Problem trying to run Assembly in Visual Studio - c

Question

I'm trying to run some assembly code in Visual Studio 2012 and call it in C just for testing purposes. As I have no experience writing assembly code I have no idea what is going wrong, so I would greatly appreciate some help!
I get the following errors trying to compile the code:
Error 5 error MSB3721: The command "ml.exe /c /nologo /Zi /Fo"Debug\callee.obj" /W3 /errorReport:prompt /Tacallee.asm" exited with code 1. C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V110\BuildCustomizations\masm.targets 49 5 ProjetoASM
Error 2 error A2206: missing operator in expression C:\Users\Suporte\Desktop\ASM\ProjetoASM\ProjetoASM\callee.asm 18 1 ProjetoASM
Error 3 error A2206: missing operator in expression C:\Users\Suporte\Desktop\ASM\ProjetoASM\ProjetoASM\callee.asm 21 1 ProjetoASM
Error 4 error A2206: missing operator in expression C:\Users\Suporte\Desktop\ASM\ProjetoASM\ProjetoASM\callee.asm 8 1 ProjetoASM
Error 1 error A2022: instruction operands must be the same size C:\Users\Suporte\Desktop\ASM\ProjetoASM\ProjetoASM\callee.asm 15 1 ProjetoASM
And the Assembly code:
PUBLIC hello_from_asm
EXTERN puts:PROC
.model flat
.data
msg db 'Hello, world!',0xa
len equ $ - msg
.code
hello_from_asm PROC
mov edx,len
mov ecx,msg
mov ebx,1
mov eax,4
int 0x80
mov eax,1
int 0x80
hello_from_asm ENDP
END
This was supposed to output a "Hello, world!", so any other ideas that might work are also welcome.
C code for completeness:
#include <stdio.h>
extern void hello_from_asm();
int main(){
printf("Hello from C");
hello_from_asm();
return 0;
}

Answer 1

Those detailed MASM error messages tell it all.
mov ecx,msg
You're required to use MASM style. The above instruction tries to load the first byte of msg in a 32-bit register. That's the "size mismatch" error.
What you need is loading the address of msg in ECX. Use
mov ecx, offset msg
The other errors might be about not recognizing the 0x hexadecimal prefix. Try using the h hexadecimal suffix instead. (0Ah, 80h)
The above is easy enough to change, and your code will assemble fine. However don't run it because the int 80h instruction is a Linux system call that is not going to work on Visual Studio 2012 (Windows).

Answer 2

Example 32 bit Visual Studio | Masm program to print "Hello World". I included the most common directives. "legacy_stdio_definitions.lib" is used for VS2015 and later, since printf and scanf were changed to be inlined with the output of the C compiler. You may not need it for VS2012.
.686p ;enable instructions
.xmm ;enable instructions
.model flat,c ;use C naming convention (stdcall is default)
; include C libraries
includelib msvcrtd
includelib oldnames
includelib legacy_stdio_definitions.lib ;for scanf, printf, ...
.data ;initialized data
pfstr db "Hello world!",0dh,0ah,0
.data? ;uinitialized data
.stack 4096 ;stack (optional, linker will default)
.code ;code
extrn printf:near
public main
main proc
push offset pfstr ; 32-bit mode uses stack args
call printf
add esp,4 ; cdecl is caller-pops
xor eax,eax ; return 0
ret
main endp
end