LD.AOUT_SO(1) BSD General Commands Manual LD.AOUT_SO(1)
ld.aout_so -- run-time link-editor
ld.aout_so is a self-contained, position independent program image providing run-time sup-
port 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 pro-
vided 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
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 loca-
tion 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-
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
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 path-
names 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 conve-
niently. The following conversions can be used:
%a The main program's name (also known as ``__progname'').
%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
%x The library's load address.
Additionally, \n and \t are recognized and have their usual mean-
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.
/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 substi-
tution of shared object dependencies or interposition of symbols.
BSD January 1, 2011 BSD