This code cannot be compiled

But this can be compiled:

#include "reg51.h"

void main(void) {
	int a = 0;
}

But this can be compiled:

#include "reg51.h"

void main(void) {
	int a = 0;
}

And these can be compiled too:

#include "reg51.h"

void main(void) {
	P1 = 0x00;
}

#include "reg51.h"

void main(void) {
	int a = 0;
	P1 = 0x00;
}

The C51 compiler basically supports the old ANSI C standard C90 with some extensions for the 8051 architecture.

The C90 standard does not allow defining local variables after executable statements within a function. In C90, the syntax rules require that all declarations within a block must come before any statements.

"P1 = 0x00;" is a statement that generates code, because P1 is already defined. After that, no definition of a local variable is allowed.

The C99 standard would allow to intermix statements and definitions, but the C51 compiler will never support this standard.