/scheduler.h"
#include "lib_mcu/reg_5131.h"
/*---------------------------------*/

char serial_data;
char data_from_adc ;
char data_save_msb;
char data_save_lsb;
int int1=1;
bit transmit_completed= 0;
int i;

/*-----------------------------------*/


void main (void)
{
...

/*------------------*/

the another file from this project is:

#include "config.h"
#include "usb_task.h"
#include "lib_mcu\usb\usb_drv.h"
#include "modules\usb_kbd_enum\usb_kbd_enum.h"
#include "modules\timer_soft\timer_soft.h"

//#include "lib_board\lcd_twi\lcd_twi.h"
//#include "lib_board\kbd_4_4\kbd_4_4.h"

/
bit   usb_connected;
bit   usb_connected_stored;
bit   usb_configured_stored;
Uchar usb_sof_counter;

Uchar usb_kbd_state;

               Uchar  transmit_no_key;
Union16 idata  scan_key;
        Uchar  key_hit;
        Uchar  shift_key;

extern         Uchar  usb_configuration_nb;
//Uchar  code    usb_keys[]= { b[1], b[2], b[3], b[4] }
extern 		char data_save_msb();
/*-------------*/
void main etc...

please look at the last reply

as stated above
"This is bog-standard 'C'; nothing specific to Keil at all - so see any 'C' textbook."

Erik

Your two files both seem to contain a definition of main. Other than that, they seem to be entirely unrelated!

What exactly is your problem?!