HP-UX Linker and Libraries User's Guide

...
}
Dynamic Path Lookup
If you specify an embedded path while building the filtered shared library using the +b linker
option, the dynamic loader attempts to use this dynamic path when it searches for the implementation
library to load. For example:
$ ld -b impl1.o -o libimpl1.sl
$ ld -b +b /path/to/implementation/libs +filter\ ./libimpl1.sl -o libfilt.sl
$ chatr +b enable libfilt.sl
$ cc prog.c libfilt.sl -o prog
$ mv libimpl1.sl /path/to/implementation/libs
$ ./prog
Thread-Private Data (TLS)
You can use TLS in the implementation libraries even though they are "`lazy-loaded". When the
filter library is loaded, space is reserved for the TLS of its implementation libraries even before they
are loaded. Because the space is reserved, filtering is not an appropriate way to reduce TLS
consumption.
Maintaining filtered shared libraries
To maintain your filtered shared libraries, you must rebuild them when you make the following
changes:
• Add or remove symbols from the set of exported symbols from the implementation libraries
• Change symbol types, for example, when an uninitialized data symbol (BSS symbol) is changed
to an initialized data symbol (or vice-versa), or a text symbol into a data symbol.
• Change the TLS (Thread Local Storage) size of an implementation library Even though the
newly-added TLS symbols may not be exported outside the implementation library, you must
rebuild the filter library so the correct amount of TLS space is reserved. To see the TLS size,
use the following command:
$ odump -sldlheader shared_library_pathname | fgrep tdsize
Function Level Versioning
A new type qualifier with syntax '__attribute__((version_id("<some version id string>"))' is supported
that can be used to facilitate shared libraries to have different versions of the same function based
on platform (11.0, 11.11 etc.) or any other factor. Existing user applications using these shared
libraries do not require recompilation when migrated to next version of the OS. Binaries will
continue to run with the older version of the function which is part of the shared library.
Example usage:
extern int y __attribute__((version_id("11.22")));
Version Control with Shared Libraries
HP-UX provides a method to support incompatible versions of shared library routines. “Library-Level
Versioning” (page 111) describes how to create multiple versions of a shared library.
NOTE: Beginning with the HP-UX 11.00 release, the linker toolset supports only library-level
versioning. Previous releases supported inter-library version control.
When to Use Shared Library Versioning
For the most part, updates to a shared library must be completely upward-compatible; that is,
updating a shared library does not usually cause problems for programs that use the library. But
sometimes - for example, if you add a new parameter to a routine - updates cause undesirable
110 Creating and Using Libraries