尝试使用MinGW编译Assembly + C ++：ccqKAvXJ.o：main.cpp :(。text + 0x18）：未定义对“ GetMagicNumber”的引用

Question

I have this simple sample:

main.cpp:

#include <iostream>

extern "C" int GetMagicNumber();

int main(void)
{
    std::cout << "The magic number is: " << GetMagicNumber();
    return 0;
}

functions.s:

.section .text

.global GetMagicNumber

GetMagicNumber:
    movl $42, %eax
    ret

and build.bat:

C:\Qt\Qt5.2.0\Tools\mingw48_32\bin\as functions.s -o functions.o
C:\Qt\Qt5.2.0\Tools\mingw48_32\bin\g++ functions.o main.cpp -o MagicNumber.exe

or

C:\Qt\Qt5.2.0\Tools\mingw48_32\bin\gcc functions.s main.cpp -lstdc++ -o MagicNumber.exe

When I try to compile with MinGW 4.8. I get:

C:\Qt\Qt5.2.0\Tools\mingw48_32\bin\gcc functions.s main.cpp -lstdc++ -o MagicNumber.exe
C:\Users\...\Local\Temp\ccqKAvXJ.o:main.cpp:(.text+0x18): undefined reference to `GetMagicNumber'
collect2.exe: error: ld returned 1 exit status

I've read a few tutorials and I watched videos and I still cannot figure out what I am doing wrong. Why do I get that undefined reference error?

Answer 1

External scope C symbols have a leading underscore, so try:

.section .text

.global _GetMagicNumber

_GetMagicNumber:
    movl $42, %eax
    ret

EDIT if you want to make the code more platform-agnostic, I use the following macro, where the assembler file has a .S extension which is processed by the C compiler, allowing the pre-processor to play along:

#ifdef __APPLE__
#define NAME(name) _##name
#else
#define NAME(name) name
#endif

.globl NAME(GetMagicNumber);
NAME(GetMagicNumber):
    ...

(see an example here ).