freebsd/share/man/man5/link.5

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.Dd October 23, 1993
.Dt LINK 5
.Os
.Sh NAME
.Nm link
.Nd dynamic loader and link editor interface
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <nlist.h>
.Fd #include <link.h>
.Sh DESCRIPTION
The include file
.Aq Pa link.h
declares several structures that are present in dynamically linked
programs and libraries.
The structures define the interface between several components of the
link-editor and loader mechanism.
The layout of a number of these
structures within the binaries resembles the a.out format in many places
as it serves such similar functions as symbol definitions (including the
accompanying string table) and relocation records needed to resolve
references to external entities.
It also records a number of data structures
unique to the dynamic loading and linking process.
These include references
to other objects that are required to complete the link-editing process and
indirection tables to facilitate
.Em Position Independent Code
(PIC for short) to improve sharing of code pages among different processes.
The collection of data structures described here will be referred to as the
.Em Run-time Relocation Section (RRS)
and is embedded in the standard text and data segments of the dynamically
linked program or shared object image as the existing
.Xr a.out 5
format offers no room for it elsewhere.
.Pp
Several utilities cooperate to ensure that the task of getting a program
ready to run can complete successfully in a way that optimizes the use
of system resources.
The compiler emits PIC code from which shared libraries
can be built by
.Xr ld 1 .
The compiler also includes size information of any initialized data items
through the .size assembler directive.
PIC code differs from conventional code
in that it accesses data variables through an indirection table, the
Global Offset Table, by convention accessible by the reserved name
.Em _GLOBAL_OFFSET_TABLE_.
The exact mechanism used for this is machine dependent, usually a machine
register is reserved for the purpose.
The rational behind this construct
is to generate code that is independent of the actual load address.
Only
the values contained in the Global Offset Table may need updating at run-time
depending on the load addresses of the various shared objects in the address
space.
.Pp
Likewise, procedure calls to globally defined functions are redirected through
the Procedure Linkage Table (PLT) residing in the data segment of the core
image.
Again, this is done to avoid run-time modifications to the text segment.
.Pp
The linker-editor allocates the Global Offset Table and Procedure Linkage Table
when combining PIC object files into an image suitable for mapping into the
process address space.
It also collects all symbols that may be needed by the
run-time link-editor and stores these along with the image's text and data bits.
Another reserved symbol,
.Em _DYNAMIC
is used to indicate the presence of the run-time linker structures.
Whenever
_DYNAMIC is relocated to 0, there is no need to invoke the run-time
link-editor.
If this symbol is non-zero, it points at a data structure from
which the location of the necessary relocation- and symbol information can
be derived.
This is most notably used by the start-up module,
.Em crt0.
The _DYNAMIC structure is conventionally located at the start of the data
segment of the image to which it pertains.
.Pp
.Sh DATA STRUCTURES
The data structures supporting dynamic linking and run-time relocation
reside both in the text and data segments of the image they apply to.
The text segments contain read-only data such as symbols descriptions and
names, while the data segments contain the tables that need to be modified by
during the relocation process.
.Pp
The _DYNAMIC symbol references a
.Fa _dynamic
structure:
.Bd -literal -offset indent
struct	_dynamic {
	int	d_version;
	struct 	so_debug *d_debug;
	union {
		struct section_dispatch_table *d_sdt;
	} d_un;
	struct  ld_entry *d_entry;
};
.Ed
.Bl -tag -width d_version
.It Fa d_version
This field provides for different versions of the dynamic linking
implementation.
The current version numbers understood by
.Xr ld 1
and 
.Xr ld.so 1
are
.Em LD_VERSION_SUN (3),
which is used by the
.Tn SunOS
4.x releases, and
.Em LD_VERSION_BSD (8),
which has been in use since
.Fx 1.1 .
.It Fa d_un
Refers to a
.Em d_version
dependent data structure.
.It Fa so_debug
this field provides debuggers with a hook to access symbol tables of shared
objects loaded as a result of the actions of the run-time link-editor.
.El
.Pp
The
.Fa section_dispatch_table
structure is the main
.Dq dispatcher
table, containing offsets into the image's segments where various symbol
and relocation information is located.
