Re: r_debug protocol

[Jim Blandy <jimb at cygnus dot com>]

> > Hello!
> > 
> > Where can I find some information about r_debug protocol
> > (link.h, struct link_map) concerning shared library support?

As far as I know, there is no real documentation for this.  My
understanding of it comes from reading the GNU libc sources.  Look in
the `elf' directory; link.h has plenty of comments.

Do you have a more specific question?


/* Rendezvous structure used by the run-time dynamic linker to communicate
   details of shared object loading to the debugger.  If the executable's
   dynamic section has a DT_DEBUG element, the run-time linker sets that
   element's value to the address where this structure can be found.  */

struct r_debug
  {
    int r_version;		/* Version number for this protocol.  */

    struct link_map *r_map;	/* Head of the chain of loaded objects.  */

    /* This is the address of a function internal to the run-time linker,
       that will always be called when the linker begins to map in a
       library or unmap it, and again when the mapping change is complete.
       The debugger can set a breakpoint at this address if it wants to
       notice shared object mapping changes.  */
    ElfW(Addr) r_brk;
    enum
      {
	/* This state value describes the mapping change taking place when
	   the `r_brk' address is called.  */
	RT_CONSISTENT,		/* Mapping change is complete.  */
	RT_ADD,			/* Beginning to add a new object.  */
	RT_DELETE,		/* Beginning to remove an object mapping.  */
      } r_state;

    ElfW(Addr) r_ldbase;	/* Base address the linker is loaded at.  */
  };

/* This is the instance of that structure used by the dynamic linker.  */
extern struct r_debug _r_debug;

/* This symbol refers to the "dynamic structure" in the `.dynamic' section
   of whatever module refers to `_DYNAMIC'.  So, to find its own
   `struct r_debug', a program could do:
     for (dyn = _DYNAMIC; dyn->d_tag != DT_NULL)
       if (dyn->d_tag == DT_DEBUG) r_debug = (struct r_debug) dyn->d_un.d_ptr;
   */

extern ElfW(Dyn) _DYNAMIC[];


/* Structure describing a loaded shared object.  The `l_next' and `l_prev'
   members form a chain of all the shared objects loaded at startup.

   These data structures exist in space used by the run-time dynamic linker;
   modifying them may have disastrous results.  */

struct link_map
  {
    /* These first few members are part of the protocol with the debugger.
       This is the same format used in SVR4.  */

    ElfW(Addr) l_addr;		/* Base address shared object is loaded at.  */
    char *l_name;		/* Absolute file name object was found in.  */
    ElfW(Dyn) *l_ld;		/* Dynamic section of the shared object.  */
    struct link_map *l_next, *l_prev; /* Chain of loaded objects.  */

    /* All following members are internal to the dynamic linker.
       They may change without notice.  */

    const char *l_libname;	/* Name requested (before search).  */
    /* Indexed pointers to dynamic section.
       [0,DT_NUM) are indexed by the processor-independent tags.
       [DT_NUM,DT_NUM+DT_PROCNUM) are indexed by the tag minus DT_LOPROC.
       [DT_NUM+DT_PROCNUM,DT_NUM+DT_PROCNUM+DT_EXTRANUM) are indexed
       by DT_EXTRATAGIDX(tagvalue) and
       [DT_NUM+DT_PROCNUM+DT_VERSIONTAGNUM,
        DT_NUM+DT_PROCNUM+DT_VERSIONTAGNUM+DT_EXTRANUM)
       are indexed by DT_EXTRATAGIDX(tagvalue) (see <elf.h>).  */

    ElfW(Dyn) *l_info[DT_NUM + DT_PROCNUM + DT_VERSIONTAGNUM + DT_EXTRANUM];
    const ElfW(Phdr) *l_phdr;	/* Pointer to program header table in core.  */
    ElfW(Addr) l_entry;		/* Entry point location.  */
    ElfW(Half) l_phnum;		/* Number of program header entries.  */

    /* Array of DT_NEEDED dependencies and their dependencies, in
       dependency order for symbol lookup.  This is null before the
       dependencies have been loaded.  */
    struct link_map **l_searchlist;
    unsigned int l_nsearchlist;

    /* We keep another list in which we keep duplicates.  This is
       needed in _dl_lookup_symbol_skip to implemented RTLD_NEXT.  */
    struct link_map **l_dupsearchlist;
    unsigned int l_ndupsearchlist;

    /* Dependent object that first caused this object to be loaded.  */
    struct link_map *l_loader;

    /* Symbol hash table.  */
    ElfW(Symndx) l_nbuckets;
    const ElfW(Symndx) *l_buckets, *l_chain;

    unsigned int l_opencount;	/* Reference count for dlopen/dlclose.  */
    enum			/* Where this object came from.  */
      {
	lt_executable,		/* The main executable program.  */
	lt_library,		/* Library needed by main executable.  */
	lt_loaded,		/* Extra run-time loaded shared object.  */
      } l_type:2;
    unsigned int l_relocated:1;	/* Nonzero if object's relocations done.  */
    unsigned int l_init_called:1; /* Nonzero if DT_INIT function called.  */
    unsigned int l_init_running:1; /* Nonzero while DT_INIT function runs.  */
    unsigned int l_global:1;	/* Nonzero if object in _dl_global_scope.  */
    unsigned int l_reserved:2;	/* Reserved for internal use.  */
  };