C归档文件(库)与Clang链接程序的基本用法

问题描述

我有一个主程序:

  #include< stdio.h>外部intint main(int argc，char ** argv){我printf("Hello％p，％p \ n"，& i，& a);返回0;} 

和一个单独的文件 foo.c ，其中包含我对变量 a 的定义:

  int a; 

如果我这样建造:

  clang -c main.c foo.clang main.o foo.o./a.out 

一切都很好.但是，如果我执行以下操作:

  ar rvs bar.a foo.o 

我收到我不明白的警告

  r-foo.o警告:/< elided>/ranlib:库警告:bar.a目录为空(库中没有目标文件成员定义全局符号) 

所以我检查我的书架，确保其中有我的符号

  nm bar.abar.a(foo.o):0000000000000004 C _a 

然后我

  clang main.o bar.a 

我收到以下错误

 体系结构x86_64的未定义符号:"_a"，引用自:_main在main-5121f1.o中ld:找不到架构x86_64的符号clang:错误:链接器命令失败，退出代码为1(使用-v查看调用) 

我想念什么?

我认为麻烦是 foo.c 中的 int a; 是一个暂定的定义.在 nm 的输出中将其标记为 C (常见).如果您明确初始化(为保持一致性，将代码初始化为 0 ；将其初始化为某个非零值以确保用于测试目的)，我相信您不会有任何问题.

我不完全相信，因为在TU结束时，暂定定义应转换为定义:

另请参见

I have a main program:

#include <stdio.h>
extern int a;
int main (int argc, char ** argv) {
    int i;
    printf ("Hello %p, %p\n", & i, & a);
    return 0;
}

and a separate file, foo.c, which contains my definition for the variable a:

int a;

If I build like this:

clang -c main.c foo.c
clang main.o foo.o
./a.out

all is well. But, if I do the following:

ar rvs bar.a foo.o

I get a warning that I don't understand

r - foo.o
warning: /<elided>/ranlib:
warning for library: bar.a the table of contents is empty
(no object file members in the library define global symbols)

so I check my library, ensuring that my symbol is in there

nm bar.a

bar.a(foo.o):
0000000000000004 C _a

and then I do

clang main.o bar.a

I get the following error

Undefined symbols for architecture x86_64:
  "_a", referenced from:
      _main in main-5121f1.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

what did I miss?

解决方案

I think the trouble is that the int a; in foo.c is a tentative definition. It is marked C (common) in the output from nm. If you explicitly initialized (to 0 for consistency; to some non-zero value to make sure for testing purposes), I believe you'd have no problem.

I'm not completely convinced because at the end of a TU, tentative definitions should be converted to definitions:

See also How do I share a variable between source files in C?

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