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.