OS X链接器无法从仅包含变量的C文件中找到符号 [英] OS X linker unable to find symbols from a C file which only contains variables

问题描述

将C库从Linux(Ubuntu)移植到OS X时，链接器出现问题.C代码是从Matlab自动生成的，因此理想情况下，我不想更改代码本身.

I am having problems with the linker when porting a C library from Linux (Ubuntu) to OS X. The C code is auto-generated from Matlab, so ideally I don't want to change the code itself.

问题似乎出在C文件中，其中仅包含未初始化的变量声明，然后其他C文件对其进行扩展以实现Matlab算法.OS X链接器显然无法识别该文件中的符号.相同的源代码在Linux上可以正常工作，因此我想了解OS X链接器的行为方式是否不同，以及是否可以传递给我一个标志来更改行为.

The problem seems to be in a C file which contains ONLY uninitialised variable declarations, which are then EXTERNed by other C files to implement the Matlab algorithms. The OS X linker is apparently unable to recognise symbols from this file. The same source code works fine on Linux, so I want to understand how the OS X linker is behaving differently, and whether there is a flag I can pass to it to change the behaviour.

静态库的构建没有错误/警告.但是，当构建引用静态库的应用程序时，会抛出以下错误消息(在OS X上):

The static library builds without errors/warnings. But when building an application which references the static library, the following error message (on OS X) is thrown:

Undefined symbols for architecture x86_64:
  "_my_var", referenced from:
      _algorithm in libtestlibrary.a(algorithm.o)
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

'nm'表明libtestlibrary.a确实包含符号_my_var.

An 'nm' shows that libtestlibrary.a does indeed contain symbol _my_var.

下面是Matlab代码的简化版本.

A much simplified version of the code from Matlab follows below.

库代码:

// DATA.C : declaration of data
#include "data.h"
int my_var;

// DATA.H - extern declaration of data
#ifndef H_DATA
#define H_DATA
extern int my_var;
#endif

// ALGORITHM.C - performs the calculation
#include "data.h"
int algorithm(int x) {
  my_var += x;
  return my_var;
}

//ALGORITHM.H - declaration of library API
#ifndef H_ALGORITHM
#define H_ALGORITHM
int algorithm(int x);
#endif

库构建命令:

gcc -c algorithm.c
gcc -c data.c
ar rcs libtestlibrary.a data.o algorithm.o

应用代码:

// MAIN.C : Code which calls into the static library
#include "algorithm.h"
int main() {
  int x = 1;
  x = algorithm(x);
  return 0;
}

应用程序构建命令:

gcc -c main.c
gcc -o testapp main.o -ltestlibrary

如果我将data.c中的定义更改为'int my_var = 0'，以便对该变量进行初始化，则该库和应用程序可以在Linux和OS X上正确构建.但是，如上所述，我没有不想更改代码，因为它是从Matlab自动生成的.

If I change the definition in data.c to 'int my_var=0', so that the variable is initialised, then the library and application build correctly on both Linux and OS X. However, as I said above, I don't want to change the code because it is auto-generated from Matlab.

提前感谢您的帮助！

推荐答案

您的问题是您没有初始化 may_var .

Your problem is that you do not initialise may_var.

如果不初始化数据符号并将其放入静态库，则会将其创建为通用符号.链接静态库时，那些不能在OS X链接器中识别.

If you do not initialise a data symbol and put it into a static library then it will be created as a common symbol. Those are not recognised my the OS X linker when linking static libraries.

如果您将其初始化(data.c):

If you initialise it (data.c):

#include "data.h"
int my_var = 0;

然后，编译器会将其放在不同的部分，并将正确链接到静态库中.

Then the compiler will put it into a different section and it will properly link in a static library.

或者，您可以将 -fno-common 选项传递给gcc

Alternatively you can pass the -fno-common option to gcc

gcc -c data.c -fno-common

然后这将指示 gcc 不要为未初始化的变量生成通用符号，然后可以在库中链接它们.

Then this will instruct gcc not to generate common symbols for uninitialised variables and then you can link them in libraries.

这是OS X上Mach-O可执行文件格式的问题，并在

This is an issue with the Mach-O executable format on OS X and is described here.

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