ldconfig使用小结
2017/11/1
DESCRIPTION ldconfig creates the necessary links and cache to the most recent shared libraries found in the directories specified on the command line, in the file /etc/ld.so.conf, and in the trusted directories (/lib and /usr/lib). The cache is used by the run-time linker, ld.so or ld-linux.so. ldconfig checks the header and filenames of the libraries it encounters when determining which versions should have their links updated. OPTIONS -v Verbose mode. Print current version number, the name of each directory as it is scanned, and any links that are created. Overrides quiet mode. -n Only process directories specified on the command line. Don’t process the trusted directories (/lib and /usr/lib) nor those specified in /etc/ld.so.conf. Implies -N. -N Don’t rebuild the cache. Unless -X is also specified, links are still updated. -X Don’t update links. Unless -N is also specified, the cache is still rebuilt. -f conf Use conf instead of /etc/ld.so.conf. -C cache Use cache instead of /etc/ld.so.cache. -r root Change to and use root as the root directory. -l Library mode. Manually link individual libraries. Intended for use by experts only. -p Print the lists of directories and candidate libraries stored in the current cache. 通常我们要引用第三方库的时候，会执行 ldconfig 来更新缓存 要注意一个细节：不正确的配置将导致 .so 文件被还原，从而不符合预期。 我们的目的：升级 libstdc++.so 到另外一个版本 下面是一个演示 当前状态 [root@dev32 ~]# ls /usr/lib64/libstdc++.so.6* -l lrwxrwxrwx 1 root root 19 Nov 1 12:00 /usr/lib64/libstdc++.so.6 -> libstdc++.so.6.0.13 -rwxr-xr-x 1 root root 989840 May 10 2016 /usr/lib64/libstdc++.so.6.0.13 计划使用的版本 [root@dev32 ~]# cp -a /usr/lib64/libstdc++.so.6.0.13 /usr/local/lib64/libstdc++.so.6.111.222 [root@dev32 ~]# ls -l /usr/local/lib64/ total 968 -rwxr-xr-x 1 root root 989840 May 10 2016 libstdc++.so.6.111.222 如果直接做一个软连接： [root@dev32 ~]# ln -sfv /usr/local/lib64/libstdc++.so.6.111.222 /usr/lib64/libstdc++.so.6 `/usr/lib64/libstdc++.so.6' -> `/usr/local/lib64/libstdc++.so.6.111.222' [root@dev32 ~]# ls /usr/lib64/libstdc++.so.6* -l lrwxrwxrwx 1 root root 39 Nov 1 12:08 /usr/lib64/libstdc++.so.6 -> /usr/local/lib64/libstdc++.so.6.111.222 -rwxr-xr-x 1 root root 989840 May 10 2016 /usr/lib64/libstdc++.so.6.0.13 看起来是符合预期，对吧，但如果我们执行： [root@dev32 ~]# ldconfig -v |grep changed ldconfig: Path `/usr/local/lib' given more than once ldconfig: Path `/usr/local/lib64' given more than once libstdc++.so.6 -> libstdc++.so.6.111.222 (changed) libstdc++.so.6 -> libstdc++.so.6.0.13 (changed) 结果出乎意外不： [root@dev32 ~]# ls /usr/lib64/libstdc++.so.6* -l lrwxrwxrwx 1 root root 19 Nov 1 12:09 /usr/lib64/libstdc++.so.6 -> libstdc++.so.6.0.13 -rwxr-xr-x 1 root root 989840 May 10 2016 /usr/lib64/libstdc++.so.6.0.13 [root@dev32 ~]# ls -l /usr/local/lib64 total 968 lrwxrwxrwx 1 root root 22 Nov 1 12:09 libstdc++.so.6 -> libstdc++.so.6.111.222 -rwxr-xr-x 1 root root 989840 May 10 2016 libstdc++.so.6.111.222 解决方案： [root@dev32 ~]# rm /usr/lib64/libstdc++.so.6* -f [root@dev32 ~]# ln -sfv /usr/local/lib64/libstdc++.so.6.111.222 /usr/lib64/libstdc++.so.6 `/usr/lib64/libstdc++.so.6' -> `/usr/local/lib64/libstdc++.so.6.111.222' [root@dev32 ~]# ls /usr/lib64/libstdc++.so.6* -l lrwxrwxrwx 1 root root 39 Nov 1 12:15 /usr/lib64/libstdc++.so.6 -> /usr/local/lib64/libstdc++.so.6.111.222 [root@dev32 ~]# ldconfig -v |grep changed ldconfig: Path `/usr/local/lib' given more than once ldconfig: Path `/usr/local/lib64' given more than once [root@dev32 ~]# ls /usr/lib64/libstdc++.so.6* -l lrwxrwxrwx 1 root root 39 Nov 1 12:15 /usr/lib64/libstdc++.so.6 -> /usr/local/lib64/libstdc++.so.6.111.222 原因分析： [root@dev32 ~]# ls /usr/local/lib64 -l total 968 lrwxrwxrwx 1 root root 22 Nov 1 12:18 libstdc++.so.6 -> libstdc++.so.6.111.222 -rwxr-xr-x 1 root root 989840 May 10 2016 libstdc++.so.6.111.222 [root@dev32 ~]# rm -fv /usr/local/lib64/libstdc++.so.6 removed `/usr/local/lib64/libstdc++.so.6' [root@dev32 ~]# ls /usr/local/lib64 -l total 968 -rwxr-xr-x 1 root root 989840 May 10 2016 libstdc++.so.6.111.222 [root@dev32 ~]# ldconfig -v |grep changed ldconfig: Path `/usr/local/lib' given more than once ldconfig: Path `/usr/local/lib64' given more than once libstdc++.so.6 -> libstdc++.so.6.111.222 (changed) [root@dev32 ~]# 上述操作显示，当执行 ldconfig 时，默认会生成一个 libstdc++.so.6 的软连接来指向具体的库文件 libstdc++.so.6 -> libstdc++.so.6.111.222 (changed) 因而当 /usr/lib64/ /usr/local/lib64 都存在一样的库文件时，将生成对应的软连接文件 注意本文开头的这段话： ldconfig checks the header and filenames of the libraries it encounters when determining which versions should have their links updated. 个人理解，源于 ldconfig 会检查库文件的 header 和 filenames 来决定做软连接的时候将使用哪个版本。 OK 让我们再做一次实验： [root@dev32 ~]# rm -fv /usr/local/lib64/libstdc++.so.6 removed `/usr/local/lib64/libstdc++.so.6' [root@dev32 ~]# ls -l /usr/local/lib64/ total 968 -rwxr-xr-x 1 root root 989840 Nov 1 11:54 libstdc++.so.6.111.222 [root@dev32 ~]# ldconfig -X -v |grep changed ldconfig: Path `/usr/local/lib' given more than once ldconfig: Path `/usr/local/lib64' given more than once [root@dev32 ~]# ls -l /usr/local/lib64/ total 968 -rwxr-xr-x 1 root root 989840 Nov 1 11:54 libstdc++.so.6.111.222 符合预期，因为我们使用这个参数 -X 来避免更新软连接 看起来，我们找到了问题所在。
