How do you use printf from Assembly?

Question

I have an MSVC++ project set up to compile and run assembly code.

In main.c:

#include <stdio.h>

void go() ;

int main()
{
  go() ; // call the asm routine
}

In go.asm:

.586
.model flat, c
.code

go PROC
  invoke puts,"hi"
  RET
go ENDP

end

But when I compile and run, I get an error in go.asm:

error A2006: undefined symbol : puts

How do I define the symbols in <stdio.h> for the .asm files in the project?

You have to somehow link to your C standard library.

Alexandre C.
– Alexandre C.

2010-12-30 15:58:35 +00:00
Commented Dec 30, 2010 at 15:58 — Alexandre C.
– Alexandre C., Commented Dec 30, 2010 at 15:58

bobobobo · Accepted Answer · 2010-12-31 05:48:16Z

1

Here's what I have.

It works!!

.586
.model flat,c

printf PROTO C :VARARG  ; The secret sauce.. a prototype of printf

.data
msgHello1 BYTE "GREETINGS AND WELCOME TO EARTH!",0 

.code

go PROC
  push OFFSET msgHello1
  call printf
  add esp, 4 ;  Stack cleaning
  RET
go ENDP

end

answered Dec 31, 2010 at 5:48

bobobobo

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1 Comment

Madhur Ahuja Over a year ago

where is the definition of printf ? If you are using C library, it needs to be defined as extern. I am guessing that crash has something to do with Calling convetion. Use ollydbg to debug ...

user257111 · Accepted Answer · 2010-12-30 16:13:27Z

I think this article explains it better than I can.

Roughly put, the assembler cannot find the symbol (function) in go.asm. You have to tell it its an external symbol.

The linked article approaches building a mixed-code app from the point of view of using assembly as the main language including running the main routine. If you're using a C based main routine, much of what is done is unnecessary, you should just need:

Assemble the assembly module with /Mx to preserve the case of nonlocal names. If using MASM version 6.0 or later, use /Cx to preserve the case of nonlocal names.

and:

Include the statement .MODEL , c in the assembly module to ensure that C naming and calling conventions are used and that the modules use the same default segments. The will be small, medium, compact, or large.

and EXTERN directives for each function you wish to call in the format: EXTERN printf:proc.

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