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LINK(5) UNIX Programmer's Manual LINK(5) NNAAMMEE lliinnkk - dynamic loader and link editor interface SSYYNNOOPPSSIISS ##iinncclluuddee <<lliinnkk..hh>> DDEESSCCRRIIPPTTIIOONN The include file <_l_i_n_k_._h> declares several structures that are present in dynamically linked programs and libraries. The structures define the in- terface between several components of the link-editor and loader mecha- nism. The layout of a number of these structures within the binaries re- sembles the a.out format in many places as it serves such similar func- tions 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 _P_o_s_i_t_i_o_n _I_n_d_e_p_e_n_d_e_n_t _C_o_d_e (PIC for short) to improve sharing of code pages among different processes. The collection of data struc- tures described here will be refered to as the _R_u_n_-_t_i_m_e _R_e_l_o_c_a_t_i_o_n _S_e_c_t_i_o_n _(_R_R_S_) and is embedded in the standard text and data segments of the dynamically linked program or shared object image as the existing a.out 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 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 ac- cessable by the reserved name ___G_L_O_B_A_L___O_F_F_S_E_T___T_A_B_L_E__. The exact mechanism used for this is machine dependent, usually a machine register is re- served for the purpose. The rational behind this construct is to generate code that is independent of the actual load address. Only the values con- tained 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 im- age's text and data bits. Another reserved symbol, ___D_Y_N_A_M_I_C is used to indicate the presence of the run-time linker structures. Whenever _DYNAM- IC is relocated to 0, there is no need to invoke the run-time link-edi- tor. 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, _c_r_t_0. The _DY- NAMIC structure is conventionally located at the start of the data seg- ment of the image to which it pertains. DDAATTAA SSTTRRUUCCTTUURREESS 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 modi- fied by during the relocation process. The _DYNAMIC symbol references a ___d_y_n_a_m_i_c 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___v_e_r_s_i_o_n This field provides for different versions of the dynamic linking implementation. The current version numbers understood by ld and ld.so are _L_D___V_E_R_S_I_O_N___S_U_N _(_3_)_, which is used by the SunOS 4.x releases, and _L_D___V_E_R_S_I_O_N___B_S_D _(_8_)_, which is currently in use by NetBSD. _d___u_n Refers to a _d___v_e_r_s_i_o_n dependent data structure. _d___d_e_b_u_g this field provides debuggers with a hook to access symbol ta- bles of shared objects loaded as a result of the actions of the run-time link-editor. _d___e_n_t_r_y this field is obsoleted by CRT interface version CRT_VER- SION_BSD4, and is replaced by the crt_ldentry in _c_r_t___l_d_s_o. The _s_e_c_t_i_o_n___d_i_s_p_a_t_c_h___t_a_b_l_e structure is the main ``dispatcher'' table, containing offsets into the image's segments where various symbol and re- location 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; }; _s_d_t___l_o_a_d_e_d A pointer to the first link map loaded (see below). This field is set by ld.so for the benefit of debuggers that may use it to load a shared object's symbol table. _s_d_t___s_o_d_s The start of a (linked) list of shared object descriptors needed by _t_h_i_s object. _s_d_t___p_a_t_h_s Library search rules. A colon separated list of directories corresponding to the --RR option of ld(1). _s_d_t___g_o_t The location of the Global Offset Table within this image. _s_d_t___p_l_t The location of the Procedure Linkage Table within this im- age. _s_d_t___r_e_l The location of an array of _r_e_l_o_c_a_t_i_o_n___i_n_f_o structures (see a.out(5)) specifying run-time relocations. _s_d_t___h_a_s_h The location of the hash table for fast symbol lookup in this object's symbol table. _s_d_t___n_z_l_i_s_t The location of the symbol table. _s_d_t___f_i_l_l_e_r_2 Currently unused. _s_d_t___b_u_c_k_e_t_s The number of buckets in _s_d_t___h_a_s_h _s_d_t___s_t_r_i_n_g_s The location of the symbol string table that goes with _s_d_t___n_z_l_i_s_t. _s_d_t___s_t_r___s_z The size of the string table. _s_d_t___t_e_x_t___s_z The size of the object's text segment. _s_d_t___p_l_t___s_z The size of the Procedure Linkage Table. A _s_o_d 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 _s_o_d___n_e_x_t) is pointed at by the _s_d_t___s_o_d_s in the sec- tion_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; }; _s_o_d___n_a_m_e The offset in the text segment of a string describing this link object. _s_o_d___l_i_b_r_a_r_y If set, _s_o_d___n_a_m_e 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 _l_i_b_<_s_o_d___n_a_m_e_>_._s_o_._n_._m. If not set, _s_o_d___n_a_m_e should point at a full path name for the desired shared object. _s_o_d___m_a_j_o_r Specifies the major version number of the shared object to load. _s_o_d___m_i_n_o_r Specifies the prefered minor version number of the shared object to load. The run-time link-editor maintains a list of structures called _l_i_n_k _m_a_p_s 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; }; _s_o_m___a_d_d_r The address at which the shared object