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take IA32 as example, and the theory is the same as that on ARM-based platforms.
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There are two articles describing the stack frame and function call on X86 platform, but list the difference between them in theory.
Article 1: ===http://www.unixwiz.net/techtips/win32-callconv-asm.html===
Intel x86 Function-call Conventions - Assembly View
Article 2: ====http://haoxiai.net/bianchengyuyan/cyuyan/11726.html======
通过内核源码看函数调用之前世今生
Article 3: ===http://book.51cto.com/art/200801/63515.htm====
逆向工程解密
------------------------------------------------------
Here quote the complete article but get the following key solutions:
a). understanding of the stack structure: where to store return address, where to start the stack frame, relationship between stack frame pointer and stack pointer;
b). difference of compiler among VC, Blorand, and GCC;
c). Stack Frame & Stack Pointer
X86: ebp, stack frame pointer
esp, stack pointer
and all the variants, parameters, return address are stored in Stack Frame.
objdump -d xxx.o will get the re-assembly code. The push operations in Linux is;
push ebp; // push current ebp to the stack
mov esp, ebp; // set new value ebp as esp (after push ebp, esp will minus 4). I think there is some difference from the diagram in Chapter 1. if the original is correct, then difference lies on who will contain the pushed ebp, previous frame or the new frame, or push ebp doesn't change esp at all. (refer to the colored text in chapter 1).
After comparison of the two article and experiment on pc with gcc, the stack frame pointer is started on 0(%ebp) but the local variants start from -8%(ebp) instead of -4(%ebp), I think the description in Article 1 is correct, at least the stack frame structure:
16(%ebp) - third function parameter 12(%ebp) - second function parameter 8(%ebp) - first function parameter 4(%ebp) - old %EIP (the function's "return address") 0(%ebp) - old %EBP (previous function's base pointer) -4(%ebp) - first local variable -8(%ebp) - second local variable -12(%ebp) - third local variable
------------------------------------------------------------------------
http://www.unixwiz.net/techtips/win32-callconv-asm.html
Intel x86 Function-call Conventions - Assembly View
One of the "big picture" issues in looking at compiled C code is the
function-calling conventions. These are the methods that a calling
function and a called function agree on how parameters and return values
should be passed between them, and how the stack is used by the function
itself. The layout of the stack constitutes the "stack frame", and
knowing how this works can go a long way to decoding how something works.
In C and modern CPU design conventions, the stack frame is a chunk of
memory, allocated from the stack, at run-time, each time a function is
called, to store its automatic variables. Hence nested or recursive calls
to the same function, each successively obtain their own separate frames.
Physically, a function's stack frame is the area between the addresses
contained in esp, the stack pointer, and ebp, the frame pointer (base
pointer in Intel terminology). Thus, if a function pushes more values
onto the stack, it is effectively growing its frame.
This is a very low-level view: the picture as seen from the C/C++
programmer is illustrated elsewhere:
• Unixwiz.net Tech Tip:
Intel x86 Function-call Conventions - C Programmer's View
For the sake of discussion, we're using the terms that the Microsoft Visual C
compiler uses to describe these conventions, even though other platforms
may use other terms.
__cdecl (pronounced see-DECK-'ll
rhymes with "heckle")
This convention is the most common because it supports semantics required
by the C language. The C language supports variadic functions (variable
argument lists, alá printf), and this means that the caller
must clean up the stack after the function call: the called function has
no way to know how to do this. It's not terribly optimal, but the C
language semantics demand it.
__stdcall
Also known as __pascal, this requires that each function take a
fixed number of parameters, and this means that the called function
can do argument cleanup in one place rather than have this be scattered
throughout the program in every place that calls it. The Win32 API
primarily uses __stdcall.
It's important to note that these are merely conventions, and any
collection of cooperating code can agree on nearly anything. There are
other conventions (passing parameters in registers, for instance)
that behave differently, and of course the optimizer can make mincemeat
of any clear picture as well.
Our focus here is to provide an overview, and not an authoritative
definition for these conventions.
