1. 什么是菱形链接(diamond link)
菱形链接(diamond link)(参考文献 1)能十分清楚的描述出我们要讨论的问题。
如上图所示，我们的程序将要使用某厂家的共享库libvendor1.so，同时也要使用另外一个厂家的共享库libvendor2.so。
libvendor1.so和libvendor2.so都将使用某知名开源共享库libopensource.so.xxx(xxx表示版本)。
但是这两个厂家提供给我们的都是自己编译维护的libopensource.so.xxx。
我们遇到的问题是：
一个厂家使用了另外一个厂家提供libopensource.so.xxx，而不是自己提供的，出现兼容性问题。
这个问题扩展展开来：
1)如果libopensource.so.xxx的版本不相同，符号绑定（binding）的是哪个版本的？
2)如果libopensource.so.xxx的版本相同，符号绑定的是哪个版本的？
3)对于问题1)和2)，采用系统默认加载和使用"dlopen"等API显式加载，又有什么不同？
如果这几个问题您没有答案或者觉得比较含糊，建议您跟随我的实验，我们一起探讨下。
因为我个人没看过连接器和加载器的源码，所以我们的探讨集中在我们看到的证据上，并试图给出一些粗浅的结论。
2.相关代码
具体代码在github中。 我们先弄清楚c文件的调用关系，以及编译脚本做了什么样的工作。
调用依赖： main.c<----vendor[1|2].c<--------opensource_v[1|2].c(函数opensource_print的不同实现)。
C源代码:
1)其中vendor1.c会被编译生成libvendor1.so，vendor2.c会被编译生成libvendor2.so；
opensource_v1.c会被编译生成./opensource_v1/libopensource.so.xxx(xxx值表示版本信息，后续实验会给定真实值)，opensource_v2.c会被编译成./opensource_v2/libopensource.so.xxx；
libvendor1.so会依赖./opensource_v1/libopensource.so.xxx, libvendor2.so会依赖./opensource_v2/libopensource.so.xxx,
2)main.c链接libvendor1.so，libvendor2.so生成可执行文件
3)main.c有两种用法，一种"general"使用系统默认的加载共享库的方法，一种"dlopen"使用dlopen等API显式加载需要的共享库。
用于控制编译的Shell脚本（每个使用一个脚本，为了便于说清）:
different_soname_without_default_symver.sh
different_soname_with_default_symver.sh
same_soname_without_default_symver.sh
same_soname_with_default_symver.sh
3.libopensource.so.xxx的版本不相同，系统如何查找依赖库和绑定符号
在这个实验里我们编译opensource_v1.c生成./opensource_v1/libopensource.so.1.0；编译opensource_v2.c生成./opensource_v2/libopensource.so.2.0。
libvendor1.so将依赖./opensource_v1/libopensource.so.1.0； libvendor2.so将依赖./opensource_v2/libopensource.so.2.0。
3.1
符号表不带版本信息的
gcc编译的符号，默认是不带版本信息的。
3.1.1
我们用different_soname_without_default_symver.sh 来编译。
[[email protected] 0004]# sh different_soname_without_default_symver.sh
Complile success
3.1.2
列出编译生成的文件
我们把libopensource.so.1.0相应3个文件放在"./opensource_v1"目录，把libopensource.so.2.0相应3个文件放在"./opensource_v2"目录：
3.1.3
用readelf查看编译生成的main，libvendor1.so，libvendor2.so
我们仅仅关注"NEEDED"，"RPATH"项。
"NEEDED"表示依赖的库，注意"NEEDED"并不是"libopensource.so.xxx.0"这样的完整名字，而是"libopensource.so.xxx"这样只包含大版本信息的名字（该名字术语叫"SONAME"），我们在编译libopensource共享库时通过"-Wl,-soname"指定"SONAME"，参考文献
2中对"SONAME" "Major version" "Minor version" 有详细介绍； 编译时不指定""-Wl,-soname"的情况，这里不再讨论了（太多头绪，太乱），可以自己修改编译脚本做实验验证。
"RPATH"表示查找依赖库会从这些列出的路径查找(另外有个环境变量LD_LIBARARY_PATH也是类似的作用)。
更多细节所请自行Google。
[[email protected] 0004]# readelf -d main
Dynamic section at offset 0xde8 contains 28 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libvendor1.so]
0x0000000000000001 (NEEDED) Shared library: [libvendor2.so]
0x0000000000000001 (NEEDED) Shared library: [libdl.so.2]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000f (RPATH) Library rpath: [./]
[[email protected] 0004]# readelf -d libvendor1.so
Dynamic section at offset 0xde8 contains 27 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libopensource.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libvendor1.so]
0x000000000000000f (RPATH) Library rpath: [./opensource_v1]
[[email protected] 0004]# readelf -d libvendor2.so
Dynamic section at offset 0xde8 contains 27 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libopensource.so.2]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libvendor2.so]
0x000000000000000f (RPATH) Library rpath: [./opensource_v2]
[[email protected] 0004]# readelf -d opensource_v1/libopensource.so.1.0
Dynamic section at offset 0xe08 contains 25 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libopensource.so.1]
[[email protected] 0004]# readelf -d opensource_v2/libopensource.so.2.0
Dynamic section at offset 0xe08 contains 25 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libopensource.so.2]
3.1.4
用nm|grep opensource_print查看编译生成的libvendor1.so和libvendor2.so, 可以看到使用相同符号"opensource_print"
[[email protected] 0004]# nm libvendor1.so |grep opensource_print
U opensource_print
[[email protected] 0004]# nm libvendor2.so |grep opensource_print
U opensource_print
3.1.5
用LD_DEBUG 来debug 依赖库和符号绑定的过程(针对默认加载动态库的情况)
[[email protected] 0004]# LD_DEBUG_OUTPUT=robin.txt LD_DEBUG=all ./main general
-----------------------general--------------------
opensource v1 print, called by vendor 1
opensource v1 print, called by vendor 2
