Cygwin环境下Make编译出现多重定义错误求助

我是Makefile新手，在Cygwin环境编译msb项目时碰到了链接错误，具体信息和相关文件内容如下：

项目根目录Makefile内容 #

CC=gcc
default: msb doc
lib: libmsb-*
    cd libmsb-*/ && make -f Makefile.in
msb: lib msb.c
    $(CC) msb.c -o msb -I libmsb-*/ -L libmsb-*/ -l msb
install: msb
    install -s msb -t $(DESTDIR)/bin/
doc: msb.1
    groff -Tlatin1 -man msb.1 | col -b > msb.1.txt
cleanup:
    rm -f libmsb-*/*.o libmsb-*/*.a
clean: cleanup
    rm -f msb
    rm -f msb.1.txt

运行make后的错误输出 #

cd libmsb-*/ && make -f Makefile.in
make[1]: Entering directory 'C:/Users/****/Desktop/msb-0.1/libmsb-0.2'
make[1]: 'libmsb.a' is up to date.
make[1]: Leaving directory 'C:/Users/****/Desktop/msb-0.1/libmsb-0.2'
gcc msb.c -o msb -I libmsb-*/ -L libmsb-*/ -l msb
msb.c: In function ‘main’:
msb.c:94:19: warning: ‘optarg’ redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
   94 |     extern char * optarg;
      |                   ^~~~~~
msb.c:95:16: warning: ‘optind’ redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
   95 |     extern int optind, optopt;
      |                ^~~~~~
msb.c:95:24: warning: ‘optopt’ redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
   95 |     extern int optind, optopt;
      |                        ^~~~~~
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_std.o):msb_std.c:(.bss+0x0): multiple definition of `msb_parities'; /tmp/cch7E9uN.o:msb.c:(.bss+0x0): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_std.o):msb_std.c:(.bss+0x4): multiple definition of `msb_lrc_flags'; /tmp/cch7E9uN.o:msb.c:(.bss+0x4): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_std.o):msb_std.c:(.bss+0x8): multiple definition of `msb_std_track_map'; /tmp/cch7E9uN.o:msb.c:(.bss+0x8): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_encode.o):msb_encode.c:(.bss+0x0): multiple definition of `msb_parities'; /tmp/cch7E9uN.o:msb.c:(.bss+0x0): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_encode.o):msb_encode.c:(.bss+0x4): multiple definition of `msb_lrc_flags'; /tmp/cch7E9uN.o:msb.c:(.bss+0x4): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_encode.o):msb_encode.c:(.bss+0x8): multiple definition of `msb_std_track_map'; /tmp/cch7E9uN.o:msb.c:(.bss+0x8): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_decode.o):msb_decode.c:(.bss+0x0): multiple definition of `msb_parities'; /tmp/cch7E9uN.o:msb.c:(.bss+0x0): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_decode.o):msb_decode.c:(.bss+0x4): multiple definition of `msb_lrc_flags'; /tmp/cch7E9uN.o:msb.c:(.bss+0x4): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_decode.o):msb_decode.c:(.bss+0x8): multiple definition of `msb_std_track_map'; /tmp/cch7E9uN.o:msb.c:(.bss+0x8): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_7811.o):msb_7811.c:(.bss+0x0): multiple definition of `msb_parities'; /tmp/cch7E9uN.o:msb.c:(.bss+0x0): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_7811.o):msb_7811.c:(.bss+0x4): multiple definition of `msb_lrc_flags'; /tmp/cch7E9uN.o:msb.c:(.bss+0x4): first defined here
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: libmsb-0.2//libmsb.a(msb_7811.o):msb_7811.c:(.bss+0x8): multiple definition of `msb_std_track_map'; /tmp/cch7E9uN.o:msb.c:(.bss+0x8): first defined here
collect2: error: ld returned 1 exit status
make: *** [Makefile:6: msb] Error 1

libmsb子目录Makefile.in内容 #

libmsb.a: msb_common.h msb_std.h msb_encode.h msb_decode.h msb_7811.h\
                      msb_std.c msb_encode.c msb_decode.c msb_7811.c
    $(CC) -c msb_std.c -o msb_std.o
    $(CC) -c msb_encode.c -o msb_encode.o
    $(CC) -c msb_decode.c -o msb_decode.o
    $(CC) -c msb_7811.c -o msb_7811.o
    ar crs libmsb.a msb_std.o msb_encode.o msb_decode.o msb_7811.o

问题分析与解决办法 #

核心问题 #

链接器报错的multiple definition是因为全局变量msb_parities、msb_lrc_flags、msb_std_track_map在msb.c和库文件libmsb.a的多个目标文件中都被定义了。这是C语言中典型的全局变量重复定义问题——当多个编译单元（.o文件）都定义了同名全局变量，链接时就会冲突。

具体修复步骤 #

1. 修改msb.c中的全局变量定义
   打开msb.c，找到这三个全局变量的定义（类似int msb_parities;的代码），将它们改为外部声明：

   extern int msb_parities;
   extern int msb_lrc_flags;
   extern int msb_std_track_map[];
   

   这样msb.c就不会再定义这些变量，而是引用库中已经定义的版本。

2. 确保库中的变量定义唯一
   检查libmsb-0.2目录下的头文件（如msb_std.h），如果这些变量在头文件中被定义（而非声明），需要修改为带extern的声明，然后在对应的.c文件（如msb_std.c）中保留唯一的定义。
   示例：

   • 头文件中改为：

     extern int msb_parities;
     extern int msb_lrc_flags;
     extern int msb_std_track_map[];
     

   • 在msb_std.c中保留：

     int msb_parities;
     int msb_lrc_flags;
     int msb_std_track_map[...]; // 填写原代码中的数组长度
     

3. 处理optarg相关警告
   报错中的optarg、optind、optopt重复声明警告，是因为msb.c手动声明了这些变量，但它们已经在<unistd.h>中定义。解决办法：

   • 删除msb.c第94、95行的手动声明
   • 在msb.c开头添加头文件包含：#include <unistd.h>

4. 重新编译
   先执行make clean清理旧的目标文件和库，再运行make重新编译。

额外优化建议 #

• 避免在Makefile中使用libmsb-*/通配符，直接写具体目录名（如libmsb-0.2），防止目录结构变化导致编译出错。
• 编译库时添加-fPIC参数（$(CC) -c -fPIC msb_std.c -o msb_std.o），生成位置无关代码，提升兼容性。

内容的提问来源于stack exchange，提问作者O'Neil Lawrence