undefined reference to `dlopen'

在编译使用 dlfcn.h 的cpp文件时报错：

/tmp/ccd4NLWa.o: In function `main':
demo.cpp:(.text+0x27): undefined reference to `dlopen'
demo.cpp:(.text+0x37): undefined reference to `dlerror'
demo.cpp:(.text+0x7c): undefined reference to `dlsym'
demo.cpp:(.text+0x85): undefined reference to `dlerror'
demo.cpp:(.text+0xc7): undefined reference to `dlclose'
demo.cpp:(.text+0x184): undefined reference to `dlsym'
demo.cpp:(.text+0x18d): undefined reference to `dlerror'
demo.cpp:(.text+0x1cf): undefined reference to `dlclose'
demo.cpp:(.text+0x320): undefined reference to `dlclose'
collect2: error: ld returned 1 exit status

解决办法：
编译的时候加上 -ldl， 比如：

g++ -o demo demo.cpp -I./include -ldl

原因 是虽然导入了相应的头文件，但并没有加上libdl.so 联合编译。