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Re: r_debug protocol
> > 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. */
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. */
/* 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. */
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. */
/* 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
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. */
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. */
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. */