Shared object dependencies

When the link editor processes an archive library, it extracts library members and copies them into the output object file. These statically linked services are available during execution without involving the dynamic linker. Shared objects also provide services, and the dynamic linker must attach the proper shared object files to the process image for execution. Thus executable and shared object files describe their specific dependencies. For further information, see ``Link editing''.

When the dynamic linker creates the memory segments for an object file, the dependencies (recorded in DT_NEEDED entries of the dynamic structure) tell what shared objects are needed to supply the program's services. By repeatedly connecting referenced shared objects and their dependencies, the dynamic linker builds a complete process image. When resolving symbolic references, the dynamic linker examines the symbol tables with a breadth-first search. It first looks at the symbol table of the executable program itself, then at the symbol tables of the DT_NEEDED entries (in order), and then at the second level DT_NEEDED entries.

Names in the dependency list are copies either of the DT_SONAME strings or the path names of the shared objects used to build the object file. For example, if the link editor builds an executable file using one shared object with a DT_SONAME entry of lib1 and another shared object library with the path name /usr/lib/lib2, the executable file will contain lib1 and /usr/lib/lib2 in its dependency list.

If a shared object name has one or more slash (/) characters anywhere in the name, such as /usr/lib/lib2 or directory/file, the dynamic linker uses that string directly as the path name. If the name has no slashes, such as lib1, three facilities specify shared object path searching, with the following caveats:

1. The dynamic array tag DT_RPATH may give a string that holds a list of directories, separated by colons (:). For example, the string /home/dir/usr/lib:/home/dir2/usr/lib: tells the dynamic linker to search first the directory /home/dir/lib, then /home/dir2/usr/lib, and then the current directory to find dependencies.

2. A variable called LD_LIBRARY_PATH in the process environment may hold a list of directories as above, optionally followed by a semicolon (;) and another directory list. The following values would be equivalent to the previous example:

   • LD_LIBRARY_PATH=/home/dir/usr/lib:/home/dir2/usr/lib:

   • LD_LIBRARY_PATH=/home/dir/usr/lib;/home/dir2/usr/lib:

   • LD_LIBRARY_PATH=/home/dir/usr/lib:/home/dir2/usr/lib:

   All LD_LIBRARY_PATH directories are searched after those from DT_RPATH. Although some programs (such as the link editor) treat the lists before and after the semicolon differently, the dynamic linker does not. Nevertheless, the dynamic linker accepts the semicolon notation, with the semantics described previously.

3. If the other two groups of directories fail to locate the desired library, the dynamic linker searches /usr/lib.