dld.so.5 (2012 03)

d
dld.so(5)
Itanium(R)-based Systems Only
dld.so(5)
function call that could potentially reference the object.
If the
-B immediate option to ld is used, the loader binds all necessary references at startup time.
This increases the startup cost of a program, but ensures that no more binding operations are required
later. Thus, better real-time response may result, and the risk of a later abort due to unresolved exter-
nals is eliminated.
The
fastbind tool can be used to improve the start-up time of programs that use shared libraries
(incomplete executables). The
fastbind tool performs analysis on the shared library routines and data
used to bind the symbols and stores this information in the executable file. The dynamic loader notices
that this information is available, and it uses this fastbind information to bind the symbols instead of the
standard search method. For more details refer to fastbind (1) and the
+help option to ld(1) or the HP-
UX Linker and Libraries User’s Guide.
Breadth-first Searching
By default, the dynamic loader does breadth-first searching when binding symbols. If the incomplete exe-
cutable was linked with
+compat or if a
shl_load() is being executed, then depth-first searching is
used. (See Depth-first Searching .) Breadth-first searching specifies that the dynamic loader looks for
symbols starting with the incomplete executable followed by all loaded shared libraries in a left to right
order until the symbol is found. For example, the incomplete executable is searched followed by all
libraries in its library load list. Then the dependent shared libraries of the first library in the library load
list is searched, followed by the dependent shared libraries of the second library in the list, and so on.
Version Control
Since code from a shared library is mapped at run time from a separate shared library file, modifications
to a shared library may alter the behavior of existing executables. In some cases, this may cause pro-
grams to operate incorrectly.
Library-level Versioning
Users can control versions of their libraries by using a naming convention,
libname.n where n is a
numeral that is incremented with every new release of the library. When using the new naming scheme,
users must specify an internal name for the shared library by using the +h internal_name option to
ld
when building the shared library. This internal name is recorded in each incomplete executable or
shared library that links with the shared library.
At run time, the loader looks at the library list recorded in the incomplete executable file or shared
library. For each library in the list that was not an internal name, the dynamic loader looks for a
.0 ver-
sion of the library (for example,
libname.0) to load. If it does not find this version, it looks for the
library name that is recorded in the list.
Explicit Loading and Binding
The duties of the dynamic loader as described above are all performed automatically, although they can
be controlled somewhat by appropriate options to
ld. The dynamic loader can also be accessed program-
matically. The routines described under shl_load (3X), dlclose (3C), dlerror (3C), dlget (3C),
dlmodinfo (3C), dlopen (3C), and dlsym (3C) provide a portable interface that allows the programmer to
explicitly attach a shared library to the process at run time, to calculate the addresses of symbols defined
within shared libraries, and to detach the library when done.
Global Symbol Table
The global symbol table mechanism is designed as a performance enhancement option. Enabling this
mechanism causes the creation of a global symbol table which speeds up symbol lookup, by eliminating
the need to scan all loaded libraries in order to find a symbol. This is particularly effective for applica-
tions with large numbers of shared libraries. This mechanism is off by default.
The global symbol table is implemented using a hash table. Under this mechanism, whenever a library is
loaded (either implicitly or by using dlopen() or shl_load()), the mechanism hashes the library’s
exports and places them into this table. When a library is unloaded, the mechanism looks up the library’s
exports in the table and removes them.
The hash table does not contain entries for symbols defined by
shl_definesym(). User-defined sym-
bols must therefore be handled separately.
Enabling the mechanism causes
dld to use more memory and impacts the performance of the dlo-
pen(), dlclose(), shl_load(), and shl_unload() API calls.
HP-UX 11i Version 3: March 2012 − 3 − Hewlett-Packard Company 3