如何从Assembly调用C函数？

Question

Here's the entire assembly program:

.model small
.stack 256

.code
start:
    call printer
    mov ax, 3       ; store 3 into ax

    mov ah, 76      ; back to DOS
    mov al, 0       ; no errors
    int 21h         ; interupt -> DOS
end start

And this is where I define the C function printer

#include <stdio.h>

void printer()
{
    printf("Hello!\n");
}

When compiling the assembly code, I get an error: undefined symbol: printer . In C I would do an #include "file.h" , how do I achieve the same result here?

Answer 1

The problem is not compilation, it's linking. You must link (using ld if on Unix/Linux) your executable including the object file with the assembly code and the object file with the C code.

Or put your assembly code into your C file using an "asm" block.

Answer 2

You should add something like

extern  _printer

on the top of your assembly and use call with this name

call _printer

Correct name of function depends on naming convention of your C compiler. Compiler may add some characters to the C name of the function.

Correct "extern" keyword depends on your assembler and it could be ".extern" or so.

Edit 1: In Turbo Assembler and for that case with function without parameter, it should be just

extrn printer

or

extrn printer:NEAR

I am not familiar with TASM.