If you can find a copy of Jeff Duntemann's "Assembly Language
Step-by-Step, 2nd Ed.", the second section deals with using assembly and
C together. A lot of this stuff is covered in G. Adam Stanislav's
tutorial on using assembly and FreeBSD, available at
http://www.int80h.org/bsdasm/
Pay careful attention to the bit on the calling convention, what
registers you have to save, setting up a stack frame, etc. That's the
bulk of what you'll need to know. The trickiest part seems to be
getting your assembler to produce an object file in the correct format.
After you have an object file, you can link it all together with gcc.
A short example exerpted from Duntemann follows:
; Build using these commands:
; nasm -f elf eatlinux.asm
; gcc eatlinux.o -o eatlinux
[SECTION .txt]
extern puts
global main ;required so linker can find entry point
main:
push ebp ;Set up stack frame for debugger
mov ebp, esp
push ebx ; Program must preserve ebp, ebx, esi, & edi
push esi
push edi
;;; everything before this is boilerplate; use it for all ordinary
apps!
push dword eatmsg ; push a 32-bit pointer to the message on the stack
call puts ; call the clib function for displaying strings
add esp, 4 ; clean stack by adjusting esp back 4 bytes
;;; everything after this is boilerplate; use it for all ordinary apps!
pop edi ; restore saved registers
pop esi
pop ebx
mov esp, ebp ; destroy stack frame before returning
pop ebp
ret
[SECTION .data] ; section containing initialized data
eatmsg: db "Eat at Joe's!",10,0
[SECTION .bss] ; section containing uninitialized data
If I recall correctly, underscores may have to be placed before function
names when coding for Windows.
A few points to remember (from Duntemann):
- Functions have to preserve the contents of EBX, ESP, EBP, ESI and
EDI. You usually do this by not using them, or saving them on the stack
and restoring them when done.
- C functions return their values in EAX if they're 32-bit, or EAX and
EDX, with the low bits in EAX and the high bits in EDX. Anything else
is returned via a pointer in EAX
- you pass parameters to a c function by pushing them in reverse order
(right to left) onto the stack.
- C functions do not clean their parameters off the stack. If you
call a C function and pass parameters onto the stack, you are
responsible for cleaning up afterwards.
If you want to call C code, or have C code call your function, just
follow these rules. With C code calling your functions, you will have
to follow these rules scrupulously, and may have to experiment with
underscores preceding your function. Once you get it figured out,
though, it's not that hard.
Stephen
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