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       dlclose, dlopen, dlmopen - open and close a shared object


       #include <dlfcn.h>

       void *dlopen(const char *filename, int flags);

       int dlclose(void *handle);

       #define _GNU_SOURCE
       #include <dlfcn.h>

       void *dlmopen (Lmid_t lmid, const char *filename, int flags);

       Link with -ldl.


       The  function dlopen() loads the dynamic shared object (shared library)
       file named by the null-terminated string filename and returns an opaque
       "handle"  for  the  loaded  object.  This handle is employed with other
       functions in the dlopen API, such as  dlsym(3),  dladdr(3),  dlinfo(3),
       and dlclose().

       If  filename is NULL, then the returned handle is for the main program.
       If filename contains a  slash  ("/"),  then  it  is  interpreted  as  a
       (relative   or  absolute)  pathname.   Otherwise,  the  dynamic  linker
       searches for the object as follows (see for further details):

       o   (ELF only) If the executable file for the calling program  contains
           a  DT_RPATH  tag,  and  does not contain a DT_RUNPATH tag, then the
           directories listed in the DT_RPATH tag are searched.

       o   If, at the time that  the  program  was  started,  the  environment
           variable  LD_LIBRARY_PATH  was defined to contain a colon-separated
           list of directories, then  these  are  searched.   (As  a  security
           measure,  this variable is ignored for set-user-ID and set-group-ID

       o   (ELF only) If the executable file for the calling program  contains
           a  DT_RUNPATH  tag,  then  the  directories  listed in that tag are

       o   The cache file  /etc/  (maintained  by  ldconfig(8))  is
           checked to see whether it contains an entry for filename.

       o   The directories /lib and /usr/lib are searched (in that order).

       If  the  object  specified by filename has dependencies on other shared
       objects, then these are also automatically loaded by the dynamic linker
       using  the  same  rules.  (This process may occur recursively, if those
       objects in turn have dependencies, and so on.)

       One of the following two values must be included in flags:

              Perform lazy binding.  Only resolve symbols  as  the  code  that
              references them is executed.  If the symbol is never referenced,
              then it is never resolved.  (Lazy binding is performed only  for
              function   references;   references   to  variables  are  always
              immediately bound when the  shared  object  is  loaded.)   Since
              glibc  2.1.1,  this  flag  is  overridden  by  the effect of the
              LD_BIND_NOW environment variable.

              If  this  value  is  specified,  or  the  environment   variable
              LD_BIND_NOW  is  set to a nonempty string, all undefined symbols
              in the shared object are resolved before dlopen()  returns.   If
              this cannot be done, an error is returned.

       Zero or more of the following values may also be ORed in flags:

              The symbols defined by this shared object will be made available
              for symbol resolution of subsequently loaded shared objects.

              This is the converse of RTLD_GLOBAL, and the default if  neither
              flag  is  specified.   Symbols defined in this shared object are
              not made available to resolve references in subsequently  loaded
              shared objects.

       RTLD_NODELETE (since glibc 2.2)
              Do not unload the shared object during dlclose().  Consequently,
              the object's static  variables  are  not  reinitialized  if  the
              object is reloaded with dlopen() at a later time.

       RTLD_NOLOAD (since glibc 2.2)
              Don't  load  the shared object.  This can be used to test if the
              object is already resident (dlopen() returns NULL if it is  not,
              or  the  object's handle if it is resident).  This flag can also
              be used to promote the flags on a shared object that is  already
              loaded.  For example, a shared object that was previously loaded
              with RTLD_LOCAL can be reopened with RTLD_NOLOAD | RTLD_GLOBAL.

       RTLD_DEEPBIND (since glibc 2.3.4)
              Place the lookup scope of the  symbols  in  this  shared  object
              ahead  of  the  global  scope.  This means that a self-contained
              object will use its own symbols in preference to global  symbols
              with  the  same name contained in objects that have already been

       If filename is NULL, then the returned handle is for the main  program.
       When  given to dlsym(), this handle causes a search for a symbol in the
       main program, followed by all shared objects loaded at program startup,
       and   then  all  shared  objects  loaded  by  dlopen()  with  the  flag

       External references in the shared object are resolved using the  shared
       objects  in  that  object's  dependency  list  and  any  other  objects
       previously opened with the RTLD_GLOBAL flag.   If  the  executable  was
       linked     with     the    flag    "-rdynamic"    (or,    synonymously,
       "--export-dynamic"), then the global symbols  in  the  executable  will
       also  be  used  to  resolve  references  in a dynamically loaded shared

