Updated: 2022/Sep/29
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LD.AOUT_SO(1) General Commands Manual LD.AOUT_SO(1)
NAME
ld.aout_so - run-time link-editor
DESCRIPTION
ld.aout_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))
contained 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
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.aout_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 automatically 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
initialization 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
complete the link-editing process before the dynamic program calls upon
services 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-
version-number>.
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,
overriding the default search path for shared
libraries.
LD_PRELOAD A colon separated list of shared object
filenames 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-
editing 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
satisfactorily versioned library could not be
found.
LD_TRACE_LOADED_OBJECTS When set, causes ld.aout_so to exit after
loading the shared objects and printing a
summary which includes the absolute pathnames of
all objects, to standard output.
LD_TRACE_LOADED_OBJECTS_FMT1
LD_TRACE_LOADED_OBJECTS_FMT2
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
"__progname").
%A The value of the environment variable
LD_TRACE_LOADED_OBJECTS_PROGNAME
%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
internal search paths that were recorded in the
executable.
LD_NOSTD_PATH When set, do not include a set of built-in
standard directory paths for searching. This
might be useful when running on a system with a
completely non-standard filesystem layout.
FILES
/var/run/ld.so.hints library location hints built by ldconfig(8)
SEE ALSO
ld(1), ld.elf_so(1), ld.so(1), link(5), ldconfig(8)
HISTORY
The shared library model employed first appeared in SunOS 4.0.
SECURITY CONSIDERATIONS
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 10.99 January 1, 2011 NetBSD 10.99