C++加载动态库的顺序

 

1. where to load dynamic so: (rpath isdetermined and recorded when compiling, it is also used to find dynamic library)

https://unix.stackexchange.com/questions/22926/where-do-executables-look-for-shared-objects-at-runtime

The shared library HOWTO explains most of the mechanisms involved, and the dynamic loader manual goes into more detail. Each unix variant has its own way, but most use the same executable format (ELF) and have similar dynamic linkers (derived from Solaris). Below I'll summarize the common behavior with a focus on Linux; check your system's manuals for the complete story.

In a nutshell, when it's looking for a dynamic library (.so file) the linker tries:

• directories listed in the LD_LIBRARY_PATH environment variable (DYLD_LIBRARY_PATH on OSX);
• directories listed in the executable's rpath;
• directories on the system search path, which (on Linux at least) consists of the entries in /etc/ld.so.conf plus /lib and /usr/lib.

The rpath is stored in the executable (it's the DT_RPATH or DT_RUNPATH dynamic attribute). It can contain absolute paths or paths starting with $ORIGIN to indicate a path relative to the location of the executable (e.g. if the executable is in /opt/myapp/bin and its rpath is $ORIGIN/../lib:$ORIGIN/../plugins then the dynamic linker will look in /opt/myapp/lib and /opt/myapp/plugins). The rpath is normally determined when the executable is compiled, with the -rpath option to ld, but you can change it afterwards with chrpath.

In the scenario you describe, if you're the developer or packager of the application and intend for it to be installed in a …/bin, …/lib structure, then link with -rpath='$ORIGIN/../lib'. If you're installing a pre-built binary on your system, either put the library in a directory on the search path (/usr/local/lib if you're the system administrator, otherwise a directory that you add to $LD_LIBRARY_PATH), or try chrpath.

 

 

2. how to find rpath form binary or library:

http://stackoverflow.com/questions/2836330/is-there-a-way-to-inspect-the-current-rpath-on-linux

For the record, here are a couple of commands that will show the rpath header.

objdump -x binary-or-library |grep RPATH

Maybe an even better way to do it is the following:

readelf -d binary-or-library |head -20

The second command also lists the direct dependencies on other libraries followed by rpath.

 

 

3. to check all load so by:

ldd your_lib_or_binary | less

 

4. offical ld man pages:

The linker uses the following search paths to locate required

       shared libraries:

       1.  Any directories specified by -rpath-link options.

       2.  Any directories specified by -rpath options.  The difference

           between -rpath and -rpath-link is that directories specified by

           -rpath options are included in the executable and used at

           runtime, whereas the -rpath-link option is only effective at

           link time. Searching -rpath in this way is only supported by

           native linkers and cross linkers which have been configured

           with the --with-sysroot option.

       3.  On an ELF system, for native linkers, if the -rpath and

           -rpath-link options were not used, search the contents of the

           environment variable "LD_RUN_PATH".

       4.  On SunOS, if the -rpath option was not used, search any

           directories specified using -L options.

       5.  For a native linker, the search the contents of the environment

           variable "LD_LIBRARY_PATH".

       6.  For a native ELF linker, the directories in "DT_RUNPATH" or

           "DT_RPATH" of a shared library are searched for shared

           libraries needed by it. The "DT_RPATH" entries are ignored if

           "DT_RUNPATH" entries exist.

       7.  The default directories, normally /lib and /usr/lib.

       8.  For a native linker on an ELF system, if the file

           /etc/ld.so.conf exists, the list of directories found in that

           file.

       If the required shared library is not found, the linker will issue

       a warning and continue with the link.