[英]How to call a C function from ASM in C?
I'm using this code but it gives me Segmentation fault
when debugging. 我正在使用此代码,但调试时会出现
Segmentation fault
。 I'm compiling with MinGW: 我正在使用MinGW进行编译:
#include <stdio.h>
#include <stdlib.h>
int CallMe(int a, int b)
{
printf("Called.");
return a + b;
}
int main()
{
printf("CallMe at 0x00%x\n", (unsigned int)&CallMe);
printf("Calling CallMe... ");
asm (
"movl $5, (%eax);\r\n"
"movl $4, (%ebx);\r\n"
"call 0x00401334;\r\n"
);
printf("\n");
return 0;
}
The function address is ok. 功能地址可以。 Any ideas?
有任何想法吗?
Thanks in advance. 提前致谢。
First problem is the call, as @EricZ points out. @EricZ指出,第一个问题是通话。
However, fixing that won't necessarily make the segfault go away. 但是,修复该问题并不一定会使该段错误消失。 Your two moves are also causing a problem:
您的两个举动也引起了问题:
movl $5, (%eax)
movl $4, (%ebx)
This is moving values 5
and 4
into locations whose addresses are in %eax
and %ebx
, respectively. 这会将值
5
和4
移动到地址分别位于%eax
和%ebx
。 In this particular program, those addresses haven't been defined. 在此特定程序中,尚未定义这些地址。 If you want to pass them as parameters, you need to push them onto the stack.
如果要将它们作为参数传递,则需要将它们压入堆栈。
pushl $5
pushl $4
call CallMe % With some compilers, this may need to be _CallMe
addl $8,%esp % assuming a 32-bit processor
Or, if the function expects them in the registers (I'm not saying it would in this case, but if ...), remove the parentheses: 或者,如果函数期望它们在寄存器中(我不是说在这种情况下会这样做,但是如果 ...),请删除括号:
movl $5, %eax
movl $4, %ebx
You might want to read up on calling conventions, eg, http://en.wikipedia.org/wiki/X86_calling_conventions . 您可能需要阅读有关调用约定的信息,例如http://en.wikipedia.org/wiki/X86_calling_conventions 。
Generally, don't call the function by its hard-coded address. 通常,请勿通过其硬编码地址来调用该函数。 Try call the function name instead.
尝试改为调用函数名称。 Function name is effectively the function address.
函数名称实际上是函数地址。
call 0x00401334
==> ==>
call CallMe
This is what is really executing in your main()
function: 这是在
main()
函数中真正执行的操作:
0x401268 <main>: push %ebp
0x401269 <main+1>: mov %esp,%ebp
0x40126b <main+3>: sub $0x8,%esp
0x40126e <main+6>: call 0x401350 <__main>
0x401273 <main+11>: add $0xfffffff8,%esp
0x401276 <main+14>: push $0x401218 <--- address of CallMe
0x40127b <main+19>: push $0x401242
0x401280 <main+24>: call 0x4013c0 <printf>
0x401285 <main+29>: add $0x10,%esp
0x401288 <main+32>: add $0xfffffff4,%esp
0x40128b <main+35>: push $0x401250
0x401290 <main+40>: call 0x4013c0 <printf>
0x401295 <main+45>: add $0x10,%esp
0x401298 <main+48>: movl $0x5,(%eax) <--- begin of your ASM block
0x40129e <main+54>: movl $0x4,(%ebx)
0x4012a4 <main+60>: call 0x8024c1 <--- eewww! wrong address
So the question is now: why a CALL 0x00401218
is assembled as CALL 0x008024C1
? 所以现在的问题是:为什么将
CALL 0x00401218
组装为CALL 0x008024C1
?
Of course, you will see also that parameters are not being passed by registers, but by pushing them into the stack. 当然,您还将看到参数不是通过寄存器传递的,而是通过将它们压入堆栈传递的。
In fact, I've got it by using this: 实际上,我通过使用以下命令获得了它:
asm (
"pushl $5;\r\n"
"pushl $4;\r\n"
"call _CallMe;\r\n"
"addl %esp,$8;\r\n"
);
The problem I had before, about stack not bein balanced, was caused by a mistake in my code, that used addl %esp,8
instead of addl %esp,$8
. 我之前遇到的关于堆栈不平衡的问题是由我的代码中的错误引起的,该错误使用
addl %esp,8
而不是addl %esp,$8
。 The former (tries to) add the contents of memory at address 8
to ESP
, instead of adding the constant 8
to ESP
. 前者(尝试)将地址
8
的内存内容添加到ESP
,而不是将常量8
添加到ESP
。
If you want to collect the result of your function and save it, for example, in a variable, you can do as this: 如果要收集函数结果并将其保存(例如,保存在变量中),可以执行以下操作:
int res;
...
...
asm (
"pushl $5;\r\n"
"pushl $4;\r\n"
"call _CallMe;\r\n"
"addl $8,%%esp;\r\n"
"movl %%eax,%0;\r\n" : "=m" (res)
);
If you really want to call the function using its address, you can do it several ways: 如果您确实想使用其地址来调用该函数,则可以采用以下几种方法:
By putting the address in a variable, say, a void
pointer: 通过将地址放在变量中,例如
void
指针:
void *addr;
addr = (void *)CallMe;
asm (
"pushl $5;\r\n"
"pushl $4;\r\n"
"calll *%1;\r\n"
"addl $8,%%esp;\r\n"
"movl %%eax,%0;\r\n" : "=m" (res) : "m" (addr)
);
Or by putting the address to call in a register: 或通过将要调用的地址放入寄存器中:
asm (
"pushl $5;\r\n"
"pushl $4;\r\n"
"movl $0x00401218,%%eax;\r\n"
"calll *%%eax;\r\n"
"addl $8,%%esp;\r\n"
"movl %%eax,%0;\r\n" : "=m" (res)
);
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.