.Bd -literal -offset indent
struct section_dispatch_table {
	struct	so_map *sdt_loaded;
	long	sdt_sods;
	long	sdt_filler1;
	long	sdt_got;
	long	sdt_plt;
	long	sdt_rel;
	long	sdt_hash;
	long	sdt_nzlist;
	long	sdt_filler2;
	long	sdt_buckets;
	long	sdt_strings;
	long	sdt_str_sz;
	long	sdt_text_sz;
	long	sdt_plt_sz;
};
.Ed
.Pp
.Bl -tag -width sdt_filler1
.It Fa sdt_loaded
A pointer to the first link map loaded (see below). This field is set by
.Nm ld.so
.It Fa sdt_sods
The start of a (linked) list of shared object descriptors needed by
.Em this
object.
.It Fa sdt_filler1
Deprecated (used by SunOS to specify library search rules).
.It Fa sdt_got
The location of the Global Offset Table within this image.
.It Fa sdt_plt
The location of the Procedure Linkage Table within this image.
.It Fa sdt_rel
The location of an array of
.Fa relocation_info
structures
.Po
see
.Xr a.out 5
.Pc
specifying run-time relocations.
.It Fa sdt_hash
The location of the hash table for fast symbol lookup in this object's
symbol table.
.It Fa sdt_nzlist
The location of the symbol table.
.It Fa sdt_filler2
Currently unused.
.It Fa sdt_buckets
The number of buckets in
.Fa sdt_hash
.It Fa sdt_strings
The location of the symbol string table that goes with
.Fa sdt_nzlist.
.It Fa sdt_str_sz
The size of the string table.
.It Fa sdt_text_sz
The size of the object's text segment.
.It Fa sdt_plt_sz
The size of the Procedure Linkage Table.
.El
.Pp
A
.Fa sod
structure describes a shared object that is needed
to complete the link edit process of the object containing it.
A list of such objects
.Po
chained through
.Fa sod_next
.Pc
is pointed at
by the
.Fa sdt_sods
in the section_dispatch_table structure.
.Bd -literal -offset indent
struct sod {
	long	sod_name;
	u_int	sod_library : 1,
		sod_reserved : 31;
	short	sod_major;
	short	sod_minor;
	long	sod_next;
};
.Ed
.Pp
.Bl -tag -width sod_library
.It Fa sod_name
The offset in the text segment of a string describing this link object.
.It Fa sod_library
If set,
.Fa sod_name
specifies a library that is to be searched for by 
.Nm ld.so .
The path name
is obtained by searching a set of directories
.Po
see also
.Xr ldconfig 8
.Pc
for a shared object matching
.Em lib\&<sod_name>\&.so.n.m.
If not set,
.Fa sod_name
should point at a full path name for the desired shared object.
.It Fa sod_major
Specifies the major version number of the shared object to load.
.It Fa sod_minor
Specifies the prefered minor version number of the shared object to load.
.El
.Pp
The run-time link-editor maintains a list of structures called
.Em link maps
to keep track of all shared objects loaded into a process' address space.
These structures are only used at run-time and do not occur within
the text or data segment of an executable or shared library.
.Bd -literal -offset indent
struct so_map {
	caddr_t	som_addr;
	char 	*som_path;
	struct	so_map *som_next;
	struct	sod *som_sod;
	caddr_t som_sodbase;
	u_int	som_write : 1;
	struct	_dynamic *som_dynamic;
	caddr_t	som_spd;
};
.Ed
.Bl -tag -width som_dynamic
.It Fa som_addr
The address at which the shared object associated with this link map has
been loaded.
.It Fa som_path
The full path name of the loaded object.
.It Fa som_next
Pointer to the next link map.
.It Fa som_sod
The
.Fa sod
structure that was responsible for loading this shared object.
.It Fa som_sodbase
Tossed in later versions the run-time linker.
.It Fa som_write
Set if (some portion of) this object's text segment is currently writable.
.It Fa som_dynamic
Pointer to this object's
.Fa _dynamic
structure.