Register use in the stack frame
In both __cdecl and __stdcall conventions,
the same set of three registers is involved in the function-call
frame:
%ESP - Stack Pointer
This 32-bit register is implicitly manipulated by several
CPU instructions (PUSH, POP, CALL, and RET among others),
it always points to the last element used on the stack
(not the first free element): this means
that the PUSH and POP operations would be specified in pseudo-C as:
*--ESP = value; // push
value = *ESP++; // pop
The "Top of the stack" is an occupied location, not a free one, and
is at the lowest memory address.
%EBP - Base Pointer
This 32-bit register is used to reference all the function parameters
and local variables in the current stack frame. Unlike the %esp register,
the base pointer is manipulated only explicitly. This is sometimes called
the "Frame Pointer".
%EIP - Instruction Pointer
This holds the address of the next CPU instruction to be
executed, and it's saved onto the stack as part of the CALL
instruction. As well, any of the "jump" instructions modify the
%EIP directly.
Assembler notation
Virtually everybody in the Intel assembler world uses the Intel
notation, but the GNU C compiler uses what they call the "AT&T syntax"
for backwards compatibility. This seems to us to be a really dumb idea,
but it's a fact of life.
There are minor notational differences between the two notations, but
by far the most annoying is that the AT&T syntax reverses the
source and destination operands. To move the immediate value 4 into
the EAX register:
mov $4, %eax // AT&T notation
mov eax, 4 // Intel notation
More recent GNU compilers have a way to generate the Intel
syntax, but it's not clear if the GNU assembler takes it. In any
case, we'll use the Intel notation exclusively.
There are other minor differences that are not of much concern to
the reverse engineer.
Calling a __cdecl function
The best way to understand the stack organization is to see each
step in calling a function with the __cdecl conventions. These
steps are taken automatically by the compiler, and though not all
of them are used in every case (sometimes no parameters, sometimes
no local variables, sometimes no saved registers), but this shows
the overall mechanism employed.
Push parameters onto the stack, from right to left
Parameters are pushed onto the stack, one at a time, from right to
left. Whether the parameters are evaluated from right to left is a
different matter, and in any case this is unspecified by the language
and code should never rely on this. The calling code must keep
track of how many bytes of parameters have been pushed onto the stack
so it can clean it up later.
Call the function
Here, the processor pushes contents of the %EIP (instruction
pointer) onto the stack, and it points to the first byte
after the CALL instruction. After this finishes, the caller has
lost control, and the callee is in charge. This step does not
change the %ebp register.
Save and update the %ebp
Now that we're in the new function, we need a new local stack
frame pointed to by %ebp, so this is done by saving the current %ebp
(which belongs to the previous function's frame) and making it point
to the top of the stack.
push ebp
mov ebp, esp // ebp « esp
Once %ebp has been changed, it can now refer directly to the
function's arguments as 8(%ebp), 12(%ebp). Note that
0(%ebp) is the old base pointer and 4(%ebp) is the old
instruction pointer.
Save CPU registers used for temporaries
![]()
If this function will use any CPU registers, it has to
save the old values first lest it walk on data used by the
calling functions. Each register to be used is pushed onto the
stack one at a time, and the compiler must remember what it
did so it can unwind it later.
Allocate local variables
The function may choose to use local stack-based variables,
and they are allocated here simply by decrementing the stack
pointer by the amount of space required. This is always done
in four-byte chunks.
Now, the local variables are located on the stack between the
%ebp and %esp registers, and though it would be possible to
refer to them as offsets from either one, by convention the %ebp
register is used. This means that -4(%ebp) refers to the first
local variable.
Perform the function's purpose
At this point, the stack frame is set up correctly, and this
is represented by the diagram to the right. All the parameters
and locals are offsets from the %ebp register:
16(%ebp)
- third function parameter
12(%ebp)
- second function parameter
8(%ebp)
- first function parameter
4(%ebp)
- old %EIP (the function's "return address")
0(%ebp)
- old %EBP (previous function's base pointer)
-4(%ebp)
- first local variable
-8(%ebp)
- second local variable
-12(%ebp)
- third local variable
The function is free to use any of the registers that had been
saved onto the stack upon entry, but it must not change the
stack pointer or all Hell will break loose upon function return.
