在编程中，库是可以在程序中重用的预编译代码的集合。库提供了可重用的函数，例程，类，数据结构等，从而简化了程序员的工作
它们可以在程序中重用。
静态库：静态库或静态链接库是一组例程，外部函数和变量，它们在编译时在调用方中解析，并由编译器，链接器或联编程序复制到目标应用程序中，从而生成目标文件和一个独立的可执行文件。该可执行文件及其编译过程都称为程序的静态生成。从历史上看，库只能是静态的。
它们通常比共享库快，因为一组常用的目标文件被放入单个库可执行文件中。一个人可以构建多个可执行文件，而无需重新编译文件。因为它是要构建的单个文件，所以链接命令的使用比共享库链接命令的使用更简单，因为您指定了静态库的名称。
共享库：
共享库是.so(或Windows .dll，或OS X .dylib)文件。
这些链接是动态链接的，只是简单地包含了库的地址(而静态链接是浪费空间的)。动态链接在运行时链接库。因此，所有功能都位于内存空间中的特殊位置，并且每个程序都可以访问它们，而无需多个副本。
| properties | Static library | Shared library |
|---|---|---|
| Linking time | It happens as the last step of the compilation process. After the program is placed in the memory | Shared libraries are added during linking process when executable file and libraries are added to the memory. |
| Means | Performed by linkers | Performed by operating System |
| Size | Static libraries are much bigger in size, because external programs are built in the executable file. | Dynamic libraries are much smaller, because there is only one copy of dynamic library that is kept in memory. |
| External file changes | Executable file will have to be recompiled if any changes were applied to external files. | In shared libraries, no need to recompile the executable. |
| Time | Takes longer to execute, because loading into the memory happens every time while executing. | It is faster because shared library code is already in the memory. |
| Compatibility | Never has compatibility issue, since all code is in one executable module. | Programs are dependent on having a compatible library. Dependent program will not work if library gets removed from the system . |
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