static link & Dynamic Link & Dynamic Load

【轉貼】 一天一成長

Static link:

Compiler時，library加入程式碼

優：快

劣：佔空間

Dynamic Link:

Compiler時，不將library加入程式碼，執行程式的時後，再將 library載入程式碼，若有多個程式共用同一個library，只需載一個library進memory

優：省空間

劣：慢

Dynamic Load:

Compiler時，不將library加入程式碼，執行程式時，遇到函式，才將library載入，用完後再free出空間

優：更省空間

劣：更慢

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Static Link

Creating a Static Library:


Compile source codes   

# gcc –c file1.c file2.c file3.c

Create a static library named libmylib.a 

# ar rcs libmylib.a file1.o file2.o file3.o

-r: 新增 *.o 到 libfun.a 中 

-c: 建立新的程式庫

-s: 將一個 object 檔的 index 寫入程式庫

Using a Static Library:

# gcc –o main main.c –L. –lmylib (lib不用打)

Parameters:

-L: the directory of the library

-l: the name of the library




Dynamic Link

Creating a Shared Library:


Compile source codes 

# gcc -c file1.c file2.c file3.c
or
加上-fPIC用來產生position-independent code
# gcc -c -fPIC file1.c file2.c file3.c


Create a shared library named libmylib.so

# gcc -shared -o libmylib.so file1.o file2.o file3.o

Using a Shared Library: 

# gcc –o main main.c –L. –lmylib

Note: Remember to put libmylib.so into PATH, ex. /usr/lib




Dynamic Load (還沒用過)

Creating a Shared Library:


Compile source codes;

# gcc –c file1.c file2.c file3.c

Create a shared library named libmylib.so 

# gcc -shared -o file1.o file2.o file3.o libmylib.so


Using a Shared Library:

(Reference: http://linux.die.net/man/3/dlopen)

Use the following function to access the shared library:

#include <dlfcn.h>

void *dlopen(const char *filename, int flag);

char *dlerror(void);

void *dlsym(void *handle, const char *symbol);

int dlclose(void *handle);

Compile: Since above function are implemented in the library libdl.a, we need to load this library

# gcc dltest.c –ldl 

Parameters:

-ldl: load the library libdl.a

ref:

1. 一天一成長
2. 井民全觀點