.It Fa som_spd
Hook for attaching private data maintained by the run-time link-editor.
.El
.Pp
Symbol description with size.
This is simply an
.Fa nlist
structure with one field
.Pq Fa nz_size
added.
Used to convey size information on items in the data segment
of shared objects.
An array of these lives in the shared object's
text segment and is addressed by the
.Fa sdt_nzlist
field of
.Fa section_dispatch_table.
.Bd -literal -offset indent
struct nzlist {
	struct nlist	nlist;
	u_long		nz_size;
#define nz_un		nlist.n_un
#define nz_strx		nlist.n_un.n_strx
#define nz_name		nlist.n_un.n_name
#define nz_type		nlist.n_type
#define nz_value	nlist.n_value
#define nz_desc		nlist.n_desc
#define nz_other	nlist.n_other
};
.Ed
.Bl -tag -width nz_size
.It Fa nlist
.Po
see
.Xr nlist 3
.Pc .
.It Fa nz_size
The size of the data represented by this symbol.
.El
.Pp
A hash table is included within the text segment of shared object to
to facilitate quick lookup of symbols during run-time link-editing.
The
.Fa sdt_hash
field of the
.Fa section_dispatch_table
structure points at an array of
.Fa rrs_hash
structures:
.Bd -literal -offset indent
struct rrs_hash {
	int	rh_symbolnum;		/* symbol number */
	int	rh_next;		/* next hash entry */
};
.Ed
.Pp
.Bl -tag -width rh_symbolnum
.It Fa rh_symbolnum
The index of the symbol in the shared object's symbol table (as given by the
.Fa ld_symbols
field).
.It Fa rh_next
In case of collisions, this field is the offset of the next entry in this
hash table bucket.
It is zero for the last bucket element.
.El
The
.Fa rt_symbol
structure is used to keep track of run-time allocated commons
and data items copied from shared objects.
These items are kept on linked list
and is exported through the
.Fa dd_cc
field in the
.Fa so_debug
structure (see below) for use by debuggers.
.Bd -literal -offset indent
struct rt_symbol {
	struct nzlist		*rt_sp;
	struct rt_symbol	*rt_next;
	struct rt_symbol	*rt_link;
	caddr_t			rt_srcaddr;
	struct so_map		*rt_smp;
};
.Ed
.Pp
.Bl -tag -width rt_scraddr
.It Fa rt_sp
The symbol description.
.It Fa rt_next
Virtual address of next rt_symbol.
.It Fa rt_link
Next in hash bucket.
Used by internally by
.Nm ld.so .
.It Fa rt_srcaddr
Location of the source of initialized data within a shared object.
.It Fa rt_smp
The shared object which is the original source of the data that this
run-time symbol describes.
.El
.Pp
The
.Fa so_debug
structure is used by debuggers to gain knowledge of any shared objects
that have been loaded in the process's address space as a result of run-time
link-editing.
Since the run-time link-editor runs as a part of process
initialization, a debugger that wishes to access symbols from shared objects
can only do so after the link-editor has been called from crt0.
A dynamically linked binary contains a
.Fa so_debug
structure which can be located by means of the
.Fa d_debug
field in
.Fa _dynamic .
.Bd -literal -offset indent
struct 	so_debug {
	int	dd_version;
	int	dd_in_debugger;
	int	dd_sym_loaded;
	char    *dd_bpt_addr;
	int	dd_bpt_shadow;
	struct rt_symbol *dd_cc;
};
.Ed
.Pp
.Bl -tag -width dd_in_debugger
.It Fa dd_version
Version number of this interface.
.It Fa dd_in_debugger
Set by the debugger to indicate to the run-time linker that the program is
run under control of a debugger.
.It Fa dd_sym_loaded
Set by the run-time linker whenever it adds symbols by loading shared objects.
.It Fa dd_bpt_addr
The address were a breakpoint will be set by the run-time linker to
divert control to the debugger.
This address is determined by the start-up
module,
.Em crt0.o,
to be some convenient place before the call to _main.
.It Fa dd_bpt_shadow
Contains the original instruction that was at
.Fa dd_bpt_addr .
