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.Dd December 17, 2010
.Dt LD.ELF_SO 1
.Os
.Sh NAME
.Nm ld.elf_so
.Nd run-time link-editor (linker)
.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 information stored in data structures within the binary (see
.Xr elf 5 )
and environment variables to determine which shared objects are needed.
These shared objects are loaded at a convenient virtual address using the
.Xr mmap 2
system call.
After all shared objects have been successfully loaded,
.Nm
proceeds to resolve external references from both
the main program and all objects loaded.
Once all required references are resolved control is
passed to the program via its entry point.
.Ss Startup
On the execution of a dynamically linked binary the kernel will load
the program and its run-time linker as specified in the PT_INTERP
section in the program header.
At this point, instead of passing control directly to the program,
the kernel passes control to the specified linker.
An auxiliary vector of information is passed that includes
the address of the program header, the size of each entry in the header,
and the number of entries.
The entry point of the program and the base address of where
.Nm
is loaded is also supplied.
.Ss Finding objects
Each
.Xr elf 5
object file may contain information in its dynamic (PT_DYNAMIC) section
about which shared objects it requires (often referred to as dependencies).
These dependencies are specified in the optional DT_NEEDED entry within
the dynamic section.
Each DT_NEEDED entry refers to a filename string of
the shared object that is to be searched for.
.Pp
The linker will search for libraries in three lists of paths:
.Bl -enum
.It
A user defined list of paths as specified in LD_LIBRARY_PATH and
.Xr ld.so.conf 5 .
.Pp
The use of ld.so.conf should be avoided as the setting of a global search
path can present a security risk.
.It
A list of paths specified within a shared object using a DT_RPATH entry in
the dynamic section.
This is defined at shared object link time.
.It
The list of default paths which is set to
.Pa /usr/lib .
.El
.Pp
Dynamic loading via
.Xr dlopen 3
uses the DT_RPATH entries of the main binary, independently of which object
the call came from.
.Pp
.Nm
will expand the following variables if present in the paths:
.Bl -tag -width $PLATFORM
.It $HWCAP
Processor hardware capabilities, for example FPU, MMX, SSE.
Currently unimplemented.
.It $ISALIST
List of instructions sets this processor can execute.
Currently unimplemented.
.It $ORIGIN
The directory of the main object.
Implemented in
.Nm
but the kernel support is currently disabled.
.It $OSNAME
The value of the
.Dv kern.ostype
.Xr sysctl 3 .
.It $OSREL
The value of the
.Dv kern.osrelease
.Xr sysctl 3 .
.It $PLATFORM
The value of the
.Dv hw.machine_arch
.Xr sysctl 3 .
.El
.Pp
Both
.Dv ${VARIABLE}
and
.Dv $VARIABLE
are recognized.
.Pp
The filename string can be considered free form, however, it will almost
always take the form lib\*[Lt]name\*[Gt].so.\*[Lt]number\*[Gt],
where name specifies the
.Sq library
name and number is conceptually the library's major version number.
.Pp
This name and another of the form lib\*[Lt]name\*[Gt].so are normally
symbolic links to the real shared object which has a filename of the form
lib\*[Lt]name\*[Gt].so.\*[Lt]major\*[Gt].\*[Lt]minor\*[Gt][.\*[Lt]teeny\*[Gt]].
This naming convention allows a versioning scheme similar to
.Xr a.out 5 .
.Ss Relocation
.Nm
will perform all necessary relocations immediately except for relocations
relating to the Procedure Linkage Table (PLT).
The PLT is used as a indirection method for procedure
calls to globally defined functions.
It allows, through the use of intermediate code, the delayed binding of
a call to a globally defined function to be performed at procedure call time.
This
.Sq lazy
method is the default (see LD_BIND_NOW).
.Ss Initialization
A mechanism is provided for initialization and termination routines
to be called, on a per-object basis before execution of the program proper
begins or after the program has completed.
This gives a shared object an opportunity to perform
any extra set-up or completion work.
.Pp
The DT_INIT and DT_FINI entries in the dynamic section specify the addresses
of the initialization and termination functions, respectively, for
the shared object.
.Nm
arranges for each initialization function to be called before control is passed
to the program and for the termination functions to be called by using
.Xr atexit 3 .
.Pp
This mechanism is exploited by the system-supplied constructor
initialization and destructor code located in
.Pa /usr/lib/crtbeginS.o
and
.Pa /usr/lib/crtendS.o .
These files are automatically included by
.Xr cc 1
and
.Xr c++ 1
in the list of object-code files passed to
.Xr ld 1
when building a shared C or C++ object.
.Sh ENVIRONMENT
If the following environment variables exist they will be used by
.Nm .
.Bl -tag -width "LD_LIBRARY_PATH"
.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 or space separated list of shared object filenames to be loaded
.Em after
the main program but
.Em before
its shared object dependencies.
Space is allowed as a separator for backwards compatibility only.
Support may be removed in a future release and should not be relied upon.
.It Ev LD_BIND_NOW
If defined immediate binding of Procedure Link Table (PLT) entries is
performed instead of the default lazy method.
.It Ev LD_DEBUG
If defined a variety of debug information will be written to the standard
error of an dynamically linked executed when it is run.
This variable is only recognized if
.Nm
was compiled with debugging support
.Sy ( -DDEBUG ) .
.El
.Sh FILES
.Bl -tag -width /etc/ld.so.conf -compact
.It Pa /etc/ld.so.conf
library location hints supplied by the system administrator.
.El
.Sh SEE ALSO
.Xr ld 1 ,
.Xr ld.aout_so 1 ,
.Xr dlfcn 3 ,
.Xr elf 5
.Sh HISTORY
The ELF shared library model employed first appeared in Sys V R4.
.Pp
The path expansion variables first appeared in Solaris 10, and
in
.Nx 5.0 .
.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.