关于LD_DEBUG

引用

LD_DEBUG
The dynamic library loader used in linux (part of glibc) has some neat tricks. One of these is that you can set an environment variable called
LD_DEBUG

to show how symbols (variables and functions, for example) are resolved for a dynamic executable. This can sometimes help resolve obscure bugs where your application isn’t doing what you expect (assuming it is caused by symbols being resolved differently to what you were expecting).

This is very useful if you get segmentation violations or aborts for a program - this can sometimes be caused by linking against the wrong version of a library. This is also a really good way to understand what happens when you run any program! It has some self-documentation - for the impatient, you can do

原文引述一下memcached的问题：

LD_DEBUG 是 glibc 中的 loader 为了方便自身调试而设置的一个环境变量。通过设置这个环境变量，可以方便的看到 loader 的加载过程。 以在 64位 centos 上编译安装 memcached 为例，安装阶段顺利，执行 memcached 命令时出现错误：

#memcached –h
/usr/local/memcached/bin/memcached: error while loading shared libraries: libevent-1.4.so.2:
cannot open shared object file: No such file or directory
#find / -name libevent-1.4.so.2
/usr/lib/libevent-1.4.so.2

设置 LD_DEBUG 变量获得更多信息:

#LD_DEBUG=help ls
Valid options for the LD_DEBUG environment variable are:

  libs        display library search paths
  reloc       display relocation processing
  files       display progress for input file
  symbols     display symbol table processing
  bindings    display information about symbol binding
  versions    display version dependencies
  all         all previous options combined
  statistics  display relocation statistics
  unused      determined unused DSOs
  help        display this help message and exit

To direct the debugging output into a file instead of standard output
a filename can be specified using the LD_DEBUG_OUTPUT environment variable.

库依赖的问题，使用 libs 参数:

#LD_DEBUG=libs /usr/local/memcached/bin/memcached –h
9808:      find library=libevent-1.4.so.2 [0]; searching
9808:      search cache=/etc/ld.so.cache
9808:      search path=/lib64/tls/x86_64:/lib64/tls:/lib64/x86_64:/lib64:/usr/lib64/tls/x86_64
/usr/lib64/tls:/usr/lib64/x86_64:/usr/lib64
(system search path)
9808:      trying file=/lib64/tls/x86_64/libevent-1.4.so.2
9808:      trying file=/lib64/tls/libevent-1.4.so.2
9808:      trying file=/lib64/x86_64/libevent-1.4.so.2
9808:      trying file=/lib64/libevent-1.4.so.2
9808:      trying file=/usr/lib64/tls/x86_64/libevent-1.4.so.2
9808:      trying file=/usr/lib64/tls/libevent-1.4.so.2
9808:      trying file=/usr/lib64/x86_64/libevent-1.4.so.2
9808:      trying file=/usr/lib64/libevent-1.4.so.2
9808:      /usr/local/memcached/bin/memcached: error while loading shared libraries: libevent-1.4.so.2: cannot open shared object file: No such file or directory

问题一目了然:

ln -s /usr/lib/libevent-1.4.so.2 /usr/lib64/libevent-1.4.so.2

参考原文：
http://grepk.com/?p=62
http://blog.chinaunix.net/u1/59481/showart_2131371.html