shared library 筆記

source: http://www.thegeekstuff.com/2012/06/linux-shared-libraries/

無聊來練一下shared library好了。

1.先寫好share.c

#include "shared.h"
unsigned int add(unsigned int a, unsigned int b)
{
    printf("\n Inside add()\n");
    return (a+b);
}

2.share.h

#include
extern unsigned int add(unsigned int a, unsigned int b);

3.編釋囉
gcc -c -Wall -Werror -fPIC share.c
gcc -shared -o libshare.so share.o

4.產生出來libshare.so囉!!!
# ls
libshare.so  share.c  share.h  share.o
5.寫一個測式的程式
include
#include"share.h"
int main(void)
{
    unsigned int a = 1;
    unsigned int b = 2;
    unsigned int result = 0;

    result = add(a,b);

    printf("\n The result is [%u]\n",result);
    return 0;
}

# gcc -L. -Wall sharetest.c -o go -lshare
-L. 在當前的目錄下，找-lshare 名稱為libshare.so的library來編。
# go
./go: error while loading shared libraries: libshare.so: cannot open shared object file: No such file or directory
go的測試程式找不到需要的libshare.so。

這時有兩個方法可以解決
 a.修改/etc/ld.so.conf 然後執行ldconfig
 vim /etc/ld.so.conf

include ld.so.conf.d/*.conf
/usr/local/openssl/lib/
/usr/lib
/home/carlosnew/Cpratic

b.export LD_LIBRARY_PATH=/home/carlosnew/Cpratic:$LD_LIBRARY_PATH

6最後一看一下執行結果!
# go

 Inside add()


 The result is [3]

 static library會佔用較多的記憶體因為把要用到的functions都先包進去了，但是執行速度會比較快，shared library執行速會比較慢，相同的function只需要一塊記憶體空間。