Release local storage
When the function allocates local, temporary space, it does so by
decrementing from the stack point by the amount space needed, and this
process must be reversed to reclaim that space. It's usually done
by adding to the stack pointer the same amount which
was subtracted previously, though a series of POP
instructions could achieve the same thing.
Restore saved registers
For each register saved onto the stack upon entry, it must be
restored from the stack in reverse order. If the "save" and "restore"
phases don't match exactly, catastrophic stack corruption will
occur.
Restore the old base pointer
The first thing this function did upon entry was save the
caller's %ebp base pointer, and by restoring it now (popping
the top item from the stack), we effectively discard the entire
local stack frame and put the caller's frame back in play.
Return from the function
This is the last step of the called function, and the RET
instruction pops the old %EIP from the stack and jumps to that
location. This gives control back to the calling function. Only
the stack pointer and instruction pointers are modified by a
subroutine return.
Clean up pushed parameters
In the __cdecl convention, the caller must clean up the
parameters pushed onto the stack, and this is done either by
popping the stack into don't-care registers (for a few parameters)
or by adding the parameter-block size to the stack pointer directly.
__cdecl -vs- __stdcall
The __stdcall convention is mainly used by the Windows API, and
it's a bit more compact than __cdecl. The main difference is that
any given function has a hard-coded set of parameters, and this cannot
vary from call to call like it can in C (no "variadic functions").
Because the size of the parameter block is fixed, the burden of cleaning
these parameters off the stack can be shifted to the called function,
instead of being done by the calling function as in __cdecl. There
are several effects of this:
the code is a tiny bit smaller, because the parameter-cleanup code is
found once — in the called function itself — rather than in every
place the function is called. These may be only a few bytes per call,
but for commonly-used functions it can add up. This presumably means
that the code may be a tiny bit faster as well.
calling the function with the wrong number of parameters is
catastrophic - the stack will be badly misaligned, and general
havoc will surely ensue.
As an offshoot of #2, Microsoft Visual C takes special care of
functions that are B{__stdcall}. Since the number of parameters is
known at compile time, the compiler encodes the parameter byte count
in the symbol name itself, and this means that calling the function
wrong leads to a link error.
For instance, the function int foo(int a, int b) would generate
— at the assembler level — the symbol "_foo@8",
where "8" is the number
of bytes expected. This means that not only will a call with 1 or 3
parameters not resolve (due to the size mismatch), but neither
will a call expecting the __cdecl parameters (which looks for _foo).
It's a clever mechanism that avoids a lot of problems.
Variations and Notes
The x86 architecture provides a number of built-in mechanisms for
assisting with frame management, but they don't seem to be commonly used
by C compilers. Of particular interest is the ENTER instruction,
which handles most of the function-prolog code.
ENTER 10,0 PUSH ebp
MOV ebp, esp
SUB esp, 10
We're pretty sure these are functionally equivalent, but our 80386
processor reference suggests that the ENTER version is more
compact (6 bytes -vs- 9) but slower (15 clocks -vs- 6). The newer
processors are probably harder to pin down, but somebody has probably
figured out that ENTER is slower. Sigh.
