链接与加载---2.dlopen共享库

1. 
1.1 共享库
有如下代码共享库与主函数中都有相同的add函数

cong@msi:/work/test/ctest/5dlopen_1$ cat libtestadd/libadd.c
int add(int a, int b)
{
    return (a+b);
}

编译生成共享库:
gcc -g -fPIC -O0   -c -o libadd.o libadd.c
gcc -fPIC -shared libadd.o -o libtestadd.so

1.2 主函数

cong@msi:/work/test/ctest/5dlopen_1$ cat main.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "libadd.h"
int add(int a, int b)
{
    printf("main.c: add\n");
    return 0;
}
int main (int argc, char *argv[])
{
    printf("add=%d\n",add(1,2));
    return 0;
}

gcc -g -O0 -I./libtestadd    -c -o main.o main.c
gcc main.o -o main -L./libtestadd/ -ltestadd  -Wl,-rpath,libtestadd
1.3 运行结果

cong@msi:/work/test/ctest/5dlopen_1$ ./main
main.c: add                    -->发现调用的是主函数的add
add=0

用ldd及readelf查看

cong@msi:/work/test/ctest/5dlopen_1$ ldd main
    linux-vdso.so.1 => (0x00007ffd975d9000)           -->这儿根本就没有去链接libtestadd.so
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f6ca45c9000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f6ca49b5000)
cong@msi:/work/test/ctest/5dlopen_1$
cong@msi:/work/test/ctest/5dlopen_1$ readelf -d ./main

Dynamic section at offset 0xe18 contains 25 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED) Shared library: [libc.so.6]   -->需要的共享库只有libc.so根本就没有用libtestadd.so
 0x000000000000000f (RPATH) Library rpath: [libtestadd]
 0x000000000000000c (INIT) 0x400428

将main.c中的add函数注掉之后，再次用ldd及readelf查看:

cong@msi:/work/test/ctest/5dlopen_1$ ldd main
    linux-vdso.so.1 => (0x00007ffd43f39000)
    libtestadd.so => libtestadd/libtestadd.so (0x00007fd3bebd0000)     -->去掉main中的add才会用到libtestadd.so这个共享库
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fd3be7e6000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fd3bedd4000)
cong@msi:/work/test/ctest/5dlopen_1$
cong@msi:/work/test/ctest/5dlopen_1$ readelf -d ./main
Dynamic section at offset 0xe08 contains 26 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED) Shared library: [libtestadd.so]           -->去掉main中的add才会用到共享库中的add
 0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
 0x000000000000000f (RPATH) Library rpath: [libtestadd]
 0x000000000000000c (INIT) 0x400580

1.4 代码下载
5dlopen_1.rar   (下载后改名为5dlopen_1.tar.gz)

---

2. 运行时加载库

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>
int add(int a, int b)
{
    printf("add in main.c\n");
    return (a+b);
}
#define TEST_LIB_PATH "./libtestadd/libtestadd.so"
int main (int argc, char *argv[])
{
    int result;
    void* dp = NULL;
    int (*add_func)(int, int);
    dp = dlopen(TEST_LIB_PATH, RTLD_LAZY );
    if(dp==NULL)
    {
        printf("dlopen failed\n");
        return 0;
    }

    add_func = dlsym(dp, "add");
    result = add_func(3, 4);
    printf("addr of add_func=%p\n", add_func);
    printf("dlopen: add_func=%d\n", result);
    dlclose(dp);

    printf("add=%d\n",add(1,2));
    return 0;
}

2.3 运行结果

cong@msi:/work/test/ctest/5dlopen$ ./main
addr of add_func=0x7f192e886675
dlopen: add_func=7    -->运行了libtestadd.so中的add
add in main.c
add=3

用dlopen之后再次用ldd及readelf查看:

cong@msi:/work/test/ctest/5dlopen$ ldd main
    linux-vdso.so.1 => (0x00007ffdea9f0000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f5d20aa9000)   -->需要用到libdl.so，但是自己的libtestadd.so没有用到
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f5d206e4000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f5d20cd4000)
cong@msi:/work/test/ctest/5dlopen$ readelf -d ./main

Dynamic section at offset 0xe18 contains 25 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED) Shared library: [libdl.so.2]    -->需要用到libdl.so，但是自己的libtestadd.so没有用到
 0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
 0x000000000000000c (INIT) 0x400588

2.4 代码下载
 5dlopen.rar(下载后改名为5dlopen.tar.gz)