首先看输出，从结果看，仅仅调用了./opensource_v1/libopensource.so.1(opensource_v1.c)里的"opensource_print函数"。
完整的LD_DEBUG输出在robin.1.txt
我们来分析robin.1.txt输出：
58行到68行，./opensource_v1/libopensource.so.1被查找到；71行到81行，./opensource_v2/libopensource.so.2被找到：
58 3774: file=libopensource.so.1 [0]; needed by ./libvendor1.so [0]
59 3774: find library=libopensource.so.1 [0]; searching
60 3774: search path=./opensource_v1/tls/x86_64:./opensource_v1/tls:./opensource_v1/x86_64:./opensource_v1 (RPATH from file ./libvendor1.so)
61 3774: trying file=./opensource_v1/tls/x86_64/libopensource.so.1
62 3774: trying file=./opensource_v1/tls/libopensource.so.1
63 3774: trying file=./opensource_v1/x86_64/libopensource.so.1
64 3774: trying file=./opensource_v1/libopensource.so.1
65 3774:
66 3774: file=libopensource.so.1 [0]; generating link map
67 3774: dynamic: 0x00007f940ce07e08 base: 0x00007f940cc07000 size: 0x0000000000201038
68 3774: entry: 0x00007f940cc07600 phdr: 0x00007f940cc07040 phnum: 7
69 3774:
70 3774:
71 3774: file=libopensource.so.2 [0]; needed by ./libvendor2.so [0]
72 3774: find library=libopensource.so.2 [0]; searching
73 3774: search path=./opensource_v2/tls/x86_64:./opensource_v2/tls:./opensource_v2/x86_64:./opensource_v2 (RPATH from file ./libvendor2.so)
74 3774: trying file=./opensource_v2/tls/x86_64/libopensource.so.2
75 3774: trying file=./opensource_v2/tls/libopensource.so.2
76 3774: trying file=./opensource_v2/x86_64/libopensource.so.2
77 3774: trying file=./opensource_v2/libopensource.so.2
78 3774:
79 3774: file=libopensource.so.2 [0]; generating link map
80 3774: dynamic: 0x00007f940cc05e08 base: 0x00007f940ca05000 size: 0x0000000000201038
81 3774: entry: 0x00007f940ca05600 phdr: 0x00007f940ca05040 phnum:
996行，libvendor1.so调用的"opensource_print"被绑定到./opensource_v1/libopensource.so.1上；1013行，libvendor2.so调用的"opensource_print"也被绑定到./opensource_v1/libopensource.so.1：
990 3774: symbol=opensource_print; lookup in file=./main [0]
991 3774: symbol=opensource_print; lookup in file=./libvendor1.so [0]
992 3774: symbol=opensource_print; lookup in file=./libvendor2.so [0]
993 3774: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
994 3774: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
995 3774: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
996 3774: binding file ./libvendor1.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘
997 3774: symbol=printf; lookup in file=./main [0]
998 3774: symbol=printf; lookup in file=./libvendor1.so [0]
999 3774: symbol=printf; lookup in file=./libvendor2.so [0]
1000 3774: symbol=printf; lookup in file=/lib64/libdl.so.2 [0]
1001 3774: symbol=printf; lookup in file=/lib64/libc.so.6 [0]
1002 3774: binding file ./opensource_v1/libopensource.so.1 [0] to /lib64/libc.so.6 [0]: normal symbol `printf‘ [GLIBC_2.2.5]
1003 3774: symbol=vendor2; lookup in file=./main [0]
1004 3774: symbol=vendor2; lookup in file=./libvendor1.so [0]
1005 3774: symbol=vendor2; lookup in file=./libvendor2.so [0]
1006 3774: binding file ./main [0] to ./libvendor2.so [0]: normal symbol `vendor2‘
1007 3774: symbol=opensource_print; lookup in file=./main [0]
1008 3774: symbol=opensource_print; lookup in file=./libvendor1.so [0]
1009 3774: symbol=opensource_print; lookup in file=./libvendor2.so [0]
1010 3774: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
1011 3774: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
1012 3774: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
1013 3774: binding file ./libvendor2.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘
1014 3774:
3.1.6
用LD_DEBUG 来debug 依赖库和符号绑定的过程(针对使用"dlopen"等API，显式加载共享库的情况)
和3.1.5差不多一样。
[[email protected] 0004]# LD_DEBUG_OUTPUT=robin.txt LD_DEBUG=all ./main dlopen
----------------------dlopen----------------------
opensource v1 print, called by vendor 1
opensource v1 print, called by vendor 2
首先看输出，从结果看，仅仅调用了./opensource_v1/libopensource.so.1(opensource_v1.c)里的"opensource_print函数"。
完整的LD_DEBUG输出在robin.2.txt
我们来分析robin.2.txt输出：
58行到68行，./opensource_v1/libopensource.so.1被查找到；71行到81行，./opensource_v2/libopensource.so.2被找到：
58 22438: file=libopensource.so.1 [0]; needed by ./libvendor1.so [0]
59 22438: find library=libopensource.so.1 [0]; searching
