NAME
link —
dynamic loader and link editor
interface
SYNOPSIS
#include <link.h>
DESCRIPTION
The include file
<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(5) 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) to improve sharing of code
pages among different processes.
The collection of data structures described here will be referred to as the
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
a.out(5) format offers no room
for it elsewhere.
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
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
_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.
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.
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,
_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,
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 descriptions 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 numbers understood by ld and
ld.so are LD_VERSION_SUN (3), which is used by the SunOS
4.x releases, and LD_VERSION_BSD (8), which is currently
in use by NetBSD.
-
-
- d_un
- Refers to a d_version dependent data
structure.
-
-
- d_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.
-
-
- d_entry
- this field is obsoleted by CRT interface version
CRT_VERSION_BSD4, and is replaced by the crt_ldentry in
crt_ldso.
The
section_dispatch_table structure is the main
“dispatcher” 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_paths;
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 below). This
field is set by ld.so(1) for
the benefit of debuggers that may use it to load a shared object's symbol
table.
-
-
- sdt_sods
- The start of a (linked) list of shared object descriptors
needed by this object.
-
-
- sdt_paths
- Library search rules. A colon separated list of directories
corresponding to the -R option of
ld(1).
-
-
- 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_unused : 31;
short sod_major;
short sod_minor;
long sod_next;
};
-
-
- sod_name
- The offset in the text segment of a string describing 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 {
void *som_addr;
char *som_path;
struct so_map *som_next;
struct sod *som_sod;
void *som_sodbase;
u_int som_write : 1;
struct _dynamic *som_dynamic;
void *som_spd;
};
-
-
- som_addr
- The address at which the shared object associated 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 in later versions the run-time linker.
-
-
- som_write
- Set if (some portion of) this object's text segment is
currently 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 number */
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 offset 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;
void *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 by internally by ld.so.
-
-
- rt_srcaddr
- Location of the source of initialized data within 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
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
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 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, 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 program.
-
-
- dd_cc
- A pointer to the linked list of run-time allocated symbols
that the debugger may be interested in.
The
ld_entry structure defines a set of service routines
within ld.so. See
dlfcn(3) for
more information.
struct ld_entry {
void *(*dlopen)(char *, int);
int (*dlclose)(void *);
void *(*dlsym)(void *, char *);
int (*dlctl)(void *, int, void *);
void (*dlexit)(void);
};
The
crt_ldso structure defines the interface between ld.so
and the start-up code in crt0.
struct crt_ldso {
int crt_ba;
int crt_dzfd;
int crt_ldfd;
struct _dynamic *crt_dp;
char **crt_ep;
void *crt_bp;
char *crt_prog;
char *crt_ldso;
char *crt_ldentry;
};
#define CRT_VERSION_SUN 1
#define CRT_VERSION_BSD2 2
#define CRT_VERSION_BSD3 3
#define CRT_VERSION_BSD4 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 NetBSD 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 debugger. See
so_debug
-
-
- crt_prog
- The name of the main program as determined by crt0
(CRT_VERSION_BSD3 only).
-
-
- crt_ldso
- The path of the run-time linker as mapped by crt0
(CRT_VERSION_BSD4 only).
-
-
- crt_ldentry
- The dlfcn(3)
entry points provided by the run-time linker (CRT_VERSION_BSD4 only).
The
hints_header and
hints_bucket
structures define the layout 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(5) 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
#define LD_HINTS_VERSION_2 2
long hh_hashtab;
long hh_nbucket;
long hh_strtab;
long hh_strtab_sz;
long hh_ehints;
long hh_dirlist;
};
-
-
- 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.
-
-
- hh_dirlist
- Offset in string table of a colon-separated list of
directories that was used in constructing the hints file. See also
ldconfig(8). This field is
only available with interface version number
LD_HINTS_VERSION_2
and higher.
/*
* 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.