       If the same shared object is  loaded  again  with  dlopen(),  the  same
       object  handle  is  returned.   The  dynamic linker maintains reference
       counts for object handles, so a dynamically loaded shared object is not
       deallocated  until  dlclose()  has  been  called on it as many times as
       dlopen() has succeeded on it.  Any initialization returns  (see  below)
       are  called  just once.  However, a subsequent dlopen() call that loads
       the same shared object with RTLD_NOW may force symbol resolution for  a
       shared object earlier loaded with RTLD_LAZY.

       If dlopen() fails for any reason, it returns NULL.

       This function performs the same task as dlopen()—the filename and flags
       arguments, as well as the return value, are the same,  except  for  the
       differences noted below.

       The  dlmopen()  function  differs  from  dlopen()  primarily in that it
       accepts an additional argument, lmid, that specifies the link-map  list
       (also  referred to as a namespace) in which the shared object should be
       loaded.  (By comparison, dlopen() adds the  dynamically  loaded  shared
       object  to  the  same  namespace  as  the  shared object from which the
       dlopen() call is made.)  The Lmid_t  type  is  an  opaque  handle  that
       refers to a namespace.

       The  lmid argument is either the ID of an existing namespace (which can
       be obtained using the dlinfo(3) RTLD_DI_LMID request)  or  one  of  the
       following special values:

              Load  the  shared  object  in  the  initial namespace (i.e., the
              application's namespace).

              Create a new namespace  and  load  the  shared  object  in  that
              namespace.   The  object  must  have  been  correctly  linked to
              reference all of the other  shared  objects  that  it  requires,
              since the new namespace is initially empty.

       If  filename  is  NULL,  then  the  only  permitted  value  for lmid is

       The  function  dlclose()  decrements  the  reference   count   on   the
       dynamically  loaded  shared  object  referred  to  by  handle.   If the
       reference count drops to zero, then the object is unloaded.  All shared
       objects that were automatically loaded when dlopen() was invoked on the
       object referred to by handle are recursively closed in the same manner.

       A successful return from dlclose() does not guarantee that the  symbols
       associated with handle are removed from the caller's address space.  In
       addition to references resulting from explicit dlopen() calls, a shared
       object  may have been implicitly loaded (and reference counted) because
       of dependencies in other shared objects.  Only when all references have
       been released can the shared object be removed from the address space.


       On  success,  dlopen()  and  dlmopen() return a non-NULL handle for the
       loaded library.  On error (file could not be found, was  not  readable,
       had the wrong format, or caused errors during loading), these functions
       return NULL.

       On success, dlclose() returns 0; on error, it returns a nonzero value.

       Errors from these functions can be diagnosed using dlerror(3).


       dlopen() and dlclose() are present in glibc 2.0 and  later.   dlmopen()
       first appeared in glibc 2.3.4.


       For   an   explanation   of   the  terms  used  in  this  section,  see

       │InterfaceAttributeValue   │
       │dlopen(), dlmopen(), dlclose() │ Thread safety │ MT-Safe │


       POSIX.1-2001 describes dlclose() and dlopen().  The dlmopen()  function
       is a GNU extension.

       The   RTLD_NOLOAD,  RTLD_NODELETE,  and  RTLD_DEEPBIND  flags  are  GNU
       extensions; the first two of these flags are also present on Solaris.


   dlmopen() and namespaces
       A link-map list defines an isolated namespace  for  the  resolution  of
       symbols  by  the  dynamic linker.  Within a namespace, dependent shared
       objects are implicitly loaded according to the usual rules, and  symbol
       references are likewise resolved according to the usual rules, but such
       resolution is confined to the definitions provided by the objects  that
       have been (explicitly and implicitly) loaded into the namespace.

       The  dlmopen()  function  permits  object-load isolation—the ability to
       load a shared object in a new namespace without exposing  the  rest  of
       the  application to the symbols made available by the new object.  Note
       that the use of the RTLD_LOCAL flag is not sufficient for this purpose,
       since it prevents a shared object's symbols from being available to any
       other shared object.  In some cases, we may want to  make  the  symbols
       provided  by  a dynamically loaded shared object available to (a subset
       of) other shared objects without exposing those symbols to  the  entire
       application.   This  can  be achieved by using a separate namespace and
       the RTLD_GLOBAL flag.

