HP-UX Reference (11i v3 07/02) - 5 Miscellaneous Topics (vol 9)

d
dld.so(5) dld.so(5)
(Itanium(R)-based System Only)
NAME
dld.so - dynamic loader
MULTITHREAD USAGE
The dynamic loader is thread-safe.
DESCRIPTION
The /usr/lib/hpux64/dld.so
shared library is the 64-bit dynamic loader. The
/usr/lib/hpux32/dld.so
shared library is the 32-bit dynamic loader. In programs that use shared
libraries,
dld.so is invoked automatically at startup time.
/usr/ccs/lib/hpux64/crt0.o
is the
64-bit run-time startup file.
/usr/ccs/lib/hpux32/crt0.o
is the 32-bit run-time startup file. The
/usr/lib/hpux32/uld.so
and /usr/lib/hpux64/uld.so
shared libraries are the 32-bit and
64-bit microloaders. In programs that use shared libraries, the microloader is invoked automatically at
startup time by exec(2). The microloader’s sole responsibility is to load the dynamic loader,
dld.so
, into
memory for program execution. The microloaders (
uld.so
) and the dynamic loaders (dld.so) are,
themselves, shared libraries, although they define no symbols for use by user programs.
Shared Libraries
Shared libraries are executable files created with the
-b option to ld (see ld(1)). They must contain
position-independent code (PIC) that can be mapped anywhere in the address space of a process and exe-
cuted with minimal relocation. PIC can use PC-relative addressing modes and/or linkage tables. The HP
compilers generate PIC by default.
Incomplete Executables
An executable program linked with one or more shared libraries is called an incomplete executable.
When creating an executable (
a.out) file from object files and libraries, the linker does not copy text
(code) or data from the shared library into the output file. Instead, the dynamic loader maps the library
into the address space of the process at run time. The linker binds all program references to shared library
routines and data to entries in a linkage table, and relies on the dynamic loader to fill in the linkage table
entries once the libraries have been mapped. This linkage table serves as a jump table for function calls.
Thread local storage
Two thread local storage models are supported: static and dynamic, which is controlled by a compiler option
+tls=static/dynamic
. The default is +tls=dynamic . Shared libraries built with the dynamic
model can be loaded using dlopen(3C) and shl_load(3X) APIs. An attempt to load a shared library built
with the static model using dlopen(3C) or shl_load(3X) APIs will result in the following error:
/usr/lib/hpux[32|64]/dld.so: Can’t shl_load() a library containing
Thread Local Storage: /usr/lib/hpux[32|64]/libcps.so.1
The dynamic loader tallies each shared library’s thread local storage size as well as the program’s thread
local storage size. When all libraries are loaded, the dynamic loader invokes an initializer in the system
library libc, which does the thread initialization, allocation of the initial thread, and sets the thread
pointer.
Loading
An incomplete executable contains a list of path names of the shared libraries searched at link time. At run
time, the dynamic loader attaches to the process all shared libraries that were linked with the program.
The dynamic loader attempts to load each library from the same directory in which it was found at link
time. It is possible to change the shared library run time search path by specifying a dynamic path list.
(For PA-RISC 32-bit compatibility mode information, see Dynamic Path List.)
The text segment of a library is shared among all processes that use it. The data and bss (uninitialized
data) segments are loaded on a page-by-page basis. When a process first accesses (reads or writes) a data
or bss page, a copy of that page is made for the process.
Dynamic Path List
For default mode libraries, the dynamic loader uses dynamic path searching to find shared libraries whose
names appear in a library list of the program or loaded shared libraries with no embedded
/ character.
Dynamic path searching is enabled by default for these libraries or executables. If ld +noenvvar is
specified, the dynamic loader does not look at any dynamic path environment variables to find dependent
shared libraries. This limits the dynamic path searching to the value of rpath (run time path or embed-
ded path set by the ld +b command) and the default directory /usr/lib/hpux64 for 64-bit libraries
or the default directory /usr/lib/hpux32 for 32-bit libraries.
118 Hewlett-Packard Company − 1 − HP-UX 11i Version 3: February 2007