path: root/manual/dynlink.texi

@node Dynamic Linker
@c @node Dynamic Linker, Internal Probes, Threads, Top
@c %MENU% Loading programs and shared objects.
@chapter Dynamic Linker
@cindex dynamic linker
@cindex dynamic loader

The @dfn{dynamic linker} is responsible for loading dynamically linked
programs and their dependencies (in the form of shared objects).  The
dynamic linker in @theglibc{} also supports loading shared objects (such
as plugins) later at run time.

Dynamic linkers are sometimes called @dfn{dynamic loaders}.

@menu
* Dynamic Linker Introspection::    Interfaces for querying mapping information.
@end menu

@node Dynamic Linker Introspection
@section Dynamic Linker Introspection

@Theglibc{} provides various functions for querying information from the
dynamic linker.

@deftypefun {int} dlinfo (void *@var{handle}, int @var{request}, void *@var{arg})
@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}}
@standards{GNU, dlfcn.h}
This function returns information about @var{handle} in the memory
location @var{arg}, based on @var{request}.  The @var{handle} argument
must be a pointer returned by @code{dlopen} or @code{dlmopen}; it must
not have been closed by @code{dlclose}.

On success, @code{dlinfo} returns 0 for most request types; exceptions
are noted below.  If there is an error, the function returns @math{-1},
and @code{dlerror} can be used to obtain a corresponding error message.

The following operations are defined for use with @var{request}:

@vtable @code
@item RTLD_DI_LINKMAP
The corresponding @code{struct link_map} pointer for @var{handle} is
written to @code{*@var{arg}}.  The @var{arg} argument must be the
address of an object of type @code{struct link_map *}.

@item RTLD_DI_LMID
The namespace identifier of @var{handle} is written to
@code{*@var{arg}}.  The @var{arg} argument must be the address of an
object of type @code{Lmid_t}.

@item RTLD_DI_ORIGIN
The value of the @code{$ORIGIN} dynamic string token for @var{handle} is
written to the character array starting at @var{arg} as a
null-terminated string.

This request type should not be used because it is prone to buffer
overflows.

@item RTLD_DI_SERINFO
@itemx RTLD_DI_SERINFOSIZE
These requests can be used to obtain search path information for
@var{handle}.  For both requests, @var{arg} must point to a
@code{Dl_serinfo} object.  The @code{RTLD_DI_SERINFOSIZE} request must
be made first; it updates the @code{dls_size} and @code{dls_cnt} members
of the @code{Dl_serinfo} object.  The caller should then allocate memory
to store at least @code{dls_size} bytes and pass that buffer to a
@code{RTLD_DI_SERINFO} request.  This second request fills the
@code{dls_serpath} array.  The number of array elements was returned in
the @code{dls_cnt} member in the initial @code{RTLD_DI_SERINFOSIZE}
request.  The caller is responsible for freeing the allocated buffer.

This interface is prone to buffer overflows in multi-threaded processes
because the required size can change between the
@code{RTLD_DI_SERINFOSIZE} and @code{RTLD_DI_SERINFO} requests.

@item RTLD_DI_TLS_DATA
This request writes the address of the TLS block (in the current thread)
for the shared object identified by @var{handle} to @code{*@var{arg}}.
The argument @var{arg} must be the address of an object of type
@code{void *}.  A null pointer is written if the object does not have
any associated TLS block.

@item RTLD_DI_TLS_MODID
This request writes the TLS module ID for the shared object @var{handle}
to @code{*@var{arg}}.  The argument @var{arg} must be the address of an
object of type @code{size_t}.  The module ID is zero if the object
does not have an associated TLS block.

@item RTLD_DI_PHDR
This request writes the address of the program header array to
@code{*@var{arg}}.  The argument @var{arg} must be the address of an
object of type @code{const ElfW(Phdr) *} (that is,
@code{const Elf32_Phdr *} or @code{const Elf64_Phdr *}, as appropriate
for the current architecture).  For this request, the value returned by
@code{dlinfo} is the number of program headers in the program header
array.
@end vtable

The @code{dlinfo} function is a GNU extension.
@end deftypefun

The remainder of this section documents the @code{_dl_find_object}
function and supporting types and constants.

