link - dynamic loader and link editor interface
#include <link.h>
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 runtime,
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.
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<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'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 runtime 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 runtime
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 runtime
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.
Only the (GNU) C compiler currently supports the creation of
shared libraries.
Other programming languages can not be used.
OpenBSD 3.6 October 23, 1993
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