Detailed explanation of command-ldconfig

ldconfig is usually run when the system starts. When the user installs a new dynamic link library, this command needs to be run manually.

grammar

ldconfig [-v|--verbose] [-n] [-N] [-X] [-f CONF] [-C CACHE] [-r ROOT] [-l] [-p|--print-cache] [-c FORMAT] [--format=FORMAT] [-V] [-?|--<span class="wp_keywordlink">help</span>|--usage] path...

Options

-v或--verbose：用此选项时，ldconfig将显示正在扫描的目录及搜索到的动态链接库，还有它所创建的连接的名字。
-n：用此选项时,ldconfig仅扫描命令行指定的目录，不扫描默认目录（/lib、/usr/lib），也不扫描配置文件/etc/ld.so.conf所列的目录。
-N：此选项指示ldconfig不重建缓存文件（/etc/ld.so.cache），若未用-X选项，ldconfig照常更新文件的连接。
-X：此选项指示ldconfig不更新文件的连接，若未用-N选项，则缓存文件正常更新。
-f CONF：此选项指定动态链接库的配置文件为CONF，系统默认为/etc/ld.so.conf。
-C CACHE：此选项指定生成的缓存文件为CACHE，系统默认的是/etc/ld.so.cache，此文件存放已排好序的可共享的动态链接库的列表。
-r ROOT：此选项改变应用程序的根目录为ROOT（是调用chroot函数实现的）。选择此项时，系统默认的配置文件/etc/ld.so.conf，实际对应的为ROOT/etc/ld.so.conf。如用-r /usr/zzz时，打开配置文件/etc/ld.so.conf时，实际打开的是/usr/zzz/etc/ld.so.conf文件。用此选项，可以大大增加动态链接库管理的灵活性。
-l：通常情况下,ldconfig搜索动态链接库时将自动建立动态链接库的连接，选择此项时，将进入专家模式，需要手工设置连接，一般用户不用此项。
-p或--print-cache：此选项指示ldconfig打印出当前缓存文件所保存的所有共享库的名字。
-c FORMAT 或 --format=FORMAT：此选项用于指定缓存文件所使用的格式，共有三种：old(老格式)，new(新格式)和compat（兼容格式，此为默认格式）。
-V：此选项打印出ldconfig的版本信息，而后退出。
-? 或 --help 或 --usage：这三个选项作用相同，都是让ldconfig打印出其帮助信息，而后退出。

A few things to note about ldconfig:

1. There is no need to modify /etc/ld.so.conf when adding things to /lib and /usr/lib, but you need to adjust ldconfig after finishing, otherwise the library will not be found.
2. When you want to add something to the above two directories, you must modify /etc/ld.so.conf and then call ldconfig, otherwise it will not be found.
3. For example, if you install a mysql to /usr/local/mysql, mysql has a lot of libraries under /usr/local/mysql/lib, then you need to add a line /usr under /etc/ld.so.conf /local/mysql/lib, save it and run ldconfig so that the new library can be found when the program is running.
4. If you want to put lib outside these two directories, but you don’t want to add things to /etc/ld.so.conf (or you don’t have permission to add things). That's okay, just export a global variable LD_LIBRARY_PATH, and then when you run the program, it will go to this directory to find the library. Generally speaking, this is only a temporary solution, used when there is no permission or temporary need.
5. These things ldconfig does are related to the running time of the program and have nothing to do with the compilation time. You still need to add -L when compiling, don't get confused.
6. In short, no matter what changes are made to the library, it is best to ldconfig, otherwise some unexpected results will occur. It won't take much time, but it will save a lot of trouble.
7. Furthermore, the file header such as libdb-4.3.so will contain information related to the library name (that is, it contains "libdb-4.3.so", which can be viewed with the strings command), so just modify the file name to impersonate someone. Already recognized libraries (such as libdb-4.8.so) will not work. For this purpose, you can directly modify the configuration information in the Makefile of the compiled library and specify a special library name.

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