How to flash error if a macro is not defined in program

Question

I want my program to flash compile time error like "LCD_PORT not defined" if it is not defined in program itself. For that I modified the header file like this

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.
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#if LCD_IO_MODE
#ifndef LCD_PORT
#error LCD_PORT not defined //(e.g. #define LCD_PORT PORTA/B/C/D)
#endif
#define LCD_DATA0_PORT LCD_PORT /**< port for 4bit data bit 0 */
#define LCD_DATA1_PORT LCD_PORT /**< port for 4bit data bit 1 */
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...

But even after defining the LCD_PORT (like in the following program), it flashes the error.

#include <avr/io.h>
#include <lcd.h>
#define LCD_PORT PORTA
int main(void)
{
 lcd_init(LCD_DISP_ON_CURSOR); 
 lcd_home();
 lcd_gotoxy(0,4);
 lcd_puts("Hello world!!");
}

• jippie: Modifying distribution header files is bad practice. Why not move these lines into your own source code?

Answer 1

The problem is likely to be that the preprocessor operates sequentially so you should perform the define before the include. Try the following:

#include <avr/io.h>
#define LCD_PORT PORTA
#include <lcd.h>

• Passerby: Exactly. The included header is processed before the #define is reached.