原来，自己在很多年前就总结了这块知识
今日，再一次进行更松散的总结。
linker在文件编译时，解决所有的外部符号；当执行时，loader(属于OS)将会重置所有的符号，并且放入合适的内存中。
linux 支持两类库: 静态库和动态库。根据程序编译阶段的不同，又可以分为动态链接库和动态加载库。
Shared library
共享库没啥特别的地方，也是一种ELF格式文件，和普通的C语言文件没有区别(编译之前)，除了没有main()函数入口。当然，要告诉gcc-fPIC选项。
vimer@host:~/test/lib$ cat libfunction.c
int double_me(int a){
return a + a;
}
// gcc -c -fPIC libfunction.c
// gcc -shared -o libmylib.so libfunction.o
vimer@host:~/test/lib$ ls
libfunction.c libfunction.o libmylib.so
当然，也可以综合起来:
gcc -shared -fPIC -o libmylib.so libfunction.c
那么， 在其他文件中，调用这个函数的方法是:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int double_me(int); // 最好放在 .h 文件内
int main(){
int i;
for(i = 0; i < 10; i++)
printf("%d'' s duoble is %d\t", i, double_me(i));
}
我们的main函数在编译阶段会link这个lib。
gcc -o main main.c -L. -lmylib
# -L: path
# -l : 自动加上lib及后缀so
但是，如果你执行试试:
vimer@host:~/test/lib$ ./main
./main: error while loading shared libraries: libmylib.so: cannot open shared object file: No such file or directory
link是没问题的，但是load是无法的。也就是说，尽管我们我们在link阶段指明了so的位置，但是load并不会得知。以上，基本上就是前面文件的内容。
LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH ./main
# 必须在一条命令下才可以
So in order to make the loader find our library, we have to tell it where to look, either by adding the correct directory to the LD_LIBRARY_PATH environment variable, or by adding it to the ldconfig paths in either /etc/ld.so.conf inside the /etc/ld.so.conf.d/ directory. Let’s try it with the environment variable for now
动态加载共享库
如果只想在加载 load 时使用共享库的函数，还是有办法的。
// dynload.c
#include <stdio.h>
#include <dlfcn.h>
int main(void) {
// handle for dynamic loading functions
void *handle;
// function pointer for the library's double_me() function
int (*double_me)(int);
// just a counter
int i;
// open our library ..hopefully
if ( (handle = dlopen("./libmylib.so", RTLD_LAZY)) == NULL) {
return 1;
}
// try to extract "double_me" symbol from the library
double_me = dlsym(handle, "double_me");
if (dlerror() != NULL) {
dlclose(handle);
return 2;
}
// use double_me() just like with a regularly linked library
for (i = 1; i <= 10; i++) {
printf("%d doubled is %d\n", i, double_me(i));
}
dlclose(handle);
return 0;
}
这段代码需要借助libdl动态库， 其实不好的一点就是得猜。
gcc -o dynload dynload.c -ldl
linux lib 管理
linux中有两种lib，一种是静态的，一种是动态的（Dynamic Shared Object–DSO）,由操作系统借助
ld.so或者ld-linux.so在运行时动态load.
- ldconfig : Updates the necessary links for the run time link bindings.
- ldd : Tells what libraries a given program needs to run.
- ltrace : A library call tracer.
- ld.so/ld-linux.so: Dynamic linker/loader.
重要的文件
- /lib/ld-linux.so.* : Execution time linker/loader.
- /etc/ld.so.conf : File containing a list of colon, space, tab, newline, or comma separated directories in which to search for libraries.
-
/etc/ld.so.cache : File containing an ordered list of libraries found in the directories specified in /*etc/ld.so.conf. This file is not in human readable format, and is not intended to be edited. This file is created by ldconfig command.
- lib*.so.version : Shared libraries stores in /lib, /usr/lib, /usr/lib64, /lib64, /usr/local/lib directories.
ldconfig
在上面的 main 例子中， ldconfig作用在libmylib.so 的目录中并不起作用，不知道为什么。
但是，可以使用下面的命令让共享库生效:
sudo vim /etc/ld.so.conf.d/mylib.conf
vimer@host:~/test/lib$ cat /etc/ld.so.conf.d/mylib.conf
/home/vimer/test/lib # 把libmylib.so所在目录放入这个自己创建的文件，ldconfig一下就可以
vimer@host:~/test/lib$ sudo ldconfig
vimer@host:~/test/lib$ ./main
0'' s duoble is 0 1'' s duoble is 2 2'' s duoble is 4 3'' s duoble is 6 4'' s duoble is 8 5'' s duoble is 10 6'' s duoble is 12 7'' s duoble is 14 8'' s duoble is 16 9'' s duoble is 18
查看ldconfig的内容使用-v选项。
sudo ldconfig -v | grep -i mylib
libmylib.so -> libmylib.so
ldd
ldd可以查看一个可执行文件所依赖的动态库
ld.so or ld-linux.so
The ld.so or / ld-linux.so used as follows by Linux:
- To load the shared libraries needed by a program.
- To prepare the program to run, and then runs it.
# cd /lib64
vimer@host:/lib64$ ./ld-linux-x86-64.so.2 --list /home/vimer/test/lib/main
linux-vdso.so.1 (0x00007fff48dfa000)
libmylib.so => /home/vimer/test/lib/libmylib.so (0x00007f9654e92000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f9654aa1000)
与ldd的作用一样
环境变量
The LD_LIBRARY_PATH can be used to set a library path for finding dynamic libraries using LD_LIBRARY_PATH, in the standard colon seperated format:
$ export LD_LIBRARY_PATH=/opt/simulator/lib:/usr/local/lib
The LD_PRELOAD allow an extra library not specified in the executable to be loaded:
$ export LD_PRELOAD=/home/vivek/dirhard/libdiehard.so
Please note that these variables are ignored when executing setuid/setgid programs.
补充
pkg-config
pkg-config的作用与ldconfig有些类似，但是pkg-config更侧重在编译、链接阶段；而ldconfig侧重链接、运行时阶段。