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11.5 Creating Portable Code

Portability is generally not one of the strengths of assembly language. Yet, writing assembly language programs for different platforms is possible, especially with nasm. I have written assembly language libraries that can be assembled for such different operating systems as Windows® and FreeBSD.

It is all the more possible when you want your code to run on two platforms which, while different, are based on similar architectures.

For example, FreeBSD is UNIX®, Linux is UNIX like. I only mentioned three differences between them (from an assembly language programmer's perspective): The calling convention, the function numbers, and the way of returning values.

11.5.1 Dealing with Function Numbers

In many cases the function numbers are the same. However, even when they are not, the problem is easy to deal with: Instead of using numbers in your code, use constants which you have declared differently depending on the target architecture:

%ifdef LINUX
%define SYS_execve  11
%else
%define SYS_execve  59
%endif

11.5.2 Dealing with Conventions

Both, the calling convention, and the return value (the errno problem) can be resolved with macros:

%ifdef LINUX

%macro  system  0
    call    kernel
%endmacro

align 4
kernel:
    push    ebx
    push    ecx
    push    edx
    push    esi
    push    edi
    push    ebp

    mov ebx, [esp+32]
    mov ecx, [esp+36]
    mov edx, [esp+40]
    mov esi, [esp+44]
    mov ebp, [esp+48]
    int 80h

    pop ebp
    pop edi
    pop esi
    pop edx
    pop ecx
    pop ebx

    or  eax, eax
    js  .errno
    clc
    ret

.errno:
    neg eax
    stc
    ret

%else

%macro  system  0
    int 80h
%endmacro

%endif

11.5.3 Dealing with Other Portability Issues

The above solutions can handle most cases of writing code portable between FreeBSD and Linux. Nevertheless, with some kernel services the differences are deeper.

In that case, you need to write two different handlers for those particular system calls, and use conditional assembly. Luckily, most of your code does something other than calling the kernel, so usually you will only need a few such conditional sections in your code.

11.5.4 Using a Library

You can avoid portability issues in your main code altogether by writing a library of system calls. Create a separate library for FreeBSD, a different one for Linux, and yet other libraries for more operating systems.

In your library, write a separate function (or procedure, if you prefer the traditional assembly language terminology) for each system call. Use the C calling convention of passing parameters. But still use EAX to pass the call number in. In that case, your FreeBSD library can be very simple, as many seemingly different functions can be just labels to the same code:

sys.open:
sys.close:
[etc...]
    int 80h
    ret

Your Linux library will require more different functions. But even here you can group system calls using the same number of parameters:

sys.exit:
sys.close:
[etc... one-parameter functions]
    push    ebx
    mov ebx, [esp+12]
    int 80h
    pop ebx
    jmp sys.return

...

sys.return:
    or  eax, eax
    js  sys.err
    clc
    ret

sys.err:
    neg eax
    stc
    ret

The library approach may seem inconvenient at first because it requires you to produce a separate file your code depends on. But it has many advantages: For one, you only need to write it once and can use it for all your programs. You can even let other assembly language programmers use it, or perhaps use one written by someone else. But perhaps the greatest advantage of the library is that your code can be ported to other systems, even by other programmers, by simply writing a new library without any changes to your code.

If you do not like the idea of having a library, you can at least place all your system calls in a separate assembly language file and link it with your main program. Here, again, all porters have to do is create a new object file to link with your main program.

11.5.5 Using an Include File

If you are releasing your software as (or with) source code, you can use macros and place them in a separate file, which you include in your code.

Porters of your software will simply write a new include file. No library or external object file is necessary, yet your code is portable without any need to edit the code.

Note: This is the approach we will use throughout this chapter. We will name our include file system.inc, and add to it whenever we deal with a new system call.

We can start our system.inc by declaring the standard file descriptors:

%define    stdin   0
%define stdout  1
%define stderr  2

Next, we create a symbolic name for each system call:

%define    SYS_nosys   0
%define SYS_exit    1
%define SYS_fork    2
%define SYS_read    3
%define SYS_write   4
; [etc...]

We add a short, non-global procedure with a long name, so we do not accidentally reuse the name in our code:

section    .text
align 4
access.the.bsd.kernel:
    int 80h
    ret

We create a macro which takes one argument, the syscall number:

%macro system  1
    mov eax, %1
    call    access.the.bsd.kernel
%endmacro

Finally, we create macros for each syscall. These macros take no arguments.

%macro sys.exit    0
    system  SYS_exit
%endmacro

%macro  sys.fork    0
    system  SYS_fork
%endmacro

%macro  sys.read    0
    system  SYS_read
%endmacro

%macro  sys.write   0
    system  SYS_write
%endmacro

; [etc...]

Go ahead, enter it into your editor and save it as system.inc. We will add more to it as we discuss more syscalls.

This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.

For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.