link(5)

NAME

link - dynamic loader and link editor interface

SYNOPSIS

#include <sys/types.h>
#include <nlist.h>
#include <link.h>

DESCRIPTION

The include file #include <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
Position Independent Code (PIC for short) to improve sharing of code
pages among different processes. The collection of data structures de
scribed here will be referred to as the Run-time Relocation Section (RRS)
and is embedded in the standard text and data segments of the dy
namically linked program or shared object image as the existing a.out(5) for
mat offers no room for it elsewhere.
Several utilities cooperate to ensure that the task of get
ting a program ready to run can complete successfully in a way that opti
mizes the use of system resources. The compiler emits PIC code from which
shared libraries can be built by ld(1). The compiler also includes
size information of any initialized data items through the .size as
sembler 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 _GLOBAL_OFF
SET_TABLE_. The exact mechanism used for this is machine dependent, usually a ma
chine register is reserved for the purpose. The rational behind this con
struct is to generate code that is independent of the actual load ad
dress. Only the values contained in the Global Offset Table may need updat
ing at run-time depending on the load addresses of the various shared ob
jects in the address space.
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 modi
fications to the text segment.
The linker-editor allocates the Global Offset Table and Pro
cedure Linkage Table when combining PIC object files into an image suitable
for mapping into the process address space. It also collects all sym
bols 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,
_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 linkeditor. 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, crt0. The _DYNAMIC structure is conventionally located at the
start of the data segment of the image to which it pertains.

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 de
scriptions and names, while the data segments contain the tables that need
to be modified by during the relocation process.
The _DYNAMIC symbol references a _dynamic structure:: struct _dynamic {
int d_version;
struct so_debug *d_debug;
union {
struct section_dispatch_table *d_sdt;

} d_un;
struct ld_entry *d_entry;; };
d_version This field provides for different versions of the
dynamic: linking implementation. The current version num; bers understood by ld(1) and ld.so(1) are LD_VERSION_SUN; (3), which is
used by the SunOS 4.x releases, and; LD_VERSION_BSD (8), which
has been in use since FreeBSD 1.1.
d_un Refers to a d_version dependent data structure.
so_debug this field provides debuggers with a hook to ac
cess symbol: tables of shared objects loaded as a result of; the actions of
the run-time link-editor.
The section_dispatch_table structure is the main ``dispatch
er'' table, containing offsets into the image's segments where various
symbol and relocation information is located.: 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;; };
sdt_loaded A pointer to the first link map loaded (see be
low). This: field is set by ld.so
sdt_sods The start of a (linked) list of shared object
descriptors: needed by this object.
sdt_filler1 Deprecated (used by SunOS to specify library
search rules).
sdt_got The location of the Global Offset Table within
this image.
sdt_plt The location of the Procedure Linkage Table
within this: image.
sdt_rel The location of an array of relocation_info
structures (see: a.out(5)) specifying run-time relocations.
sdt_hash The location of the hash table for fast symbol
lookup in: this object's symbol table.
sdt_nzlist The location of the symbol table.
sdt_filler2 Currently unused.
sdt_buckets The number of buckets in sdt_hash
sdt_strings The location of the symbol string table that
goes with: sdt_nzlist.
sdt_str_sz The size of the string table.
sdt_text_sz The size of the object's text segment.
sdt_plt_sz The size of the Procedure Linkage Table.
A 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
(chained through sod_next) is pointed at by the sdt_sods in
the section_dispatch_table structure.: struct sod {
long sod_name;
u_int sod_library : 1,
sod_reserved : 31;

short sod_major;
short sod_minor;
long sod_next;; };
sod_name The offset in the text segment of a string de
scribing this: link object.
sod_library If set, sod_name specifies a library that is to
be searched: for by ld.so. The path name is obtained by; searching a set
of directories (see also ldconfig(8)) for a; shared object
matching lib<sod_name>.so.n.m. If not set,; sod_name should
point at a full path name for the desired; shared object.
sod_major Specifies the major version number of the
shared object to: load.
sod_minor Specifies the preferred minor version number of
the shared: object to load.
The run-time link-editor maintains a list of structures
called 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.: 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;; };
som_addr The address at which the shared object associ
ated with this: link map has been loaded.
som_path The full path name of the loaded object.
som_next Pointer to the next link map.
som_sod The sod structure that was responsible for
loading this: shared object.
som_sodbase Tossed out in later versions of the run-time
linker.
som_write Set if (some portion of) this object's text
segment is cur: rently writable.
som_dynamic Pointer to this object's _dynamic structure.
som_spd Hook for attaching private data maintained by
the run-time: link-editor.
Symbol description with size. This is simply an nlist
structure with one
field (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 sdt_nzlist
field of
section_dispatch_table.: 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
};
nlist (see nlist(3)).
nz_size The size of the data represented by this symbol.
A hash table is included within the text segment of shared
object to facilitate quick lookup of symbols during run-time link
editing. The
sdt_hash field of the section_dispatch_table structure
points at an array
of rrs_hash structures:: struct rrs_hash {
int rh_symbolnum; /* symbol num