associated with this link map has been loaded. _s_o_m___p_a_t_h The full path name of the loaded object. _s_o_m___n_e_x_t Pointer to the next link map. _s_o_m___s_o_d The _s_o_d structure that was responsible for loading this shared object. _s_o_m___s_o_d_b_a_s_e Tossed in later versions the run-time linker. _s_o_m___w_r_i_t_e Set if (some portion of) this object's text segment is cur- rently writable. _s_o_m___d_y_n_a_m_i_c Pointer to this object's ___d_y_n_a_m_i_c structure. _s_o_m___s_p_d Hook for attaching private data maintained by the run-time link-editor. Symbol description with size. This is simply an _n_l_i_s_t structure with one field (_n_z___s_i_z_e) added. Used to convey size information on items in the data segment of shared objects. An array of these lives in the shared ob- ject's text segment and is addressed by the _s_d_t___n_z_l_i_s_t field of _s_e_c_t_i_o_n___d_i_s_p_a_t_c_h___t_a_b_l_e. 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 }; _n_l_i_s_t (see nlist(5)). _n_z___s_i_z_e 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 _s_d_t___h_a_s_h field of the _s_e_c_t_i_o_n___d_i_s_p_a_t_c_h___t_a_b_l_e structure points at an array of _r_r_s___h_a_s_h structures: struct rrs_hash { int rh_symbolnum; /* symbol number */ int rh_next; /* next hash entry */ }; _r_h___s_y_m_b_o_l_n_u_m The index of the symbol in the shared object's symbol table (as given by the _l_d___s_y_m_b_o_l_s field). _r_h___n_e_x_t 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 _r_t___s_y_m_b_o_l structure is used to keep track of run-time allocated com- mons and data items copied from shared objects. These items are kept on linked list and is exported through the _d_d___c_c field in the _s_o___d_e_b_u_g 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; }; _r_t___s_p The symbol description. _r_t___n_e_x_t Virtual address of next rt_symbol. _r_t___l_i_n_k Next in hash bucket. Used by internally by ld.so. _r_t___s_r_c_a_d_d_r Location of the source of initialized data within a shared object. _r_t___s_m_p The shared object which is the original source of the data that this run-time symbol describes. The _s_o___d_e_b_u_g 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 _s_o___d_e_b_u_g structure which can be located by means of the _d___d_e_b_u_g field in ___d_y_n_a_m_i_c. 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; }; _d_d___v_e_r_s_i_o_n Version number of this interface. _d_d___i_n___d_e_b_u_g_g_e_r Set by the debugger to indicate to the run-time linker that the program is run under control of a debugger. _d_d___s_y_m___l_o_a_d_e_d Set by the run-time linker whenever it adds symbols by loading shared objects. _d_d___b_p_t___a_d_d_r The address were a breakpoint will be set by the the run- time linker to divert control to the debugger. This ad- dress is determined by the start-up module, _c_r_t_0_._o_, to be some convenient place before the call to _main. _d_d___b_p_t___s_h_a_d_o_w Contains the original instruction that was at _d_d___b_p_t___a_d_d_r. The debugger is expected to put this in- struction back before continuing the program. _d_d___c_c A pointer to the linked list of run-time allocated sym- bols that the debugger may be interested in. The _l_d___e_n_t_r_y 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) __P((void)); }; The _c_r_t___l_d_s_o 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 _c_r_t___b_a The virtual address at which ld.so was loaded by crt0. _c_r_t___d_z_f_d 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. _c_r_t___l_d_f_d Contains an open file descriptor that was used by crt0 to load ld.so. _c_r_t___d_p A pointer to main's ___d_y_n_a_m_i_c structure. _c_r_t___e_p A pointer to the environment strings. _c_r_t___b_p The address at which a breakpoint will be placed by the run- time linker if the main program is run by a debugger. See _s_o___d_e_b_u_g _c_r_t___p_r_o_g The name of the main program as determined by crt0 (CRT_VER- SION_BSD3 only). _c_r_t___l_d_s_o The path of the run-time linker as mapped by crt0 (CRT_VER- SION_BSD4 only). _c_r_t___l_d_e_n_t_r_y The dlfcn(3) entry points provided by the run-time linker (CRT_VERSION_BSD4 only). The _h_i_n_t_s___h_e_a_d_e_r and _h_i_n_t_s___b_u_c_k_e_t structures define the layout of the li- brary hints, normally found in ``/var/run/ld.so.hints,'' 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 ``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; }; _h_h___m_a_g_i_c Hints file magic number. _h_h___v_e_r_s_i_o_n Interface version number. _h_h___h_a_s_h_t_a_b Offset of hash table. _h_h___s_t_r_t_a_b Offset of string table. _h_h___s_t_r_t_a_b___s_z Size of strings. _h_h___e_h_i_n_t_s 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; }; _h_i___n_a_m_e_x Index of the string identifying the library. _h_i___p_a_t_h_x Index of the string representing the full path name of the li- brary. _h_i___d_e_w_e_y The version numbers of the shared library. _h_i___n_d_e_w_e_y The number of valid entries in _h_i___d_e_w_e_y. _h_i___n_e_x_t Next bucket in case of hashing collisions. CCAAVVEEAATTSS Only the (GNU) C compiler currently supports the creation of shared li- braries. Other programming languages can not be used. BSD Experimental October 23, 1993 7