How to write a C function called from assembly code
I need to write a C function that will be called from build code in linux kernel.
What special issues should I consider?
I mean, but can anyone provide more details:
(1) Calling agreement
Make sure the caller is complete and the person being called with a good handshake. But which convention should I use? How can I declare a function c and declare it in assembler?
(2) Protect registers
Some registers must be saved in the assembly before being called. I roughly remember that they are eax, ecx and edx. But I don't need to save them unless I need to reference the old value before calling the C function, right?
What other problems?
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As your tagged quest, linux-kernel
look at the following kernel header: arch / x86 / include / asm / calling.h .
3 x86 function call convention, 64-bit:
4 -------------------------------------
5 arguments | callee-saved | extra caller-saved | return
6 [callee-clobbered] | | [callee-clobbered] |
7 ---------------------------------------------------------------------------
8 rdi rsi rdx rcx r8-9 | rbx rbp [*] r12-15 | r10-11 | rax, rdx [**]
9
10 ( rsp is obviously invariant across normal function calls. (gcc can 'merge'
11 functions when it sees tail-call optimization possibilities) rflags is
12 clobbered. Leftover arguments are passed over the stack frame.)
13
14 [*] In the frame-pointers case rbp is fixed to the stack frame.
15
16 [**] for struct return values wider than 64 bits the return convention is a
17 bit more complex: up to 128 bits width we return small structures
18 straight in rax, rdx. For structures larger than that (3 words or
19 larger) the caller puts a pointer to an on-stack return struct
20 [allocated in the caller stack frame] into the first argument - i.e.
21 into rdi. All other arguments shift up by one in this case.
22 Fortunately this case is rare in the kernel.
23
24 For 32-bit we have the following conventions - kernel is built with
25 -mregparm=3 and -freg-struct-return:
26
27 x86 function calling convention, 32-bit:
28 ----------------------------------------
29 arguments | callee-saved | extra caller-saved | return
30 [callee-clobbered] | | [callee-clobbered] |
31 -------------------------------------------------------------------------
32 eax edx ecx | ebx edi esi ebp [*] | <none> | eax, edx [**]
33
34 ( here too esp is obviously invariant across normal function calls. eflags
35 is clobbered. Leftover arguments are passed over the stack frame. )
36
37 [*] In the frame-pointers case ebp is fixed to the stack frame.
38
39 [**] We build with -freg-struct-return, which on 32-bit means similar
40 semantics as on 64-bit: edx can be used for a second return value
41 (i.e. covering integer and structure sizes up to 64 bits) - after that
42 it gets more complex and more expensive: 3-word or larger struct returns
43 get done in the caller frame and the pointer to the return struct goes
44 into regparm0, i.e. eax - the other arguments shift up and the
45 function register parameters degenerate to regparm=2 in essence.
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