Linux dynamic library search path

2013.08.23 updated

Concept 1: The shared library is called a dynamic library.

In Linux, the executable file or dynamic library in ELF format has two fields: DT_RPATH and DT_RUNPATH (this is added to the new version ).

// This method is only written to the DT_RPATH domain gcc-g-Wall-o prog. c-Wl,-rpath,/home/dir1-ldemo // This method is written into DT_RPATH and DT_RUNPATHgcc-g-Wall-o prog. c-Wl, -- enable-new-dtags-Wl,-rpath,/home/dir1-ldemo

Compilation is divided into several stages.-W indicates the stage in which the following parameters are added,-Wl indicates that the following parameters are added in the link stage, and-rpath specifies the runtime dynamic path parameter for the linker.

The reason why the fields DT_RPATH and DT_RUNPATH need to be written later is that the previous version does not have DT_RUNTIME and is compatible with the old version.

2. The search path sequence of the Linux dynamic library is as follows:

0> If the DT_RUNPATH field is empty and DT_RPATH is not empty, search for the path list specified by DT_RPATH (multiple paths are separated by colons );

1> the dynamic library search path specified by the Environment Variable LD_LIBRARY_PATH. Multiple paths can be separated;

2> If DT_RUNPATH is not empty, search for the path list corresponding to DT_RUNPATH;

3> in/etc/ld. so. specified in the cache file. This file is a binary file. If you want to add it, edit the configuration file/etc/ld first. so. conf (the file is text), and then run ldconfig to generate a new/etc/ld. so. cache;

4> default dynamic library search path/lib;

5> default dynamic library search path/usr/lib.

The following are the suggestions for using LD_LIBRARY_PATH:

1> do not set LD_LIBRARY_PATH globally

2> if you must publish your code using a dynamic library and allow users to customize the installation path

• Use the. o file to publish and reconnect them to the correct installation path during installation.
• In the executable program, write a very lengthy, non-existent path (-Wl,-rpath), and then use a binary editor to correct the path during installation.

3> If LD_LIBRARY_PATH must be used, it must be packaged and used, for example, shell scripts.

Here is an example of my personal experience.

An Installed Software Package A is built on Platform 1.0. All dynamic library files of the Platform are located in/opt/lib64/

A new software package B is built on the Platform 2.0. All dynamic libraries of the Platform are located under the/lib directory of the software.

Start Software B and find the running error. Run the ldd command and find that the connected dynamic library is incorrect. The lib in the local directory should be connected, but the/opt/lib64/

It still cannot be started by setting LD_LIBRARY_PATH, because the path has been written to the executable file. You can use the strings command to view the strings in the binary.

Two solutions were found later:

1> use the binary editor to rewrite the path in the executable file. Change/opt/lib64 to/opt/lib65, so that/opt/lib65 cannot be found during running, then LD_LIBRARY_PATH takes effect.

2> before compiling B, modify the path in configure. ac so that the parameter passed to-Wl and-rpath is an impossible path.