60 22438: search path=./opensource_v1/tls/x86_64:./opensource_v1/tls:./opensource_v1/x86_64:./opensource_v1 (RPATH from file ./libvendor1.so)
61 22438: trying file=./opensource_v1/tls/x86_64/libopensource.so.1
62 22438: trying file=./opensource_v1/tls/libopensource.so.1
63 22438: trying file=./opensource_v1/x86_64/libopensource.so.1
64 22438: trying file=./opensource_v1/libopensource.so.1
65 22438:
66 22438: file=libopensource.so.1 [0]; generating link map
67 22438: dynamic: 0x00007f41a353de08 base: 0x00007f41a333d000 size: 0x0000000000201038
68 22438: entry: 0x00007f41a333d600 phdr: 0x00007f41a333d040 phnum: 7
69 22438:
70 22438:
71 22438: file=libopensource.so.2 [0]; needed by ./libvendor2.so [0]
72 22438: find library=libopensource.so.2 [0]; searching
73 22438: search path=./opensource_v2/tls/x86_64:./opensource_v2/tls:./opensource_v2/x86_64:./opensource_v2 (RPATH from file ./libvendor2.so)
74 22438: trying file=./opensource_v2/tls/x86_64/libopensource.so.2
75 22438: trying file=./opensource_v2/tls/libopensource.so.2
76 22438: trying file=./opensource_v2/x86_64/libopensource.so.2
77 22438: trying file=./opensource_v2/libopensource.so.2
78 22438:
79 22438: file=libopensource.so.2 [0]; generating link map
80 22438: dynamic: 0x00007f41a333be08 base: 0x00007f41a313b000 size: 0x0000000000201038
81 22438: entry: 0x00007f41a313b600 phdr: 0x00007f41a313b040 phnum: 7
1033行，libvendor1.so调用的"opensource_print"被绑定到./opensource_v1/libopensource.so.1上；1072行，libvendor2.so调用的"opensource_print"也被绑定到./opensource_v1/libopensource.so.1：
1027 22438: symbol=opensource_print; lookup in file=./main [0]
1028 22438: symbol=opensource_print; lookup in file=./libvendor1.so [0]
1029 22438: symbol=opensource_print; lookup in file=./libvendor2.so [0]
1030 22438: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
1031 22438: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
1032 22438: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
1033 22438: binding file ./libvendor1.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘
1066 22438: symbol=opensource_print; lookup in file=./main [0]
1067 22438: symbol=opensource_print; lookup in file=./libvendor1.so [0]
1068 22438: symbol=opensource_print; lookup in file=./libvendor2.so [0]
1069 22438: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
1070 22438: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
1071 22438: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
1072 22438: binding file ./libvendor2.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘
3.1.7
推测结论
!!!!!!猜测: 根据3.1.5和3.1.6，虽然./opensource_v1/libopensource.so.1和./opensource_v2/libopensource.so.2都被查找到，但是./opensource_v1/libopensource.so.1的位置靠前，所以符号"opensource_print"先在./opensource_v1/libopensource.so.1中被查找到，并绑定；一旦查找到一个，就不再查找。
!!!!!!我们是有证据支持这个猜测的。
编辑different_soname_without_default_symver.sh，仅仅改变"-lvendor2","-lvendor1"的顺序，让"-lvendor2"靠前，如下：
#main.c
#gcc -Wl,-rpath=./ -o main main.c -L. -lvendor1 -lvendor2 -ldl
gcc -Wl,-rpath=./ -o main main.c -L. -lvendor2 -lvendor1 -ldl
重新编译：
[[email protected] 0004]# sh different_soname_without_default_symver.sh
Complile success
重新查看"readelf -d main", 和之前的3.1.3比较，看到"libverndor2.so"位置被提前了：
[[email protected] 0004]# readelf -d main
Dynamic section at offset 0xde8 contains 28 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libvendor2.so]
0x0000000000000001 (NEEDED) Shared library: [libvendor1.so]
0x0000000000000001 (NEEDED) Shared library: [libdl.so.2]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000f (RPATH) Library rpath: [./]
运行程序，./opensource_v2/libopensource.so.2里的"opensource_print"被绑定了(此处，略去LD_DEBUG步骤，有兴趣可以自己试)：
[[email protected] 0004]# ./main general
-----------------------general--------------------
opensource v2 print, called by vendor 1
opensource v2 print, called by vendor 2
!!!!!!推论的延伸: 如果不使用libopensource.so.2这样和libopensource.so.1混淆的名字，而是使用一个其他的名字(例如librobin.so.2)，和本次测试是一样的结果(此处不再给出测试结果）。可以用different_soname_without_default_symver_2.sh 来编译做实验
3.2
符号表带版本信息的
编译时指定"-Wl,--default-symver"，那么编译出的符号是带版本信息的。
3.2.1
我们用different_soname_with_default_symver.sh 来编译
[[email protected] 0004]# sh different_soname_with_default_symver.sh
Complile success
3.2.2
列出编译生成的文件
略，和3.1.2一样.
