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@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. 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.
@end vtable
The @code{dlinfo} 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
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