====http://haoxiai.net/bianchengyuyan/cyuyan/11726.html======
作者：杨小华
栈（Stack）：一个有序的积累或堆积
对每一位孜孜不倦的程序员来说，栈已深深的烙在其脑海中，甚至已经发生变异。栈可以用来传递函数参数、存储局部变量、以及存储返回值的信息、还可以用于保存
寄存器的值以供恢复之用。
在X86平台上（又称之为IA32），应用程序借用栈来支持函数（又称为过程）调用，变量的存储按后进先出（LIFO）的方式进行。一、 栈帧布局
在具体讲解函数调用之前，我们先来明确栈的几个概念：满栈与空栈，升序栈与降序栈。
满栈是指栈指针指向上次写的最后一个数据单元，而空栈的栈指针指向第一个空闲单元。一个降序栈是在内存中反向增长(就是从应用程序空间结束处开始反向增
长)，而升序栈在内存中正向增长。 RISC机器使用传统的满降序栈(FD Full
Descending)。如果使用符合IA32规定的编译器，它通常把你的栈指针设置在应用程序空间的结束处并接着使用一个满降序栈。用来存放一个函数的
局部变量、参数、返回地址和其它临时变量的栈区域称为栈帧（stack frame），如图1所示。
![]()
图
1 栈帧结构
栈帧布局的设计要考虑到指令集的体系结构特征和被编译的程序设计语言的特征。但是，计算机的制造者常常规定一种用于其体系结构的“标准”栈帧布局，以便被
所有的程序设计语言编译器采纳。这种栈帧布局对于某些特定的程序设计语言或编译器可能并不是最方便的，但是通过这种“标准”布局，用不同程序设计语言编写
的函数得以相互调用。当P调用Q时，Q的参数是放在P的帧中的。另外，当P调用Q时，P中的下一条指令地址将被压入栈中，形成P的栈帧的末尾，具体可参见
图1，返回地址就是当程序从Q返回时应该继续执行的地方。Q的栈帧从保存帧指针的位置开始，后面开始保存其他寄存器的值。Q也会用栈帧来保存其他不能存放
在寄存器中的局部变量。如果函数要返回整数或指针的话，常用寄存器%eax来保存返回值。当程序执行时，栈指针是可以移动的，因此大多数信息的访问都是相
对于帧指针(%ebp)的。二、 寄存器使用惯例假设函数P（……）调用函数Q（a1，……，an），我们称P是调用者（caller），Q是被调用者（callee）。如果必须被调用者保存和恢复的寄存器，我们称之为调用者保护的寄存器（caller-save）；如果是被调用者的责任，则称之为被调用者保护的寄存器（callee-
save）。程序寄存器组是唯一一个被所有函数共享的资源。虽然在给定时刻只能有一个函数是活动的，但是我们必须保证当一个函数调用另一个函数时，被调用
者不会覆盖某个调用者稍后会使用的寄存器的值。为此，任何一个平台都会制订一套标准，让所有的函数都必须遵循，包括程序库中的函数。但在大多数计算机系统
结构中，调用者保护的寄存器和被调用者保护的寄存器的概念并不是由硬件来实现的，而是机器参考手册中规定的一种约定。比如，在ARM体系平台中，所有的函
数调用必须遵守ARM 过程调用标准(APCS，ARM Procedure Call
Standard)。该标准提供了一套紧凑的代码编写机制，定义的函数可以与其他语言编写的函数交织在一起。其他函数可以编译自 C、
Pascal、也可以是用汇编语言写成的函数。同理，IA32平台也采用了一套统一的寄存器使用惯例。根据惯例，寄存器%eax、%edx、%ecx被划
分为调用者保存。当函数P（调用者）调用Q（被调用者）时，Q可以覆盖这些寄存器的值，而不会破坏任何P所需要的数据。另
外，%ebx、%esi、%edi、%ebp被划分为被调用者保存，这意味着Q必须在覆盖他们之前，将这些寄存器的值保存到栈中，并在返回前恢复他们。三、 参数传递惯例大
约在1960年之前，参数传递不是通过栈来传递的，而是通过一块静态分配的存储空间来传递的，这种方法阻碍了递归函数的使用。从20世纪70年代开始，大
多数调用约定函数参数的传递通过栈来实现（因为访问寄存器比访问存储器要快的多），同时也会导致一些不必要的存储器访问。对实际程序的研究表明，很少有函
数的参数个数是超过4个，并且极少有6个的。因此，现代计算机中的参数传递约定都规定，一个函数的前k个参数（典型的，k=4或者k=6）放在寄存器中传
递，剩余的参数则放在存储器中传递。在ARM体系平台中，APCS就明确规定：1) 前 4 个整数实参(或者更少!)被装载到 R0
– R4寄存器中。 2) 前 4 个浮点实参(或者更少!)被装载到 f0 - f3寄存器中。 3)
其他任何实参(如果有的话)存储在内存中，用进入函数时紧接在栈指针所指向的空间。换句话说，其余的参数被压入栈顶。但在IA32平台上，参数传递不是完
全通过寄存器来实现的，而是通过栈帧来实现的。根据不同的调用方式，参数在栈帧的存放方式又有一点差别，区别如下表所示： 调用方式 参数在堆栈里的次序 操作方式 _cdecl 第一个参数在低位地址 调用者 _stdcall 第一个参数在低位地址 被调用者 _fastcall 编译器指定 被调用者 _pascal 第一个参数在高位地址 被调用者 Borland