3.2.3
用readelf查看编译生成的main，libvendor1.so，libvendor2.so
可以看出和3.1.3是一样的。
[[email protected] 0004]# readelf -d main
Dynamic section at offset 0x1de8 contains 28 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libvendor1.so]
0x0000000000000001 (NEEDED) Shared library: [libvendor2.so]
0x0000000000000001 (NEEDED) Shared library: [libdl.so.2]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000f (RPATH) Library rpath: [./]
[[email protected] 0004]# readelf -d libvendor1.so
Dynamic section at offset 0xdc8 contains 29 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libopensource.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libvendor1.so]
0x000000000000000f (RPATH) Library rpath: [./opensource_v1]
[[email protected] 0004]# readelf -d libvendor2.so
Dynamic section at offset 0xdc8 contains 29 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libopensource.so.2]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libvendor2.so]
0x000000000000000f (RPATH) Library rpath: [./opensource_v2]
3.2.4
用nm|grep opensource_print查看编译生成的libvendor1.so和libvendor2.so, 可以看到不同的符号"[email protected]@libopensource.so.1"和[email protected]@libopensource.so.2
[[email protected] 0004]# nm libvendor1.so |grep opensource_print
U [email protected]@libopensource.so.1
[[email protected] 0004]# nm libvendor2.so |grep opensource_print
U [email protected]@libopensource.so.2
3.2.5
用LD_DEBUG 来debug 依赖库和符号绑定的过程(针对默认加载动态库的情况)
[[email protected] 0004]# LD_DEBUG_OUTPUT=robin.txt LD_DEBUG=all ./main general
-----------------------general--------------------
opensource v1 print, called by vendor 1
opensource v2 print, called by vendor 2
首先看输出，从结果看，libvendor1.so调用了./opensource_v1/libopensource.so.1里的"opensource_print"(opensource_v1.c)；libvendor2.so调用了./opensource_v2/libopensource.so.2里的"opensource_print"(opensource_v2.c)。
完整的LD_DEBUG输出在robin.3.txt
我们来分析robin.3.txt输出：
58行到68行，./opensource_v1/libopensource.so.1被查找到；71行到81行，./opensource_v2/libopensource.so.2被查找到：
58 409: file=libopensource.so.1 [0]; needed by ./libvendor1.so [0]
59 409: find library=libopensource.so.1 [0]; searching
60 409: search path=./opensource_v1/tls/x86_64:./opensource_v1/tls:./opensource_v1/x86_64:./opensource_v1 (RPATH from file ./libvendor1.so)
61 409: trying file=./opensource_v1/tls/x86_64/libopensource.so.1
62 409: trying file=./opensource_v1/tls/libopensource.so.1
63 409: trying file=./opensource_v1/x86_64/libopensource.so.1
64 409: trying file=./opensource_v1/libopensource.so.1
65 409:
66 409: file=libopensource.so.1 [0]; generating link map
67 409: dynamic: 0x00007ffa01a9ade8 base: 0x00007ffa0189a000 size: 0x0000000000201038
68 409: entry: 0x00007ffa0189a640 phdr: 0x00007ffa0189a040 phnum: 7
69 409:
70 409:
71 409: file=libopensource.so.2 [0]; needed by ./libvendor2.so [0]
72 409: find library=libopensource.so.2 [0]; searching
73 409: search path=./opensource_v2/tls/x86_64:./opensource_v2/tls:./opensource_v2/x86_64:./opensource_v2 (RPATH from file ./libvendor2.so)
74 409: trying file=./opensource_v2/tls/x86_64/libopensource.so.2
75 409: trying file=./opensource_v2/tls/libopensource.so.2
76 409: trying file=./opensource_v2/x86_64/libopensource.so.2
77 409: trying file=./opensource_v2/libopensource.so.2
78 409:
79 409: file=libopensource.so.2 [0]; generating link map
80 409: dynamic: 0x00007ffa01898de8 base: 0x00007ffa01698000 size: 0x0000000000201038
81 409: entry: 0x00007ffa01698640 phdr: 0x00007ffa01698040 phnum: 7
1000行，可以看到libvendor1.so调用的"opensource_print"被绑定到./opensource_v1/libopensource.so.1上；1018行，libvendor2.so调用的"opensource_print"被绑定到./opensource_v2/libopensource.so.2上:
994 409: symbol=opensource_print; lookup in file=./main [0]
995 409: symbol=opensource_print; lookup in file=./libvendor1.so [0]
996 409: symbol=opensource_print; lookup in file=./libvendor2.so [0]
997 409: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
998 409: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
999 409: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
1000 409: binding file ./libvendor1.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘ [libopensource.so.1]
