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LD.AOUT_SO(1) NetBSD General Commands Manual LD.AOUT_SO(1)
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ld.aout_so -- run-time link-editor
ld.aout_so is a self-contained, position independent program image pro-
viding 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.aout_so pro-
ceeds 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.aout_so looks for a symbol named .init in each object's sym-
bol 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 automat-
ically 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.aout_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.aout_so are needed
(see link(5) for details). crt0 passes control to rtld's entry point
before the program's main() routine is called. Thus, ld.aout_so can com-
plete the link-editing process before the dynamic program calls upon ser-
vices of any dynamic library.
To quickly locate the required shared objects in the filesystem,
ld.aout_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.aout_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.aout_so to exit after load-
ing 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.aout_so does not process any inter-
nal 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 8.0 January 1, 2011 NetBSD 8.0