关于LD_DEBUG （转载）

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