和 GNU 编译器使用 _cdecl 方式,而 Microsoft 使用 _stdcall
方式。通过这里可以发现，当今两大主流的编译器，第一个参数都在低地址，也就是说第一个参数是最后一个压栈的，当被调用函数被调用时，此时就赋值给了第一
个形参。四、 Linux内核源码研究下面我们以Linux内核中断处理源代码来研究函数调用的本质。本文
所提及的内核代码版本为2.6.10，读者需了解GCC汇编语言的基础知识。当某个中断发生后，将会执行push $i-256,jmp
common_interrupt指令，此时将会跳转到common_interrupt处执行。 文件名：arch/i386/kernel/entry.S（说明：前面的数字表示行号）
359 ALIGN 360 common_interrupt: 361
SAVE_ALL 362 movl %esp,%eax 363
call do_IRQ 364 jmp ret_from_intr 这里主要的操作是宏操作SAVE_ALL，就是所谓的“保存现场”，把中断发生前夕所有寄存器的内容都保存在堆栈中，待中断服务完毕返回之前再来“恢复现场”。
文件名：arch/i386/kernel/irq.c 48 fastcall unsigned int
do_IRQ(struct pt_regs *regs) 49 { 50 /*
high bits used in ret_from_ code */ //取得中断向量号 51 int
irq = regs->orig_eax & 0xff; 52 #ifdef CONFIG_4KSTACKS
53 union irq_ctx *curctx, *irqctx; 54
u32 *isp; 55 #endif …… 107 } 下面我们来分析SAVE_ALL和do_IRQ 函数中的struct pt_regs结构体。 文件名：arch/i386/kernel/entry.S 文件名：include/asm-i386/ptrace.h
84 #define SAVE_ALL \ 85 cld; \ 86
pushl %es; \ 87 pushl %ds; \ 88
pushl %eax; \ 89 pushl %ebp; \ 90
pushl %edi; \ 91 pushl %esi; \ 92
pushl %edx; \ 93 pushl %ecx; \ 94
pushl %ebx; \ 95 movl $(__USER_DS), %edx; \
96 movl %edx, %ds; \ 97 movl %edx,
%es;
26 struct pt_regs { 27 long ebx;
28 long ecx; 29 long edx;
30 long esi; 31 long edi;
32 long ebp; 33 long eax;
34 int xds; 35 int xes;
36 long orig_eax; 37 long eip;
38 int xcs; 39 long eflags;
40 long esp; 41 int xss; 42
};
从以上代码可以看出，SAVE_ALL中，依次将寄存器中的值压入栈中，其中最后一个压入的是pushl %ebx；我们观察struct
pt_regs结构体，可以看出该结构体的第一个成员变量就是long ebx；然后就依次对应。其中long orig_eax;的所对应的push
语句为：push $i-256。是在jmp common_interrupt之前压入的。细心的读者可能还会产生另外一个疑问： struct
pt_regs结构体中最后几个变量怎么在SAVE_ALL中没有对应的push。因为在进入中断服务程序之前，已经将这部分值压进栈中了，这是由硬件来
完成的。 从以上可以看出，在IA32平台上，参数的传递不是通过寄存器来实现的，而是通过栈帧来实现。但这并不是在Linux
操作系统上如此，而是所有的IA32平台都是如此，不管是何种操作系统，何种编译器，都得遵循前面所提及的规范。
下面我们仍以中断为例，在内核中调用者是如何保护寄存器的。
48 fastcall unsigned int do_IRQ(struct pt_regs *regs)
49 { …… 73 #ifdef CONFIG_4KSTACKS ……
92 asm volatile( 93
" xchgl %%ebx,%%esp \n" 94
" call __do_IRQ \n" 95
" movl %%ebx,%%esp \n" 96
: "=a" (arg1), "=d" (arg2), "=b" (ebx)
97 : "0" (irq), "1" (regs), "2" (isp)
98 : "memory", "cc", "ecx" 99
); …… 101 #endif
上述代码中的第一个冒号表示输出值，第二个冒号表示是输入值，第三个冒号后表示将被破坏的部分，需要恢复的值。在该段代码中，将会调用
