| LINK(5) |
AerieBSD 1.0 Refernce Manual |
LINK(5) |
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 for short) 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 symbol descriptions and
names, while the data segments contain the tables that need to be modified
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(1)
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
OpenBSD.
- 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 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_reserved : 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(1).
The path name
is obtained by searching a set of directories
(
see also
ldconfig(8)
)
for a shared object matching
lib.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's 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 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 of 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 objects 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 in a linked list and are 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(1).
- 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 where 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(1).
See
dlfcn(3)
for more information.
struct ld_entry {
void *(*dlopen)(const char *, int);
int (*dlclose)(void *);
void *(*dlsym)(void *, const char *);
int (*dlctl)(void *, int, void *);
void (*dlexit)(void);
void (*dlrsrvd[3])(void);
};
The
crt_ldso
structure defines the interface between
ld.so(1)
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;
caddr_t crt_bp;
char *crt_prog;
char *crt_ldso;
struct ld_entry *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(1)
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
OpenBSD
systems it contains \-1.
- crt_ldfd
-
Contains an open file descriptor that was used by
crt0
to load
ld.so(1).
- 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(1)
to quickly locate the shared object images in the filesystem.
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.
CAVEATS
Only the (GNU) C compiler currently supports the creation of shared libraries.
Other programming languages can not be used.
| AerieBSD 1.0 Reference Manual |
August 26 2008 |
LINK(5) |