HP-UX Reference (11i v2 04/09) - 5 Miscellaneous Topics (vol 9)

d
dld.sl(5) dld.sl(5)
(PA-RISC System Only)
Library-level Versioning
The second way for users to version their libraries is by using a new 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 will look at the shared 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 will look
for a
.0 version of the library (e.g.
libname.0) to load. If it does not find this version, it will look for
the library name that is recorded in the list.
PA-RISC 32-bit 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 reserved symbol
_ _dld_loc
, which is defined in crt0.o, points to a jump table within
the dynamic loader. The routines described under shl_load (3X) 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.
PA-RISC 64-bit 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. Instead, this mechanism allows the
dynamic loader to scan one table which contains the symbol information from all the loaded libraries.
This is particularly effective for applications 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
export symbols and places them into this table. When a library is unloaded, the mechanism looks up the
library’s export symbols 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.
The global symbol table mechanism can force the dynamic loader (
dld.sl) to perform a large number of
hashing operations to locate symbols. Performing this hash function can cost considerable time, espe-
cially when symbol names are very long (C++ programs). To speed up the loader, you can off-load com-
puting hash values to the linker by using the +gst option. This causes the linker to compute the hash
value for all symbols exported from libraries listed on the link line, and store the hash values in the exe-
cutable. At run time, the loader then builds the global symbol table in memory and reads the stored hash
values from the executable as each library is loaded. If you do not specify +gst at link time, you can use
the +gst flag option of the chatr (1) command to enable the global symbol table mechanism, which
causes the loader to build the table and compute the hash values at run time.
Use the GST options (with the
ld and chatr commands), +gst, +gstbuckets (PA-RISC 32-bit only),
+gstsize, +nodynhash (PA-RISC 64-bit systems only), and +plabel_cache, (PA-RISC 32-bit sys-
tems only) to control the behavior of the global symbol table hash mechanism. You can use the
+gstsize and +nodynhash linker/chatr options to control the behavior of the global symbol table hash
mechanism. With the GST options, you can tune the size of the hash table and number of buckets per
entry to reach a balance of performance and memory use. To maximize for performance, tune the table
size for an average chain length of one. For maximum memory use, at the expense of performance, tune
the size of the table to minimize the number of empty entries. In general, use prime numbers for the
Section 5−−86 Hewlett-Packard Company − 4 − HP-UX 11i Version 2: September 2004