ber */
int rh_next; /* next hash

entry */; };
rh_symbolnum The index of the symbol in the shared object's
symbol table: (as given by the ld_symbols field).
rh_next In case of collisions, this field is the off
set of the next: entry in this hash table bucket. It is zero; for the last
bucket element.
The 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 dd_cc field in the
so_debug
structure (see below) for use by debuggers.: 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;; };
rt_sp The symbol description.
rt_next Virtual address of next rt_symbol.
rt_link Next in hash bucket. Used internally by ld.so.
rt_srcaddr Location of the source of initialized data with
in a shared: object.
rt_smp The shared object which is the original source
of the data: that this run-time symbol describes.
The so_debug structure is used by debuggers to gain knowl
edge of any shared objects that have been loaded in the process's ad
dress 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-edi
tor has been called from crt0. A dynamically linked binary contains a
so_debug structure which can be located by means of the d_debug field in
_dynamic.: 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;; };
dd_version Version number of this interface.
dd_in_debugger Set by the debugger to indicate to the run
time linker: that the program is run under control of a; debugger.
dd_sym_loaded Set by the run-time linker whenever it adds
symbols by: loading shared objects.
dd_bpt_addr The address where a breakpoint will be set
by the run: time linker to divert control to the debug; ger. This
address is determined by the start-up mod; ule, crt0.o, to
be some convenient place before the call to; _main.
dd_bpt_shadow Contains the original instruction that was
at: dd_bpt_addr. The debugger is expected to; put this
instruction back before continuing the pro; gram.
dd_cc A pointer to the linked list of run-time al
located sym: bols that the debugger may be interested in.
The ld_entry structure defines a set of service routines
within ld.so.: struct ld_entry {
void *(*dlopen)(char *, int);
int (*dlclose)(void *);
void *(*dlsym)(void *, char *);
char *(*dlerror)(void);; };
The crt_ldso structure defines the interface between the
start-up code in
crt0 and ld.so.: 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
crt_ba The virtual address at which ld.so was loaded by
crt0.
crt_dzfd On SunOS systems, this field contains an open file
descriptor: to ``/dev/zero'' used to get demand paged zeroed; pages. On
FreeBSD systems it contains -1.
crt_ldfd Contains an open file descriptor that was used by
crt0 to load: ld.so.
crt_dp A pointer to main's _dynamic structure.
crt_ep A pointer to the environment strings.
crt_bp The address at which a breakpoint will be placed
by the run: time linker if the main program is run by a debug; ger. See
so_debug
crt_prog The name of the main program as determined by crt0
(CRT_VER: SION_BSD3 only).
crt_ldso The path of the run-time linker as mapped by crt0
(CRT_VER: SION_BSD4 only).
The hints_header and hints_bucket structures define the lay
out of the library hints, normally found in ``/var/run/ld.so.hints'',
which is used
by ld.so to quickly locate the shared object images in the
file system. The organization of the hints file is not unlike that of an
``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.: 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;; };
hh_magic Hints file magic number.
hh_version Interface version number.
hh_hashtab Offset of hash table.
hh_strtab Offset of string table.
hh_strtab_sz Size of strings.
hh_ehints Maximum usable offset in hints file.: /*
* 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;; };
hi_namex Index of the string identifying the library.
hi_pathx Index of the string representing the full path
name of the: library.
hi_dewey The version numbers of the shared library.
hi_ndewey The number of valid entries in hi_dewey.
hi_next Next bucket in case of hashing collisions.

CAVEATS

Only the (GNU) C compiler currently supports the creation of
shared libraries. Other programming languages cannot be used.
BSD October 23, 1993

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In section

See also

link(5)

NAME

SYNOPSIS

DESCRIPTION

DATA STRUCTURES

CAVEATS