1001 409: symbol=printf; lookup in file=./main [0]
1002 409: symbol=printf; lookup in file=./libvendor1.so [0]
1003 409: symbol=printf; lookup in file=./libvendor2.so [0]
1004 409: symbol=printf; lookup in file=/lib64/libdl.so.2 [0]
1005 409: symbol=printf; lookup in file=/lib64/libc.so.6 [0]
1006 409: binding file ./opensource_v1/libopensource.so.1 [0] to /lib64/libc.so.6 [0]: normal symbol `printf‘ [GLIBC_2.2.5]
1007 409: symbol=vendor2; lookup in file=./main [0]
1008 409: symbol=vendor2; lookup in file=./libvendor1.so [0]
1009 409: symbol=vendor2; lookup in file=./libvendor2.so [0]
1010 409: binding file ./main [0] to ./libvendor2.so [0]: normal symbol `vendor2‘ [libvendor2.so]
1011 409: symbol=opensource_print; lookup in file=./main [0]
1012 409: symbol=opensource_print; lookup in file=./libvendor1.so [0]
1013 409: symbol=opensource_print; lookup in file=./libvendor2.so [0]
1014 409: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
1015 409: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
1016 409: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
1017 409: symbol=opensource_print; lookup in file=./opensource_v2/libopensource.so.2 [0]
1018 409: binding file ./libvendor2.so [0] to ./opensource_v2/libopensource.so.2 [0]: normal symbol `opensource_print‘ [libopensource.so.2]
3.2.6
用LD_DEBUG 来debug 依赖库和符号绑定的过程(针对使用"dlopen"等API，显式加载共享库的情况)
和3.2.5差不多一样。
[[email protected] 0004]# LD_DEBUG_OUTPUT=robin.txt LD_DEBUG=all ./main dlopen
-----------------------general--------------------
opensource v1 print, called by vendor 1
opensource v2 print, called by vendor 2
首先看输出，从结果看，libvendor1.so调用了./opensource_v1/libopensource.so.1里的"opensource_print"(opensource_v1.c)；libvendor2.so调用了./opensource_v2/libopensource.so.2里的"opensource_print"(opensource_v2.c)。
完整的LD_DEBUG输出在robin.4.txt
我们来分析robin.4.txt输出：
58行到68行，./opensource_v1/libopensource.so.1被查找到；71行到81行，./opensource_v2/libopensource.so.2被查找到：
58 409: file=libopensource.so.1 [0]; needed by ./libvendor1.so [0]
59 409: find library=libopensource.so.1 [0]; searching
60 409: search path=./opensource_v1/tls/x86_64:./opensource_v1/tls:./opensource_v1/x86_64:./opensource_v1 (RPATH from file ./libvendor1.so)
61 409: trying file=./opensource_v1/tls/x86_64/libopensource.so.1
62 409: trying file=./opensource_v1/tls/libopensource.so.1
63 409: trying file=./opensource_v1/x86_64/libopensource.so.1
64 409: trying file=./opensource_v1/libopensource.so.1
65 409:
66 409: file=libopensource.so.1 [0]; generating link map
67 409: dynamic: 0x00007ffa01a9ade8 base: 0x00007ffa0189a000 size: 0x0000000000201038
68 409: entry: 0x00007ffa0189a640 phdr: 0x00007ffa0189a040 phnum: 7
69 409:
70 409:
71 409: file=libopensource.so.2 [0]; needed by ./libvendor2.so [0]
72 409: find library=libopensource.so.2 [0]; searching
73 409: search path=./opensource_v2/tls/x86_64:./opensource_v2/tls:./opensource_v2/x86_64:./opensource_v2 (RPATH from file ./libvendor2.so)
74 409: trying file=./opensource_v2/tls/x86_64/libopensource.so.2
75 409: trying file=./opensource_v2/tls/libopensource.so.2
76 409: trying file=./opensource_v2/x86_64/libopensource.so.2
77 409: trying file=./opensource_v2/libopensource.so.2
78 409:
79 409: file=libopensource.so.2 [0]; generating link map
80 409: dynamic: 0x00007ffa01898de8 base: 0x00007ffa01698000 size: 0x0000000000201038
81 409: entry: 0x00007ffa01698640 phdr: 0x00007ffa01698040 phnum: 7
1037行，可以看到libvendor1.so调用的"opensource_print"被绑定到./opensource_v1/libopensource.so.1上；1077行，libvendor2.so调用的"opensource_print"被绑定到./opensource_v2/libopensource.so.2上:
1031 24314: symbol=opensource_print; lookup in file=./main [0]
1032 24314: symbol=opensource_print; lookup in file=./libvendor1.so [0]
1033 24314: symbol=opensource_print; lookup in file=./libvendor2.so [0]
1034 24314: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
1035 24314: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
1036 24314: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