The debugger is expected to put this instruction back before continuing the
program.
.It Fa dd_cc
A pointer to the linked list of run-time allocated symbols that the debugger
may be interested in.
.El
.Pp
The
.Em ld_entry
structure defines a set of service routines within
.Nm ld.so .
.\" See
.\" .Xr libdl.a
.\" for more information.
.Bd -literal -offset indent
struct ld_entry {
	void	*(*dlopen)(char *, int);
	int	(*dlclose)(void *);
	void	*(*dlsym)(void *, char *);
	char	*(*dlerror)(void);
};
.Ed
.Pp
The
.Fa crt_ldso
structure defines the interface between the start-up code in crt0 and
.Nm ld.so .
.Bd -literal -offset indent
struct crt_ldso {
	int		crt_ba;
	int		crt_dzfd;
	int		crt_ldfd;
	struct _dynamic	*crt_dp;
	char		**crt_ep;
	caddr_t		crt_bp;
	char		*crt_prog;
	char		*crt_ldso;
	struct ld_entry	*crt_ldentry;
};
#define CRT_VERSION_SUN		1
#define CRT_VERSION_BSD_2	2
#define CRT_VERSION_BSD_3	3
#define	CRT_VERSION_BSD_4	4
.Ed
.Bl -tag -width crt_dzfd
.It Fa crt_ba
The virtual address at which
.Nm ld.so
was loaded by crt0.
.It Fa crt_dzfd
On SunOS systems, this field contains an open file descriptor to
.Dq Pa /dev/zero
used to get demand paged zeroed pages.
On
.Fx
systems it contains -1.
.It Fa crt_ldfd
Contains an open file descriptor that was used by crt0 to load
.Nm ld.so .
.It Fa crt_dp
A pointer to main's
.Fa _dynamic
structure.
.It Fa crt_ep
A pointer to the environment strings.
.It Fa crt_bp
The address at which a breakpoint will be placed by the run-time linker
if the main program is run by a debugger.
See
.Fa so_debug
.It Fa crt_prog
The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only).
.It Fa crt_ldso
The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only).
.El
.Pp
The
.Fa hints_header
and
.Fa hints_bucket
structures define the layout of the library hints, normally found in
.Dq Pa /var/run/ld.so.hints ,
which is used by 
.Nm ld.so
to quickly locate the shared object images in the
filesystem.
The organization of the hints file is not unlike that of an
.Dq a.out
object file, in that it contains a header determining the offset and size
of a table of fixed sized hash buckets and a common string pool.
.Bd -literal -offset indent
struct hints_header {
	long		hh_magic;
#define HH_MAGIC	011421044151
	long		hh_version;
#define LD_HINTS_VERSION_1	1
	long		hh_hashtab;
	long		hh_nbucket;
	long		hh_strtab;
	long		hh_strtab_sz;
	long		hh_ehints;
};
.Ed
.Bl -tag -width hh_strtab_sz
.It Fa hh_magic
Hints file magic number.
.It Fa hh_version
Interface version number.
.It Fa hh_hashtab
Offset of hash table.
.It Fa hh_strtab
Offset of string table.
.It Fa hh_strtab_sz
Size of strings.
.It Fa hh_ehints
Maximum usable offset in hints file.
.El
.Pp
.Bd -literal -offset indent
/*
 * Hash table element in hints file.
 */
struct hints_bucket {
	int		hi_namex;
	int		hi_pathx;
	int		hi_dewey[MAXDEWEY];
	int		hi_ndewey;
#define hi_major hi_dewey[0]
#define hi_minor hi_dewey[1]
	int		hi_next;
};
.Ed
.Bl -tag -width hi_ndewey
.It Fa hi_namex
Index of the string identifying the library.
.It Fa hi_pathx
Index of the string representing the full path name of the library.
.It Fa hi_dewey
The version numbers of the shared library.
.It Fa hi_ndewey
The number of valid entries in
.Fa hi_dewey .
.It Fa hi_next
Next bucket in case of hashing collisions.
.El
.Sh CAVEATS
Only the (GNU) C compiler currently supports the creation of shared libraries.
Other programming languages can not be used.