__do_IRQ（）函数，由于在调用完该函数后，还会用到ecx的值，所有需要保存将ecx的值。那么在__do_IRQ（）函数中就可以毫无忌惮的使
用ecx寄存器。 从上例还可以发现，函数的输出值往往都存贮在寄存器eax中。第一个冒号后的"=a"
(arg1)表示将变量arg1和eax绑定，也就是arg1 = eax。五、 案例分析前不久，笔者在逛CSDN论坛时，发现不少人对如下题目展开了激烈的讨论。
#include low_to_up(char in); void main() {
printf("%c\n",low_to_up('d')); } low_to_up(char in) { char ch;
if(in>='a' && in 我们先在VC++ 6.0中反汇编这段代码：
1: #include 2: low_to_up(char in); 3: 4: void
main() 5: { 00401020 push ebp 00401021 mov ebp,esp
00401023 sub esp,40h 00401026 push ebx 00401027
push esi 00401028 push edi 00401029 lea
edi,[ebp-40h] 0040102C mov ecx,10h 00401031 mov
eax,0CCCCCCCCh 00401036 rep stos dword ptr [edi] 6:
printf("%c\n",low_to_up('d')); 00401038 push #64h
d的ASC码 (1处) 0040103A call @ILT+5(low_to_up) (0040100a)
此时把eax的值压栈，然后调用printf函数，所以此时打印的就是d。
0040103F add esp,4 00401042 push
eax # (5处)
00401043 push offset string "%c\n" (0042001c) 00401048
call printf (004010e0) 0040104D add esp,8 7: }
00401050 pop edi 00401051 pop esi 00401052
pop ebx 00401053 add esp,40h 00401056 cmp
ebp,esp 00401058 call __chkesp (00401160) 0040105D
mov esp,ebp 0040105F pop ebp 00401060 ret 8: 9:
low_to_up(char in) 10: { 00401080 push ebp 前一个栈帧的内容 …… 64h 返回地址 ebp (ch) ……
当前栈的内容 这里的ebp+8的内容就是64h，此时eax就是64。 00401081 mov ebp,esp
00401083 sub esp,44h
00401086 push ebx 00401087 push esi 00401088
push edi 00401089 lea edi,[ebp-44h] 0040108C
mov ecx,11h 00401091 mov eax,0CCCCCCCCh 00401096
rep stos dword ptr [edi] 11: char ch; 12: if(in>='a'
&& in
此时的edx是计算机后的值，但eax的值仍旧还是没有变。然后存储到ebp-4中，就是ch中。
13: ch=in-'a'+'A';
004010AA movsx edx,byte ptr [ebp+8] # (3处)
004010AE sub edx,20h 004010B1 mov byte ptr
[ebp-4],dl 14: else 004010B4 jmp low_to_up+3Ah
(004010ba)
15: return(ch);
004010B6 movsx eax,byte ptr [ebp-4]
16: } 004010BA pop edi # 恢复寄存器的值，做返回处理 (7处)
004010BB pop esi 004010BC pop ebx 004010BD
mov esp,ebp 004010BF pop ebp 004010C0 ret
从以上汇编可以看出，被调用者保存的寄存器在这里体现的酣畅淋漓。在被调用者函数中，根本不会管调用者保存的寄存器，直接使用。这里还体现了常用eax来
存贮返回值。由于是使用eax做返回值，所以在5处，直接将eax压栈，所以该程序在VC++6.0中，打印”d”。由于在low_to_up（）函数
中，在比较过程中借用了eax寄存器。按正常的流程，在执行完if语句后，应该有一条恰当的退出语句，所以导致没有执行7处的汇编语句，把正确的值存储到
eax中。从而阴差阳错的，在执行printf时，打印了错误的值。这有可能是VC++编译器的bug？GCC的编译器似乎技高一筹，表现的更尽人意。下
面我们来分析GCC反汇编后的代码（阅读这部分代码需了解GCC汇编语法）： .file "csdn.c" .text
前一个栈帧的内容 …… 100 返回地址 ebp …… esp-> (ch) ……
