Libraries

Static Libraries

a collection of ordinary object files (目标文件的集合)
loaded at program link time (链接阶段加载)
不再那么重要的优势
- 节省编译时间
- 理论上稍快的执行速度
use the -l option to specify the library
- the -l option is a linker option, and thus needs to be placed AFTER the name of the file to be compiled

Shared Libraries

libraries that are loaded by programs when they start (程序启动时加载)
soname - real name - linker name
- a symbolic link to the shared library's "real name''

Thus, /usr/lib/libreadline.so.3 is a fully-qualified soname, which ldconfig would set to be a symbolic link to some realname like /usr/lib/libreadline.so.3.0. There should also be a linker name, /usr/lib/libreadline.so which could be a symbolic link referring to /usr/lib/libreadline.so.3.

Dynamically Loaded (DL) Libraries

libraries that are loaded at times other than during the startup of a program (程序启动之后才加载的)
- they permit waiting to load the plugin until it's needed
built as standard object files or standard shared libraries
- there is an API for opening a library, looking up symbols, handling errors, and closing the library

makefile中一些编译器选项

CFLAGS CXXFLAGS

CFLAGS 表示用于 C 编译器的选项
CXXFLAGS 表示用于 C++ 编译器的选项
这两个变量实际上涵盖了编译和汇编两个步骤
指定头文件(.h文件)的路径
- CFLAGS=-I/usr/include -I/path/include

(安装一个包时会在安装路径下建立一个include目录，当安装过程中出现问题时，试着把以前安装的包的include目录加入到该变量中来)

LDFLAGS

gcc等编译器会用到的一些优化参数
指定库文件的位置 ( ~~静态库文件~~ 连接阶段使用)
LDFLAGS=-L/usr/lib -L/path/to/your/lib

(每安装一个包都几乎一定的会在安装目录里建立一个lib目录。如果明明安装了某个包，而安装另一个包时出现找不到已经安装的包时，可以把那个包的lib路径加入的LDFALGS中试一下)

LIBS

告诉链接器要链接哪些库文件
LIBS = -lpthread -liconv

(简单地说，LDFLAGS/-L是告诉链接器从哪里寻找库文件，而LIBS/-l是告诉链接器要链接哪些库文件。不过使用时 链接阶段 这两个参数都会加上，所以你即使将这两个的值互换，也没有问题)

运行时的链接操作

1

有时候LDFLAGS指定-L虽然能让链接器找到库进行链接，但是运行时链接器却找不到这个库，如果要让软件运行时库文件的路径也得到扩展，那么我们需要增加这两个库给"-Wl,R"：

LDFLAGS = -L/var/xxx/lib -L/opt/mysql/lib -Wl,R/var/xxx/lib -Wl,R/opt/mysql/lib

如果在执行./configure以前设置环境变量export LDFLAGS="-L/var/xxx/lib -L/opt/mysql/lib -Wl,R/var/xxx/lib -Wl,R/opt/mysql/lib" ，注意 设置环境变量等号两边不可以有空格，而且要加上引号（shell的用法）。
那么执行configure以后，Makefile将会设置这个选项，链接时会有这个参数，编译出来的可执行程序的库文件搜索路径就得到扩展了。

2 -wl,-rpath=<link_path>

-Wl,-rpath,/usr/local/thrift0.11.0/lib:/usr/local/lib_boost_1_68_0:/usr/local/lib:/usr/local/lib64
使用场景
- A库依赖B库，只链接了A库时
- 指定程序运行过程中查找库的路径

Add a directory to the runtime library search path. This is used when linking an ELF executable with shared objects. All -rpath arguments are concatenated and passed to the runtime linker, which uses them to locate shared objects at runtime. The -rpath option is also used when locating shared objects which are needed by shared objects explicitly included in the link;

LD_LIBRARY_PATH环境变量

~~指定的共享库/动态库 搜索路径 (运行阶段使用)~~
echo "export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib64/:/usr/local/lib/:/usr/lib64/:/usr/lib/:/lib/:/lib64/ :/usr/local/cuda-8.0/lib64/" >> /home/worker/.bashrc

In Linux, the environment variable LD_LIBRARY_PATH is a colon-separated set of directories where libraries should be searched for first, before the standard set of directories;

LD_LIBRARY_PATH环境变量指定的是链接时查找库的顺序，还是运行时查找库的顺序？
配置文件/etc/ld.so.conf 与 ldconf
- 待补充

Makefile 中一些编译器选项