@deftp {Data Type} {struct dl_find_object}
@standards{GNU, dlfcn.h}
This structure contains information about a main program or loaded
object.  The @code{_dl_find_object} function uses it to return
result data to the caller.

@table @code
@item unsigned long long int dlfo_flags
Currently unused and always 0.

@item void *dlfo_map_start
The start address of the inspected mapping.  This information comes from
the program header, so it follows its convention, and the address is not
necessarily page-aligned.

@item void *dlfo_map_end
The end address of the mapping.

@item struct link_map *dlfo_link_map
This member contains a pointer to the link map of the object.

@item void *dlfo_eh_frame
This member contains a pointer to the exception handling data of the
object.  See @code{DLFO_EH_SEGMENT_TYPE} below.

@end table

This structure is a GNU extension.
@end deftp

@deftypevr Macro int DLFO_STRUCT_HAS_EH_DBASE
@standards{GNU, dlfcn.h}
On most targets, this macro is defined as @code{0}.  If it is defined to
@code{1}, @code{struct dl_find_object} contains an additional member
@code{dlfo_eh_dbase} of type @code{void *}.  It is the base address for
@code{DW_EH_PE_datarel} DWARF encodings to this location.

This macro is a GNU extension.
@end deftypevr

@deftypevr Macro int DLFO_STRUCT_HAS_EH_COUNT
@standards{GNU, dlfcn.h}
On most targets, this macro is defined as @code{0}.  If it is defined to
@code{1}, @code{struct dl_find_object} contains an additional member
@code{dlfo_eh_count} of type @code{int}.  It is the number of exception
handling entries in the EH frame segment identified by the
@code{dlfo_eh_frame} member.

This macro is a GNU extension.
@end deftypevr

@deftypevr Macro int DLFO_EH_SEGMENT_TYPE
@standards{GNU, dlfcn.h}
On targets using DWARF-based exception unwinding, this macro expands to
@code{PT_GNU_EH_FRAME}.  This indicates that @code{dlfo_eh_frame} in
@code{struct dl_find_object} points to the @code{PT_GNU_EH_FRAME}
segment of the object.  On targets that use other unwinding formats, the
macro expands to the program header type for the unwinding data.

This macro is a GNU extension.
@end deftypevr

@deftypefun {int} _dl_find_object (void *@var{address}, struct dl_find_object *@var{result})
@standards{GNU, dlfcn.h}
@safety{@mtsafe{}@assafe{}@acsafe{}}
On success, this function returns 0 and writes about the object
surrounding the address to @code{*@var{result}}.  On failure, -1 is
returned.

The @var{address} can be a code address or data address.  On
architectures using function descriptors, no attempt is made to decode
the function descriptor.  Depending on how these descriptors are
implemented, @code{_dl_find_object} may return the object that defines
the function descriptor (and not the object that contains the code
implementing the function), or fail to find any object at all.

On success @var{address} is greater than or equal to
@code{@var{result}->dlfo_map_start} and less than
@code{@var{result}->dlfo_map_end}, that is, the supplied code address is
located within the reported mapping.

This function returns a pointer to the unwinding information for the
object that contains the program code @var{address} in
@code{@var{result}->dlfo_eh_frame}.  If the platform uses DWARF
unwinding information, this is the in-memory address of the
@code{PT_GNU_EH_FRAME} segment.  See @code{DLFO_EH_SEGMENT_TYPE} above.
In case @var{address} resides in an object that lacks unwinding information,
the function still returns 0, but sets @code{@var{result}->dlfo_eh_frame}
to a null pointer.

@code{_dl_find_object} itself is thread-safe.  However, if the
application invokes @code{dlclose} for the object that contains
@var{address} concurrently with @code{_dl_find_object} or after the call
returns, accessing the unwinding data for that object or the link map
(through @code{@var{result}->dlfo_link_map}) is not safe.  Therefore, the
application needs to ensure by other means (e.g., by convention) that
@var{address} remains a valid code address while the unwinding
information is processed.

This function is a GNU extension.
@end deftypefun


@c FIXME these are undocumented:
@c dladdr
@c dladdr1
@c dlclose
@c dlerror
@c dlmopen
@c dlopen
@c dlsym
@c dlvsym