1037 24314: binding file ./libvendor1.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘ [libopensource.so.1]
1070 24314: symbol=opensource_print; lookup in file=./main [0]
1071 24314: symbol=opensource_print; lookup in file=./libvendor1.so [0]
1072 24314: symbol=opensource_print; lookup in file=./libvendor2.so [0]
1073 24314: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
1074 24314: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
1075 24314: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
1076 24314: symbol=opensource_print; lookup in file=./opensource_v2/libopensource.so.2 [0]
1077 24314: binding file ./libvendor2.so [0] to ./opensource_v2/libopensource.so.2 [0]: normal symbol `opensource_print‘ [libopensource.so.2]
3.2.7
推测结论
!!!!!!猜测：对比3.2.4和3.1.4 "nm"输出，可以看到当编译时设定""-Wl,--default-symver"，那么编译出的符号是有版本信息的，"[email protected]@libopensource.so.1" 和 "[email protected]@libopensource.so.2" 是能找到其对应的正确的共享库的。
4.
libopensource.so.xxx的版本相同，系统如何查找依赖库和绑定符号
在这个实验里我们编译opensource_v1.c生成./opensource_v1/libopensource.so.1.0；编译opensource_v2.c生成./opensource_v2/libopensource.so.1.0。
libvendor1.so将依赖./opensource_v1/libopensource.so.1.0； libvendor2.so将依赖./opensource_v2/libopensource.so.1.0。
4.1
符号表不带版本信息的
gcc编译的符号，默认是不带版本信息的。
4.1.1
我们用same_soname_without_default_symver.sh 来编译。
[[email protected] 0004]# sh same_soname_without_default_symver.sh
Complile success
4.1.2
列出编译生成的文件
libvendor1.so使用的libopensource.so.1.0相应3个文件放在"./opensource_v1"目录;
libvendor2.so使用的libopensource.so.1.0相应3个文件放在"./opensource_v2"目录;
4.1.3
用readelf查看编译生成的main，libvendor1.so，libvendor2.so
[[email protected] 0004]# readelf -d main
Dynamic section at offset 0xde8 contains 28 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libvendor1.so]
0x0000000000000001 (NEEDED) Shared library: [libvendor2.so]
0x0000000000000001 (NEEDED) Shared library: [libdl.so.2]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000f (RPATH) Library rpath: [./]
[[email protected] 0004]# readelf -d libvendor1.so
Dynamic section at offset 0xde8 contains 27 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libopensource.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libvendor1.so]
0x000000000000000f (RPATH) Library rpath: [./opensource_v1]
[[email protected] 0004]# readelf -d libvendor2.so
Dynamic section at offset 0xde8 contains 27 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libopensource.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libvendor2.so]
0x000000000000000f (RPATH) Library rpath: [./opensource_v2]
4.1.4
用nm|grep opensource_print查看编译生成的libvendor1.so和libvendor2.so, 可以看到使用相同符号"opensource_print"
[[email protected] 0004]# nm libvendor1.so |grep opensource_print
U opensource_print
[[email protected] 0004]# nm libvendor2.so |grep opensource_print
U opensource_print
4.1.5
用LD_DEBUG 来debug 依赖库和符号绑定的过程(针对默认加载动态库的情况)
[[email protected] 0004]# LD_DEBUG_OUTPUT=robin.txt LD_DEBUG=all ./main general
-----------------------general--------------------
opensource v1 print, called by vendor 1
opensource v1 print, called by vendor 2
首先看输出，从结果看，仅仅调用了./opensource_v1/libopensource.so.1(opensource_v1.c)里的"opensource_print函数"。
完整的LD_DEBUG输出在robin.5.txt
我们来分析robin.5.txt输出：
58行到68行，./opensource_v1/libopensource.so.1被查找到；./opensource_v2/libopensource.so.1却没有被查找：
58 24862: file=libopensource.so.1 [0]; needed by ./libvendor1.so [0]
59 24862: find library=libopensource.so.1 [0]; searching
60 24862: search path=./opensource_v1/tls/x86_64:./opensource_v1/tls:./opensource_v1/x86_64:./opensource_v1 (RPATH from file ./libvendor1.so)
61 24862: trying file=./opensource_v1/tls/x86_64/libopensource.so.1
62 24862: trying file=./opensource_v1/tls/libopensource.so.1
63 24862: trying file=./opensource_v1/x86_64/libopensource.so.1
64 24862: trying file=./opensource_v1/libopensource.so.1
65 24862:
66 24862: file=libopensource.so.1 [0]; generating link map
67 24862: dynamic: 0x00007fda2f686e08 base: 0x00007fda2f486000 size: 0x0000000000201038
68 24862: entry: 0x00007fda2f486600 phdr: 0x00007fda2f486040 phnum: 7
