HP-UX Linker and Libraries User's Guide

The following flags do not affect relocation processing, but provide the following features:
RTLD_NODELETE: Under this mode, the specified object and its dependencies behave as if they
were built with -B nodelete . An explicit unload using dlclose or shl_load returns success
silently without detaching the shared library for the process. Subsequently, the shared library handle
is valid only for shl_findsym. It stays invalid for dlsym, dlclose, and shl_unload until the next
explicit load using shl_load or dlclose.
RTLD_NOLOAD: Under this mode, the specified object is not loaded into the process's address
space, but a valid handle is returned if the object already exists in the process address space. If
the specified object does not already exist, then an error is returned. RTLD_NOLOAD can be used
to query the presence, or for overriding the modes of an existing object.
If a file is specified in multiple dlopen invocations, mode is interpreted at each invocation. Note,
however, that once RTLD_NOW has been specified, the linker operation completes all relocations,
rendering any further RTLD_NOW operations redundant and any further RTLD_LAZY operations
irrelevant. Similarly note that once you specify RTLD_GLOBAL, the shared library maintains the
RTLD_GLOBAL status regardless of any previous or future specification of RTLD_LOCAL, as long
as the shared library remains in the address space [see dlclose(3C)].
RTLD_VERBOSE: Under this mode, the dynamic loader displays verbose messages when binding
symbols on PA32 systems. If RTLD_VERBOSE is ORed with the flags parameter, the dynamic
loader displays verbose messages for all the unresolved symbols. You can use this option to view
the symbols, which are not bound. Typically, RTLD_VERBOSE is used with RTLD_NOW to debug
unresolved symbols. For example:
...
void *dl =dlopen("unsat.sl", RTLD_NOW | RTLD_VERBOSE);
...
NOTE: The RTLD_VERBOSE flag for dlopen routine, and the BIND_VERBOSE flag for shl_load
routine can also be enabled by using the _HP_DLDOPTS environment variable (instead of
specifying the flags in the dlopen, and shl_load routines). This is particularly useful for existing
programs to enable RTLD_VERBOSE, and BIND_VERBOSE without any change in code. To set
the RTLD_VERBOSE and the BIND_VERBOSE flags by using _HP_DLDOPTS, enter the following
command at the HP-UX prompt:
$ export _HP_DLOPTS=-load_verbose
Symbols introduced into a program through calls to dlopen may be used in relocation activities.
Symbols so introduced may duplicate symbols already defined by the program or previous dlopen
operations. To resolve the ambiguities such a situation may present, the resolution of a symbol
reference to a symbol definition is based on a symbol resolution order. Two such resolution orders
are defined: load and dependency ordering.
• Load order establishes an ordering among symbol definitions using the temporal order in
which the shared libraries containing the definitions were loaded, such that the definition first
loaded has priority over definitions added later. Load ordering is used in relocation processing.
• Dependency ordering uses a "breadth-first" order starting with a given shared library, then
all of its dependencies, then any dependents of those, iterating until all dependencies are
satisfied.
The dlsym function uses dependency ordering, except when the global symbol shared library is
obtained via a dlopen operation on file with a value 0. The dlsym function uses load ordering
on the global symbol shared library.
When a dlopen operation first makes it accessible, a shared library and its dependent shared
libraries are added in dependency order. Once all shared libraries are added, relocations are
performed using load order. Note that if a shared library and its dependencies have been loaded
by a previous dlopen invocation or on startup, the load and dependency order may yield different
resolutions.
The dlopen Shared Library Management Routines 151