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The include
file 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 (PIC for short) to improve sharing
of code pages among different processes. The collection of data structures
described here will be refered to as the and is embedded in the standard
text and data segments of the dynamically linked program or shared object
image as the existing format offers no room for it elsewhere. Several
utilities co-operate 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 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 accessable by the reserved name 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, 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, 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 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 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;
}; This field provides for different versions of the dynamic linking
implementation. The current version numbers understood by ld and ld.so are
which is used by the SunOS 4.x releases, and which is currently in use
by NetBSD. Refers to a dependent data structure. 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. this field is obsoleted by CRT
interface version CRT_VERSION_BSD4, and is replaced by the crt_ldentry
in The structure is the main 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;
}; A pointer to the first link map loaded (see below). This field is
set by for the benefit of debuggers that may use it to load a shared object’s
symbol table. The start of a (linked) list of shared object descriptors
needed by object. Library search rules. A colon separated list of directories
corresponding to the option of The location of the Global Offset Table
within this image. The location of the Procedure Linkage Table within this
image. The location of an array of structures see specifying run-time
relocations. The location of the hash table for fast symbol lookup in this
object’s symbol table. The location of the symbol table. Currently unused.
The number of buckets in The location of the symbol string table that
goes with The size of the string table. The size of the object’s text
segment. The size of the Procedure Linkage Table. A structure descibes
a shared object that is needed to complete the link edit process of the
object containing it. A list of such objects chained through is pointed
at by the 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;
}; The offset in the text segment of a string describing this link
object. If set, 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
for a shared object matching If not set, should point at a full path
name for the desired shared object. Specifies the major version number
of the shared object to load. Specifies the prefered minor version number
of the shared object to load. The run-time link-editor maintains a list
of structures called 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;
}; The address at which the shared object associated with this link
map has been loaded. The full path name of the loaded object. Pointer to
the next link map. The structure that was responsible for loading this
shared object. Tossed in later versions the run-time linker. Set if (some
portion of) this object’s text segment is currently writable. Pointer to
this object’s structure. Hook for attaching private data maintained by
the run-time link-editor. Symbol description with size. This is simply an
structure with one field 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 field of 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 }; see The size of the data represented
by this symbol. 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 field of the structure points at an array of structures: struct
rrs_hash { int rh_symbolnum; /* symbol number */
int rh_next; /* next hash entry */
}; The index of the symbol in the shared object’s symbol table (as
given by the field). 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 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 field in the 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;
}; The symbol description. Virtual address of next rt_symbol. Next
in hash bucket. Used by internally by ld.so. Location of the source of initialized
data within a shared object. The shared object which is the original source
of the data that this run-time symbol describes. The 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 structure which can be located by means of the field in 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;
}; Version number of this interface. Set by the debugger to indicate
to the run-time linker that the program is run under control of a debugger.
Set by the run-time linker whenever it adds symbols by loading shared objects.
The address were a breakpoint will be set by the the run-time linker to
divert control to the debugger. This address is determined by the start-up
module, to be some convenient place before the call to _main. Contains
the original instruction that was at The debugger is expected to put this
instruction back before continuing the program. A pointer to the linked
list of run-time allocated symbols that the debugger may be interested in.
The structure defines a set of service routines within ld.so. See for
more information. struct ld_entry { void *(*dlopen)(char *, int);
int (*dlclose)(void *);
void *(*dlsym)(void *, char *);
int (*dlctl)(void *, int, void *);
void (*dlexit) __P((void));
};
The 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;
caddr_t 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 The virtual address at which ld.so was loaded
by crt0. On SunOS systems, this field contains an open file descriptor
to used to get demand paged zeroed pages. On NetBSD systems it contains
-1. Contains an open file descriptor that was used by crt0 to load ld.so.
A pointer to main’s structure. A pointer to the environment strings. The
address at which a breakpoint will be placed by the run-time linker if the
main program is run by a debugger. See The name of the main program as
determined by crt0 (CRT_VERSION_BSD3 only). The path of the run-time linker
as mapped by crt0 (CRT_VERSION_BSD4 only). The entry points provided by
the run-time linker (CRT_VERSION_BSD4 only). The and structures define
the layout of the library hints, normally found in which is used by ld.so
to quickly locate the shared object images in the filesystem. The organization
of the hints file is not unlike that of an 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;
}; Hints file magic number. Interface version number. Offset of hash
table. Offset of string table. Size of strings. 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;
}; Index of the string identifying the library. Index of the string
representing the full path name of the library. The version numbers of
the shared library. The number of valid entries in 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.
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