Why I am not getting any output to the console?

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I am trying to use subroutine calls using msvc inline assembly. This is the code:

C

int main() {
	int a = 4, b = 2, c = 1;
	__asm {
	LOC_FUN:
		mov eax, 6
		mov ebx, 21
		mov ecx, 32
	    ret
		
	MAIN :
		xor eax, eax
		xor ebx, ebx
		xor ecx, ecx
	    call LOC_FUN

		mov a, eax
		mov b, ebx
		mov c, ecx
	};
	printf("%d\n", a);
	printf("%d\n", b);
	printf("%d\n", c);

	return 0;
}

What I have tried:

I have tried to comment the LOC_FUN subroutine definition and subroutine call, and that outputs the value of a, b, c to the console. I think I am making a mistake in defining subroutine, or calling it. Please help in letting me know what I am doing wrong.

Posted 21-Oct-23 4:07am

Shobhit_23

Updated 24-Oct-23 11:08am

Deeksha Shenoy

v3

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Dave Kreskowiak 21-Oct-23 11:34am

Just to add what 'Griff said below, use the debugger. That would have shown you what's going on immediately just by running the code and stepping through it one line at a time.

The debugger is there to debug YOU and your understanding of the code. Use it.

OriginalGriff 21-Oct-23 12:07pm

It's been a looong time since I last wrote any assembler code - this reminds me of how much I miss it!

Dave Kreskowiak 21-Oct-23 12:15pm

Yeah, same. The last thing I wrote was text and graphics routines way back in the Windows-on-DOS days.

OriginalGriff 21-Oct-23 16:45pm

Central bootload and Threading OS embedded in an Industrial Inkjet printer for products as they went down production lines. I still see my handiwork in supermarkets when I go shopping! UI in C++, interrupts / drivers / OS in ARM assembler.

Dave Kreskowiak 21-Oct-23 17:15pm

Cool stuff!

OriginalGriff 22-Oct-23 1:49am

Fun, but high pressure work to stupidly short deadlines and an hour each way commuting (by motorcycle - it would have been two in a car), doubled when someone did something even more stupid than normal and managed to block the road. At least once a week. :sigh: Well glad to be out of it!

Shobhit_23 21-Oct-23 13:28pm

Thanks for the suggestion, I will definitely use it.

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OriginalGriff · Accepted Answer · 2023-10-21T05:13:00

First off, you don't need to clear eax, ebx, or ecx - your function always overwrites what is there so it's irrelevant what they contain on entry.

Secondly, your assembly code is within a function body, so the assembler label LOC_FUN just marks a point in the assembler code, not a "new function" start in the C sense.
You need to jump over the function to your actual call - because otherwise the ret is encountered without a call, and the app crashes or exits, providing no printout.

Additionally, Assembler labels aren't case sensitive: so having yours called MAIN may confuse things with the C code main function.

Try this:

C/ASM

#include <stdio.h>
int main()
	{
	int a = 4, b = 2, c = 1;
	__asm 
		{
			jp START
		LOC_FUN:
			mov eax, 6
			mov ebx, 21
			mov ecx, 32
			ret

		START :
			call LOC_FUN
			mov a, eax
			mov b, ebx
			mov c, ecx
		};
	printf("%d\n", a);
	printf("%d\n", b);
	printf("%d\n", c);

    return 0;
	}