905行,libvendor1.so调用的"opensource_print"被绑定到./opensource_v1/libopensource.so.1上；922行，libvendor2.so调用的"opensource_print"也被绑定到./opensource_v1/libopensource.so.1：
899 24862: symbol=opensource_print; lookup in file=./main [0]
900 24862: symbol=opensource_print; lookup in file=./libvendor1.so [0]
901 24862: symbol=opensource_print; lookup in file=./libvendor2.so [0]
902 24862: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
903 24862: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
904 24862: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
905 24862: binding file ./libvendor1.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘
906 24862: symbol=printf; lookup in file=./main [0]
907 24862: symbol=printf; lookup in file=./libvendor1.so [0]
908 24862: symbol=printf; lookup in file=./libvendor2.so [0]
909 24862: symbol=printf; lookup in file=/lib64/libdl.so.2 [0]
910 24862: symbol=printf; lookup in file=/lib64/libc.so.6 [0]
911 24862: binding file ./opensource_v1/libopensource.so.1 [0] to /lib64/libc.so.6 [0]: normal symbol `printf‘ [GLIBC_2.2.5]
912 24862: symbol=vendor2; lookup in file=./main [0]
913 24862: symbol=vendor2; lookup in file=./libvendor1.so [0]
914 24862: symbol=vendor2; lookup in file=./libvendor2.so [0]
915 24862: binding file ./main [0] to ./libvendor2.so [0]: normal symbol `vendor2‘
916 24862: symbol=opensource_print; lookup in file=./main [0]
917 24862: symbol=opensource_print; lookup in file=./libvendor1.so [0]
918 24862: symbol=opensource_print; lookup in file=./libvendor2.so [0]
919 24862: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
920 24862: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
921 24862: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
922 24862: binding file ./libvendor2.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘
4.1.6
用LD_DEBUG 来debug 依赖库和符号绑定的过程(针对使用"dlopen"等API，显式加载共享库的情况)
和4.1.5差不多一样。
[[email protected] 0004]# LD_DEBUG_OUTPUT=robin.txt LD_DEBUG=all ./main dlopen
----------------------dlopen----------------------
opensource v1 print, called by vendor 1
opensource v1 print, called by vendor 2
首先看输出，从结果看，仅仅调用了./opensoure_v1/libopensource.so.1(opensource_v1.c)里的"opensource_print函数"。
完整的LD_DEBUG输出在robin.6.txt
我们来分析robin.6.txt输出：
58行到68行，./opensource_v1/libopensource.so.1被查找到；./opensource_v2/libopensource.so.1没有被查找：
58 25068: file=libopensource.so.1 [0]; needed by ./libvendor1.so [0]
59 25068: find library=libopensource.so.1 [0]; searching
60 25068: search path=./opensource_v1/tls/x86_64:./opensource_v1/tls:./opensource_v1/x86_64:./opensource_v1 (RPATH from file ./libvendor1.so)
61 25068: trying file=./opensource_v1/tls/x86_64/libopensource.so.1
62 25068: trying file=./opensource_v1/tls/libopensource.so.1
63 25068: trying file=./opensource_v1/x86_64/libopensource.so.1
64 25068: trying file=./opensource_v1/libopensource.so.1
65 25068:
66 25068: file=libopensource.so.1 [0]; generating link map
67 25068: dynamic: 0x00007fcc7608ce08 base: 0x00007fcc75e8c000 size: 0x0000000000201038
68 25068: entry: 0x00007fcc75e8c600 phdr: 0x00007fcc75e8c040 phnum: 7
942行，libvendor1.so调用的"opensource_print"被绑定到./opensource_v1/libopensource.so.1上；981行，libvendor2.so调用的"opensource_print"也被绑定到./opensource_v1/libopensource.so.1：
936 25068: symbol=opensource_print; lookup in file=./main [0]
937 25068: symbol=opensource_print; lookup in file=./libvendor1.so [0]
938 25068: symbol=opensource_print; lookup in file=./libvendor2.so [0]
939 25068: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
940 25068: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
941 25068: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
942 25068: binding file ./libvendor1.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘
975 25068: symbol=opensource_print; lookup in file=./main [0]
976 25068: symbol=opensource_print; lookup in file=./libvendor1.so [0]
977 25068: symbol=opensource_print; lookup in file=./libvendor2.so [0]
978 25068: symbol=opensource_print; lookup in file=/lib64/libdl.so.2 [0]
979 25068: symbol=opensource_print; lookup in file=/lib64/libc.so.6 [0]
