通过一个简单的例子，体验一下Linux动态库soname的使用。
假设有一个动态库：libbar.so.1.1.0，其对应的三个名称如下。
- realname：libbar.so.1.1.0
- soname：libbar.so.1
- linkname：libbar.so
先生成一个libbar.so，通过-Wl,-soname指定soname为libbar.so.1。
$ g++ -fPIC -shared -Wl,-soname,libbar.so.1 -o libbar.so.1.1.0
$ readelf -d libbar.so.1.1.0
Dynamic section at offset 0x520 contains 24 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libstdc++.so.6]
0x0000000000000001 (NEEDED) Shared library: [libm.so.6]
0x0000000000000001 (NEEDED) Shared library: [libgcc_s.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libbar.so.1]
0x000000000000000c (INIT) 0x398
0x000000000000000d (FINI) 0x4d8
0x000000006ffffef5 (GNU_HASH) 0x120
0x0000000000000005 (STRTAB) 0x248
0x0000000000000006 (SYMTAB) 0x158
0x000000000000000a (STRSZ) 160 (bytes)
0x000000000000000b (SYMENT) 24 (bytes)
0x0000000000000003 (PLTGOT) 0x2006f8
0x0000000000000002 (PLTRELSZ) 24 (bytes)
0x0000000000000014 (PLTREL) RELA
0x0000000000000017 (JMPREL) 0x380
0x0000000000000007 (RELA) 0x320
0x0000000000000008 (RELASZ) 96 (bytes)
0x0000000000000009 (RELAENT) 24 (bytes)
0x000000006ffffffe (VERNEED) 0x300
0x000000006fffffff (VERNEEDNUM) 1
0x000000006ffffff0 (VERSYM) 0x2e8
0x000000006ffffff9 (RELACOUNT) 1
0x0000000000000000 (NULL) 0x0
使用ldconfig生成soname对应的符号链接文件。
$ ldconfig -n /tmp/
$ ll libbar.so.1
lrwxrwxrwx 1 henshao users 15 Nov 28 01:00 libbar.so.1 -> libbar.so.1.1.0
接着写一个程序使用libbar.so。
#链接需要linkname，否则会提示找不到动态库。
$ gcc foo.c -o foo -L. -lbar
/usr/bin/ld: cannot find -lbar
collect2: ld returned 1 exit status
#生成linkname的服务链接文件
$ ln -s libbar.so.1 libbar.so
#这次编译能成功
$ gcc foo.c -o foo -L. -lbar
#可见foo依赖的是soname，而不是linkname。
$ ldd foo
linux-vdso.so.1 => (0x00007fffa27ff000)
/opt/alisentry/$LIB/alisentry_exec.so => /opt/alisentry/lib64/alisentry_exec.so (0x00007fabfda17000)
/opt/alisentry/$LIB/alisentry_kill.so => /opt/alisentry/lib64/alisentry_kill.so (0x00007fabfd814000)
libbar.so.1 => not found
libc.so.6 => /lib64/libc.so.6 (0x00007fabfd4b0000)
libdl.so.2 => /lib64/libdl.so.2 (0x00007fabfd2ac000)
/lib64/ld-linux-x86-64.so.2 (0x00007fabfdc1d000)
为什么主程序会依赖soname，而不是linkname和realname？这是因为Linux的动态库的命名格式是libbar.so.x.y.z，最后一个z版本的变动一定是兼容的。y版本升级一般向前兼容。所以这个y和z不能写死。x版本变动一般是不兼容升级。所以使用soname是最为合理的。
再做一个有意思的测试。将libbar.so.1重命名为aaa，然后同样创建aaa的符号链接为libbar.so。
$ ll libbar.so* aaa
lrwxrwxrwx 1 henshao users 3 Nov 28 01:28 libbar.so -> aaa
lrwxrwxrwx 1 henshao users 15 Nov 28 01:27 aaa -> libbar.so.1.1.0
-rwxr-xr-x 1 henshao users 5301 Nov 28 01:27 libbar.so.1.1.0
有趣的是，foo依然知道依赖libbar.so.1，而不是aaa。
$ gcc foo.c -o foo -L. -lbar
$ ldd foo
linux-vdso.so.1 => (0x00007fff19ce7000)
/opt/alisentry/$LIB/alisentry_exec.so => /opt/alisentry/lib64/alisentry_exec.so (0x00007fb07c389000)
/opt/alisentry/$LIB/alisentry_kill.so => /opt/alisentry/lib64/alisentry_kill.so (0x00007fb07c186000)
libbar.so.1 => not found
