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LD.AOUT_SO(1) NetBSD General Commands Manual LD.AOUT_SO(1)
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ld.so -- run-time link-editor
ld.so is a self-contained, position independent program image providing
run-time support for loading and link-editing shared objects into a
process' address space. It uses the data structures (see link(5)) con-
tained within dynamically linked programs to determine which shared
libraries are needed and loads them at a convenient virtual address using
the mmap(2) system call.
After all shared libraries have been successfully loaded, ld.so proceeds
to resolve external references from both the main program and all objects
loaded. A mechanism is provided for initialization routines to be
called, on a per-object basis, giving a shared object an opportunity to
perform any extra set-up, before execution of the program proper begins.
ld.so looks for a symbol named .init in each object's symbol table. If
present, this symbol is assumed to represent a C-function declared as
void .init(void), which is then called. Similarly, a void .fini(void)
function is called just before an object is unloaded from the process
address space as a result of calling dlclose(3). Note that while an
object's .init is always called, whether the object is loaded automati-
cally at program startup or programmatically by using dlopen(3), the
.fini function is called only on `last dlclose(3)'.
This mechanism is exploited by the system-supplied C++ constructor ini-
tialization code located in /usr/lib/c++rt.o. This file should be
included in the list of object-code files passed to ld(1) when building a
shared C++ library.
ld.so is itself a shared object that is initially loaded by the startup
module crt0. Since a.out(5) formats do not provide easy access to the
file header from within a running process, crt0 uses the special symbol
_DYNAMIC to determine whether a program is in fact dynamically linked or
not. Whenever the linker ld(1) has relocated this symbol to a location
other than 0, crt0 assumes the services of ld.so are needed (see link(5)
for details). crt0 passes control to rtld's entry point before the pro-
gram's main() routine is called. Thus, ld.so can complete the link-edit-
ing process before the dynamic program calls upon services of any dynamic
To quickly locate the required shared objects in the filesystem, ld.so
may use a ``hints'' file, prepared by the ldconfig(8) utility, in which
the full path specification of the shared objects can be looked up by
hashing on the 3-tuple <library-name, major-version-number, minor-
ld.so recognizes a number of environment variables that can be used to
modify its behavior as follows:
LD_LIBRARY_PATH A colon separated list of directories, overrid-
ing the default search path for shared
LD_PRELOAD A colon separated list of shared object file-
names to be loaded after the main program but
before its shared object dependencies.
LD_WARN_NON_PURE_CODE When set, issue a warning whenever a link-edit-
ing operation requires modification of the text
segment of some loaded object. This is usually
indicative of an incorrectly built library.
LD_SUPPRESS_WARNINGS When set, no warning messages of any kind are
issued. Normally, a warning is given if satis-
factorily versioned library could not be found.
LD_TRACE_LOADED_OBJECTS When set, causes ld.so to exit after loading the
shared objects and printing a summary which
includes the absolute pathnames of all objects,
to standard output.
When set, these variables are interpreted as
format strings a la printf(3) to customize the
trace output and are used by ldd(1)'s -f option
and allows ldd(1) to be operated as a filter
more conveniently. The following conversions
can be used:
%a The main program's name (also known as
%A The value of the environment variable
%o The library name.
%m The library's major version number.
%n The library's minor version number.
%p The full pathname as determined by rtld's
library search rules.
%x The library's load address.
Additionally, \n and \t are recognized and have
their usual meaning.
LD_NO_INTERN_SEARCH When set, ld.so does not process any internal
search paths that were recorded in the exe-
LD_NOSTD_PATH When set, do not include a set of built-in stan-
dard directory paths for searching. This might
be useful when running on a system with a com-
pletely non-standard filesystem layout.
/var/run/ld.so.hints library location hints built by ldconfig(8)
ld(1), ld.elf_so(1), ld.so(1), link(5), ldconfig(8)
The shared library model employed first appeared in SunOS 4.0.
The environment variables LD_LIBRARY_PATH and LD_PRELOAD are not honored
when executing in a set-user-ID or set-group-ID environment. This action
is taken to prevent malicious substitution of shared object dependencies
or interposition of symbols.
NetBSD 4.0 March 24, 2000 NetBSD 4.0