980 25068: symbol=opensource_print; lookup in file=./opensource_v1/libopensource.so.1 [0]
981 25068: binding file ./libvendor2.so [0] to ./opensource_v1/libopensource.so.1 [0]: normal symbol `opensource_print‘
4.1.7
推测结论
!!!!!!猜测: 根据4.1.5和4.1.6，只有./opensource_v1/libopensource.so.1被查找，所以只有./opensource_v1/libopensource.so.1里的"opensource_print"会被绑定
我们是有证据支持这个猜测的。
编辑same_soname_without_default_symver.sh，仅仅改变"-lvendor2","-lvendor1"的顺序，让"-lvendor2"靠前，如下：
#main.c
#gcc -Wl,-rpath=./ -o main main.c -L. -lvendor1 -lvendor2 -ldl
gcc -Wl,-rpath=./ -o main main.c -L. -lvendor2 -lvendor1 -ldl
重新编译：
[[email protected] 0004]# sh same_soname_without_default_symver.sh
Complile success
重新查看"readelf -d main", 和之前的4.1.3比较，看到"libverndor2.so"位置被提前了：
[[email protected] 0004]# readelf -d main
Dynamic section at offset 0xde8 contains 28 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libvendor2.so]
0x0000000000000001 (NEEDED) Shared library: [libvendor1.so]
0x0000000000000001 (NEEDED) Shared library: [libdl.so.2]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000f (RPATH) Library rpath: [./]
运行程序，发现./opensource_v2/libopensource.so.1里的"opensource_print"被绑定(此处，略去LD_DEBUG步骤，有兴趣可以自己试)：
[[email protected] 0004]# ./main general
-----------------------general--------------------
opensource v2 print, called by vendor 1
opensource v2 print, called by vendor 2
4.2
符号表带版本信息的
编译时指定"-Wl,--default-symver"，那么编译出的符号是带版本信息的。
4.2.1
我们用same_soname_with_default_symver.sh 来编译
[[email protected] 0004]# sh same_soname_with_default_symver.sh
Complile success
4.2.2
列出编译生成的文件
略，和4.1.2一样.
4.2.3
用readelf查看编译生成的main，libvendor1.so，libvendor2.so
可以看出和4.1.3是一样的。
[[email protected] 0004]# readelf -d main
Dynamic section at offset 0x1de8 contains 28 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libvendor1.so]
0x0000000000000001 (NEEDED) Shared library: [libvendor2.so]
0x0000000000000001 (NEEDED) Shared library: [libdl.so.2]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000f (RPATH) Library rpath: [./]
[[email protected] 0004]# readelf -d libvendor1.so
Dynamic section at offset 0xdc8 contains 29 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libopensource.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libvendor1.so]
0x000000000000000f (RPATH) Library rpath: [./opensource_v1]
[[email protected] 0004]# readelf -d libvendor2.so
Dynamic section at offset 0xdc8 contains 29 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libopensource.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libvendor2.so]
0x000000000000000f (RPATH) Library rpath: [./opensource_v2]
4.2.4
用nm|grep opensource_print查看编译生成的libvendor1.so和libvendor2.so, 可以看到相同的符号"[email protected]@libopensource.so.1"
[[email protected] 0004]# nm libvendor1.so |grep opensource_print
U [email protected]@libopensource.so.1
[[email protected] 0004]# nm libvendor2.so |grep opensource_print
U [email protected]@libopensource.so.1
!!!!!! libvendor1.so和libvendor2.so使用了相同版本的libopensource.so.1，并且使用了相同的符号[email protected]@libopensource.so.1，所以实验4.2和4.1是等价的实验，没必要再做下去了。 不过我还是列出了后续实验的LD_DEBUG的输出，供参考。
[[email protected] 0004]# LD_DEBUG_OUTPUT=robin.txt LD_DEBUG=all ./main general
-----------------------general--------------------
opensource v1 print, called by vendor 1
opensource v1 print, called by vendor 2
对应的完整的LD_DEBUG输出为robin.7.txt
[[email protected] 0004]# LD_DEBUG_OUTPUT=robin.txt LD_DEBUG=all ./main dlopen
-----------------------general--------------------
opensource v1 print, called by vendor 1
opensource v1 print, called by vendor 2
对应的完整的LD_DEBUG输出为robin.8.txt
5.
结论
1)对于不同版本的libopensource.so.xxx共享库，两个版本的共享库都会被查找到，但有先后顺序；最终绑定的"opensource_print"符号，是在先被查找到的共享库里的。
这个情况延伸开来，如果其他完全不相干的共享库里有同名符号"opensource_print"，那么到底绑定哪个"opensource_print"，也是和共享库被查找到顺序有关。
这个问题可以通过编译时指定"-Wl,--default-symver"来解决。
2)对于相同版本的lbopensource.so.xxx共享库，只有其中的一个会被查找，只有这个被查找到的共享库里的符号被绑定。
遇到这样的问题时，尝试下LD_PRELOAD，但不一定能完全解决问题。
6.
参考文献
[1] 一篇blog，https://blog.habets.se/2012/05/Shared-libraries-diamond-problem
[2] Shared libarary versions, http://bottomupcs.sourceforge.net/csbu/x4012.htm
[3] Google