libc.so.6 => /lib64/libc.so.6 (0x00007fb07be22000)
libdl.so.2 => /lib64/libdl.so.2 (0x00007fb07bc1e000)
/lib64/ld-linux-x86-64.so.2 (0x00007fb07c58f000)
在Linux系统里面找个实际的例子看看。wget依赖libz.so.1。
$ ldd /usr/bin/wget
linux-vdso.so.1 => (0x00007fff631ff000)
/opt/alisentry/$LIB/alisentry_exec.so => /opt/alisentry/lib64/alisentry_exec.so (0x00007fe75c93e000)
/opt/alisentry/$LIB/alisentry_kill.so => /opt/alisentry/lib64/alisentry_kill.so (0x00007fe75c73b000)
libssl.so.6 => /lib64/libssl.so.6 (0x00007fe75c4e1000)
libcrypto.so.6 => /lib64/libcrypto.so.6 (0x00007fe75c190000)
libdl.so.2 => /lib64/libdl.so.2 (0x00007fe75bf8c000)
libz.so.1 => /usr/lib64/libz.so.1 (0x00007fe75bd78000)
librt.so.1 => /lib64/librt.so.1 (0x00007fe75bb6f000)
libc.so.6 => /lib64/libc.so.6 (0x00007fe75b816000)
libgssapi_krb5.so.2 => /usr/lib64/libgssapi_krb5.so.2 (0x00007fe75b5e8000)
libkrb5.so.3 => /usr/lib64/libkrb5.so.3 (0x00007fe75b353000)
libcom_err.so.2 => /lib64/libcom_err.so.2 (0x00007fe75b150000)
libk5crypto.so.3 => /usr/lib64/libk5crypto.so.3 (0x00007fe75af2b000)
/lib64/ld-linux-x86-64.so.2 (0x00007fe75cb44000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fe75ad10000)
libkrb5support.so.0 => /usr/lib64/libkrb5support.so.0 (0x00007fe75ab07000)
libkeyutils.so.1 => /lib64/libkeyutils.so.1 (0x00007fe75a905000)
libresolv.so.2 => /lib64/libresolv.so.2 (0x00007fe75a6f0000)
libselinux.so.1 => /lib64/libselinux.so.1 (0x00007fe75a4d7000)
libsepol.so.1 => /lib64/libsepol.so.1 (0x00007fe75a291000)
$ll /usr/lib64/libz.so*
lrwxrwxrwx 1 root root 19 Dec 3 2015 /usr/lib64/libz.so -> ../../lib64/libz.so
lrwxrwxrwx 1 root root 21 Dec 3 2015 /usr/lib64/libz.so.1 -> ../../lib64/libz.so.1
lrwxrwxrwx 1 root root 25 Dec 3 2015 /usr/lib64/libz.so.1.2.3 -> ../../lib64/libz.so.1.2.3
$ ll /lib64/libz*
lrwxrwxrwx 1 root root 13 Dec 3 2015 /lib64/libz.so -> libz.so.1.2.3
lrwxrwxrwx 1 root root 13 Dec 3 2015 /lib64/libz.so.1 -> libz.so.1.2.3
-rwxr-xr-x 1 root root 83280 May 28 2014 /lib64/libz.so.1.2.3
$ readelf -d /usr/lib64/libz.so.1.2.3
Dynamic section at offset 0x139f8 contains 21 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000e (SONAME) Library soname: [libz.so.1]
0x000000000000000c (INIT) 0x1b48
0x000000000000000d (FINI) 0xc958
0x000000006ffffef5 (GNU_HASH) 0x158
0x0000000000000005 (STRTAB) 0xdd8
0x0000000000000006 (SYMTAB) 0x3e8
0x000000000000000a (STRSZ) 1148 (bytes)
0x000000000000000b (SYMENT) 24 (bytes)
0x0000000000000003 (PLTGOT) 0x213bc8
0x0000000000000002 (PLTRELSZ) 1200 (bytes)
0x0000000000000014 (PLTREL) RELA
0x0000000000000017 (JMPREL) 0x1698
0x0000000000000007 (RELA) 0x1368
0x0000000000000008 (RELASZ) 816 (bytes)
0x0000000000000009 (RELAENT) 24 (bytes)
0x000000006ffffffe (VERNEED) 0x1328
0x000000006fffffff (VERNEEDNUM) 1
0x000000006ffffff0 (VERSYM) 0x1254
0x000000006ffffff9 (RELACOUNT) 26
0x0000000000000000 (NULL) 0x0
