{"topic":{"id":12744,"title":"想请教一个关于 link 的问题","slug":"topic","userId":3722,"categoryId":58,"createdAt":"2024-05-29 15:32:14.61654","bumpedAt":"2024-05-30 05:08:35.179836","views":258,"postsCount":2,"highestPostNumber":2,"likeCount":0,"closed":false,"pinned":false,"pinnedGlobally":false,"pinnedAt":null,"pinnedUntil":null,"isBanner":false,"excerpt":"正常输出 1000 报错 /usr/bin/ld: /tmp/ccnihdJU.o:(.data+0x0): multiple definition of `a'; /tmp/cc8sytkl.o:(.bss+0x0): first defined here collect2: error: ld returned 1 exit status 两次编译命令都是 g++ main.cpp test.cpp -o mytest.o 第二张图…","deletedAt":null,"isAnonymous":false,"isPrivate":false,"isMembersOnly":false,"category":{"id":58,"name":"计算机系统导论","slug":"ics","color":"3AB54A"},"author":{"id":3722,"username":"worfsmile","name":"_ZSH","avatarUrl":"/uploads/image/c4/1f/c41fedce37237fb44d264c83a1abf4c378269fdbbdd56243ea58763c07ce3823.avif"},"realAuthor":{"id":3722,"username":"worfsmile","name":"_ZSH","avatarUrl":"/uploads/image/c4/1f/c41fedce37237fb44d264c83a1abf4c378269fdbbdd56243ea58763c07ce3823.avif"},"tags":[]},"posts":[{"id":34577,"postNumber":1,"raw":"![84f4880974b80b2d30676e24a01ed42b.JPG|402x500](/uploads/image/83/a2/83a21189981d51bb0f27a4e88884f6bd02326005fa81ea4b15851076e441d290.avif)\n\n正常输出 1000\n![undefined|402x500](/uploads/image/6e/82/6e82d47fe500b588f64fb095afc06f0d90b2e22890674f6da3c5f7d65e710e6d.avif)\n\n报错\n/usr/bin/ld: /tmp/ccnihdJU.o:(.data+0x0): multiple definition of `a'; /tmp/cc8sytkl.o:(.bss+0x0): first defined here\ncollect2: error: ld returned 1 exit status\n\n两次编译命令都是\ng++ main.cpp test.cpp -o mytest.o\n\n第二张图改用\ng++ test.cpp -c test.o\ng++ main.cpp -c main.o\ng++ main.o test.o -o mytest.o\n\n还是\n/usr/bin/ld: test.o:(.data+0x0): multiple definition of `a'; main.o:(.bss+0x0): first defined here\ncollect2: error: ld returned 1 exit status\n\n想问问第二张图 main.cpp 的 a 为什么不是弱变量 local var 应该绑定到 test.cpp 的 a 上，是我什么地方理解错了吗","cooked":"

$\"84f4880974b80b2d30676e24a01ed42b.JPG\"$

正常输出 1000
\n $\"undefined\"$

报错
\n/usr/bin/ld: /tmp/ccnihdJU.o:(.data+0x0): multiple definition of `a'; /tmp/cc8sytkl.o:(.bss+0x0): first defined here
\ncollect2: error: ld returned 1 exit status

两次编译命令都是
\ng++ main.cpp test.cpp -o mytest.o

第二张图改用
\ng++ test.cpp -c test.o
\ng++ main.cpp -c main.o
\ng++ main.o test.o -o mytest.o

还是
\n/usr/bin/ld: test.o:(.data+0x0): multiple definition of `a'; main.o:(.bss+0x0): first defined here
\ncollect2: error: ld returned 1 exit status

想问问第二张图 main.cpp 的 a 为什么不是弱变量 local var 应该绑定到 test.cpp 的 a 上，是我什么地方理解错了吗

\n","createdAt":"2024-05-29 15:32:14.758314","updatedAt":"2024-05-29 15:32:14.758314","replyToPostNumber":null,"replyCount":0,"likeCount":0,"wordCount":118,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":3722,"username":"worfsmile","name":"_ZSH","avatarUrl":"/uploads/image/c4/1f/c41fedce37237fb44d264c83a1abf4c378269fdbbdd56243ea58763c07ce3823.avif","admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}},{"id":34595,"postNumber":2,"raw":"把所有的源文件都改成.c 就能正确链接了。\n\n问题的根源是 C++ 编译器一般不会生成 common symbol（就算使用 `-fcommon`）。但是 C 会，因为它要支持从 Fortran port 来的程序，而 common symbol 是 Fortran 的首创。\n\n对这个问题可以详见：https://stackoverflow.com/q/10411052\n\n你也可以使用`readelf -s`观察符号的 index 是否为 `COM`，从而确定编译器是否生成了一个 common symbol。","cooked":"

把所有的源文件都改成.c 就能正确链接了。

问题的根源是 C++ 编译器一般不会生成 common symbol（就算使用 -fcommon）。但是 C 会，因为它要支持从 Fortran port 来的程序，而 common symbol 是 Fortran 的首创。

对这个问题可以详见：https://stackoverflow.com/q/10411052

你也可以使用readelf -s观察符号的 index 是否为 COM，从而确定编译器是否生成了一个 common symbol。

\n","createdAt":"2024-05-30 05:08:35.179836","updatedAt":"2024-06-05 03:12:34.401634","replyToPostNumber":null,"replyCount":0,"likeCount":0,"wordCount":114,"liked":false,"bookmarked":false,"polls":[],"reactions":[],"wiki":false,"version":1,"publicVersion":1,"deletedAt":null,"isAnonymous":false,"user":{"id":3625,"username":"yhdang","name":"yhdang","avatarUrl":null,"admin":false,"moderator":false,"signatureRaw":null,"signatureCooked":null}}],"hasMore":false,"lastReadPostNumber":2,"hasUrlPostNumber":true}