LINK(5)

NAME
link - dynamic loader and link editor interface

SYNOPSIS
#include <sys/types.h>
#include <nlist.h>
#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 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 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(1) and ld.so(1) are LD_VERSION_SUN (3), which is used by the SunOS 4.x releases, and LD_VERSION_BSD (8), which has been in use since FreeBSD 1.1.

d_un
    Refers to a d_version dependent data structure.

so_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.

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_filler1;
    	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

sdt_sods
    The start of a (linked) list of shared object descriptors needed by this object.

sdt_filler1
    Deprecated (used by SunOS to specify library search rules).

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. 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 {
    	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 out 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 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;
    	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.

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.

    struct ld_entry {
    	void	*(*dlopen)(char *, int);
    	int	(*dlclose)(void *);
    	void	*(*dlsym)(void *, char *);
    	char	*(*dlerror)(void);
    };

The crt_ldso structure defines the interface between the start-up code in crt0 and ld.so.

    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_BSD_2	2
    #define CRT_VERSION_BSD_3	3
    #define	CRT_VERSION_BSD_4	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 FreeBSD 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).

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” 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;
    };

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.

    /*
     * 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 cannot be used.