test env:

the source code coming with apue 3e and my code

Centos 7: kernel 4.17.2-1. x86_64 GNU/Linux

gcc 4.85

静态库 :

#build lib
gcc -ansi -I../include -Wall -DLINUX -D_GNU_SOURCE  -c err.c 
ar rcs ../libapue/libapue.a err.o


#List symbols in [file(s)] (a.out by default).
nm -s libapue.a 

Archive index:
err_ret in err.o
err_sys in err.o
err_cont in err.o
err_exit in err.o
err_dump in err.o
err_msg in err.o
err_quit in err.o

err.o:
.............


#link with static lib  -L指定库的搜索路径  -lapue 来表示链接 libapue.a

gcc -ansi -I../include -Wall -DLINUX -D_GNU_SOURCE  ls1.c -o ls1  -L../libapue -lapue 

动态库 ,不带版本号的

#build
gcc -shared -fPIC -o libapue.so err.c


#link:
gcc -I../include -o ls1 -L../libapue ls1.c -lapue

#run
./ls1 .
./ls1: error while loading shared libraries: libapue.so: cannot open shared object file: No such file or directory
set the right env
#to fix path issue
export LD_LIBRARY_PATH="$HOME/base/clang/apue/libapue"


#查看程序对动态库的依赖:
ldd ls1

动态库的版本

Using the GNU Compiler Collection (GCC): Link Options

#filename: gcc -O 后的文件名

#soname: （ Short for shared object name）， with major version,
           gcc 命令中通过  -Wl,-soname 指定
#link filename  gcc -l 参数所用

#build lib with soname and filename
gcc -shared -fPIC -Wl,-soname=libapue.so.0 -o libapue.so.0.1 err.o

#check via soname
readelf -d libapue.so.0

Dynamic section at offset 0x1df8 contains 25 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000000000000e (SONAME)             Library soname: [libapue.so.0]



#创建 link to soname
ldconfig -n .
ln -sf libapue.so.0 libapue.so
ls -l

lrwxrwxrwx. 1 alex alex    12 8月   1 14:14 libapue.so -> libapue.so.0
lrwxrwxrwx. 1 alex alex    14 8月   1 14:15 libapue.so.0 -> libapue.so.0.1
-rwxrwxr-x. 1 alex alex 12928 8月   1 14:10 libapue.so.0.1

#linking with link's name
gcc -Wall -I../include -L../libapue  ls1.c -lapue -o ls1

还有一种动态链接库的用法是有程序自己加载: Dynamically loaded/unloaded and linked during execution