.\"	$NetBSD: ld.aout_so.1,v 1.10 2011/01/01 16:34:52 plunky Exp $
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.\" by Paul Kranenburg.
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.Dd January 1, 2011
.Dt LD.AOUT_SO 1
.Os
.Sh NAME
.Nm ld.aout_so
.Nd run-time link-editor
.Sh DESCRIPTION
.Nm
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
.Po
see
.Xr link 5
.Pc
contained within dynamically linked programs to determine which shared
libraries are needed and loads them at a convenient virtual address
using the
.Xr mmap 2
system call.
.Pp
After all shared libraries have been successfully loaded,
.Nm
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.
.Nm
looks for a symbol named
.Em .init
in each object's symbol table.
If present, this symbol is assumed to represent a C-function declared as
.Ft void
.Fn .init "void" ,
which is then called.
Similarly, a
.Ft void
.Fn .fini "void"
function is called just before an object is unloaded from the process
address space as a result of calling
.Xr dlclose 3 .
Note that while an object's
.Em .init
is always called, whether the object is loaded automatically at program
startup or programmatically by using
.Xr dlopen 3 ,
the
.Em .fini
function is called only on
.Sq last Xr dlclose 3 .
.Pp
This mechanism is exploited by the system-supplied C++ constructor
initialization code located in
.Pa /usr/lib/c++rt.o .
This file should be included in the list of object-code files passed to
.Xr ld 1
when building a shared C++ library.
.Pp
.Nm
is itself a shared object that is initially loaded by the startup module
.Em crt0 .
Since
.Xr a.out 5
formats do not provide easy access to the file header from within a running
process,
.Em crt0
uses the special symbol
.Va _DYNAMIC
to determine whether a program is in fact dynamically linked or not.
Whenever the linker
.Xr ld 1
has relocated this symbol to a location other than 0,
.Em crt0
assumes the services of
.Nm
are needed
.Po
see
.Xr link 5
for details
.Pc .
.Em crt0
passes control to
.Nm rtld Ns 's
entry point before the program's
.Fn main
routine is called.
Thus,
.Nm
can complete the link-editing process before the dynamic program calls upon
services of any dynamic library.
.Pp
To quickly locate the required shared objects in the filesystem,
.Nm
may use a
.Dq hints
file, prepared by the
.Xr ldconfig 8
utility, in which the full path specification of the shared objects can be
looked up by hashing on the 3-tuple
.Ao
library-name, major-version-number, minor-version-number
.Ac Ns \&.
.Pp
.Nm
recognizes a number of environment variables that can be used to modify
its behavior as follows:
.Pp
.\" intentionally not the longest
.Bl -tag -width LD_TRACE_LOADED_OBJECTS
.It Ev LD_LIBRARY_PATH
A colon separated list of directories, overriding the default search path
for shared libraries.
.It Ev LD_PRELOAD
A colon separated list of shared object filenames to be loaded
.Em after
the main program but
.Em before
its shared object dependencies.
.It Ev 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.
.It Ev 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.
.It Ev LD_TRACE_LOADED_OBJECTS
When set, causes
.Nm
to exit after loading the shared objects and printing a summary which includes
the absolute pathnames of all objects, to standard output.
.It Ev LD_TRACE_LOADED_OBJECTS_FMT1
.It Ev LD_TRACE_LOADED_OBJECTS_FMT2
When set, these variables are interpreted as format strings a la
.Xr printf 3
to customize the trace output and are used by
.Xr ldd 1 Ns 's
.Fl f
option and allows
.Xr ldd 1
to be operated as a filter more conveniently.
The following conversions can be used:
.Bl -tag -width xxxx
.It \&%a
The main program's name
.Po also known as
.Dq __progname
.Pc .
.It \&%A
The value of the environment variable
.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME
.It \&%o
The library name.
.It \&%m
The library's major version number.
.It \&%n
The library's minor version number.
.It \&%p
The full pathname as determined by
.Nm rtld Ns 's
library search rules.
.It \&%x
The library's load address.
.El
.Pp
Additionally,
.Sy \en
and
.Sy \et
are recognized and have their usual meaning.
.It Ev LD_NO_INTERN_SEARCH
When set,
.Nm
does not process any internal search paths that were recorded in the
executable.
.It Ev 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.
.El
.Sh FILES
.Bl -tag -width /var/run/ld.so.hintsXXX -compact
.It Pa /var/run/ld.so.hints
library location hints built by
.Xr ldconfig 8
.El
.Sh SEE ALSO
.Xr ld 1 ,
.Xr ld.elf_so 1 ,
.Xr ld.so 1 ,
.Xr link 5 ,
.Xr ldconfig 8
.Sh HISTORY
The shared library model employed first appeared in SunOS 4.0.
.Sh SECURITY CONSIDERATIONS
The environment variables
.Ev LD_LIBRARY_PATH
and
.Ev 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.