当前栈的内容 这里的ebp+8的内容就是100，此时ch被存储在ebp-8处。
.globl low_to_up .type low_to_up, @function low_to_up:
pushl %ebp movl %esp, %ebp
subl $8, %esp
movl 8(%ebp), %eax # (2处) movb %al,
-1(%ebp) cmpb $96, -1(%ebp) jle .L2
cmpb $122, -1(%ebp) jg .L2 movzbl -1(%ebp),
%eax subb $32, %al 此处运行完if语句的内容，但没有往ch里面的赋值
movb %al, -2(%ebp) # (3处) jmp .L3 .L2:
movsbl -2(%ebp),%eax movl %eax, -8(%ebp) # (5处)
jmp .L1 .L3:
.L1:
从ch中获取值，存储到eax中，然后返回
movl -8(%ebp), %eax # (4处)
leave
ret .size low_to_up, .-low_to_up .section
.rodata .LC0: .string "%c\n" .text .globl main
.type main, @function main: pushl %ebp
movl %esp, %ebp subl $8, %esp andl $-16,
%esp movl $0, %eax subl %eax, %esp
movl $100, (%esp) #将d的值压入到栈中，然后调用low_to_up（）函数 (1处)
call low_to_up movl %eax, 4(%esp) # (6处)
movl $.LC0, (%esp) call printf movl $0,
%eax leave ret .size main, .-main
.section .note.GNU-stack,"",@progbits .ident "GCC: (GNU)
3.3.5 (Debian 1:3.3.5-13)"
从以上代码可以看出，调用函数时，GCC编译器和VC++的编译器所做的处理差不多。但GCC把局部变量ch压在ebp-8处。然后用ebp-
1/ebp-2做临时存储。我们从5处可以发现，在else语句中，编译器会将ebp-2的值压入到eax中，然后又将eax中的值在压入到ebp-8
处，即ch的栈中。当返回时，会从ebp-8处取出ch的值，然后赋值给eax，退栈返回。在6处，将eax的值压栈，然后调用printf，输出值。由
于在if条件中，并没有将值存储到ch中，所以在low_to_up函数返回时，当执行4处的汇编代码时，将ch处的随机值放到了eax中，所以该程序在
Linux环境下，输出值为随机值。
笔者猜测，如果在L3处的加上一段和L2处相同的代码，那么就可以获得正确的值。笔者然后就在代码中的if语句条件中加入了return
语句，GCC反汇编后的代码验证了笔者的猜测，代码如下：
low_to_up: pushl %ebp movl %esp, %ebp
subl $8, %esp movl 8(%ebp), %eax movb %al,
-1(%ebp) cmpb $96, -1(%ebp) jle .L2
cmpb $122, -1(%ebp) jg .L2 movzbl -1(%ebp),
%eax subb $32, %al movb %al, -2(%ebp)
jmp .L3 .L2: movsbl -2(%ebp),%eax movl
%eax, -8(%ebp) jmp .L1 .L3: movsbl -2(%ebp),%eax
movl %eax, -8(%ebp) .L1: movl -8(%ebp), %eax
leave ret 六、 总结综上所述，当我们了解了函数调用规律后，结合汇编代码，在实际项目开发过程中可以定位一些让人比较琢磨不透的问题。也不要在项目中采用案例中的程序，给项目移植性造成困难。为什么在这个平台下好好的，怎么到了另外一个平台就出错了？七、 参考文献[1]
Andrew W.Appel，赵克佳等译 《现代编译原理C语言描述》 人民邮电出版社 2006[2] Randal
E.Bryant等，龚奕利等译 《深入理解计算机系统》 中国电力出版社 2004[3] Agner Fog，云风译
《怎样优化Pentium系列处理器的代码》替换www.codingnow.com/2000/download
/cpentopt.htm[4] Linus Torvalds， Linux内核源代码（2.6.10版本） 替换www.kernel.org
本文来自ChinaUnix博客，如果查看原文请点：http://blog.chinaunix.net/u2/87831/showart_1859396.html |
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发表于 2009-03-11 16:44