       The dlmopen() function also can be used  to  provide  better  isolation
       than  the  RTLD_LOCAL  flag.  In particular, shared objects loaded with
       RTLD_LOCAL may be promoted to RTLD_GLOBAL if they are  dependencies  of
       another  shared  object  loaded  with RTLD_GLOBAL.  Thus, RTLD_LOCAL is
       insufficient to isolate a loaded shared object except in the (uncommon)
       case   where   one   has   explicit  control  over  all  shared  object

       Possible uses of dlmopen() are plugins where the author of the  plugin-
       loading  framework can't trust the plugin authors and does not wish any
       undefined symbols from the plugin framework to be  resolved  to  plugin
       symbols.   Another  use  is  to  load  the  same object more than once.
       Without the use of  dlmopen(),  this  would  require  the  creation  of
       distinct  copies  of the shared object file.  Using dlmopen(), this can
       be achieved by loading the  same  shared  object  file  into  different

       The glibc implementation supports a maximum of 16 namespaces.

   Initialization and finalization functions
       Shared      objects      may     export     functions     using     the
       __attribute__((constructor)) and  __attribute__((destructor))  function
       attributes.    Constructor   functions  are  executed  before  dlopen()
       returns,  and  destructor  functions  are  executed  before   dlclose()
       returns.    A  shared  object  may  export  multiple  constructors  and
       destructors, and priorities can be associated  with  each  function  to
       determine the order in which they are executed.  See the gcc info pages
       (under "Function attributes") for further information.

       An older method of (partially) achieving the same result is via the use
       of two special symbols recognized by the linker: _init and _fini.  If a
       dynamically loaded shared object exports a routine named _init(),  then
       that  code  is  executed after loading a shared object, before dlopen()
       returns.  If the shared object exports a routine  named  _fini(),  then
       that  routine  is  called  just before the object is unloaded.  In this
       case, one must avoid linking against the system  startup  files,  which
       contain  default versions of these files; this can be done by using the
       gcc(1) -nostartfiles command-line option.

       Use of _init and _fini is now deprecated in favor of the aforementioned
       constructors  and  destructors,  which  among  other advantages, permit
       multiple initialization and finalization functions to be defined.

       Since glibc 2.2.3, atexit(3) can be used to register  an  exit  handler
       that is automatically called when a shared object is unloaded.

       These functions are part of the dlopen API, derived from SunOS.


       The  program below loads the (glibc) math library, looks up the address
       of the cos(3) function, and prints the cosine of 2.0.  The following is
       an example of building and running the program:

           $ cc dlopen_demo.c -ldl
           $ ./a.out

   Program source
       #include <stdio.h>
       #include <stdlib.h>
       #include <dlfcn.h>
       #include <gnu/lib-names.h>  /* Defines LIBM_SO (which will be a
                                      string such as "") */
           void *handle;
           double (*cosine)(double);
           char *error;

           handle = dlopen(LIBM_SO, RTLD_LAZY);
           if (!handle) {
               fprintf(stderr, "%s\n", dlerror());

           dlerror();    /* Clear any existing error */

           cosine = (double (*)(double)) dlsym(handle, "cos");

           /* According to the ISO C standard, casting between function
              pointers and 'void *', as done above, produces undefined results.
              POSIX.1-2003 and POSIX.1-2008 accepted this state of affairs and
              proposed the following workaround:

                  *(void **) (&cosine) = dlsym(handle, "cos");

              This (clumsy) cast conforms with the ISO C standard and will
              avoid any compiler warnings.

              The 2013 Technical Corrigendum to POSIX.1-2008 (a.k.a.
              POSIX.1-2013) improved matters by requiring that conforming
              implementations support casting 'void *' to a function pointer.
              Nevertheless, some compilers (e.g., gcc with the '-pedantic'
              option) may complain about the cast used in this program. */

           error = dlerror();
           if (error != NULL) {
               fprintf(stderr, "%s\n", error);

           printf("%f\n", (*cosine)(2.0));


       As  at  glibc  2.21,  specifying  the  RTLD_GLOBAL  flag  when  calling
       dlmopen() generates an error.  Furthermore, specifying RTLD_GLOBAL when
       calling  dlopen()  results  in a program crash (SIGSEGV) if the call is
       made from any object loaded in  a  namespace  other  than  the  initial


       ld(1),  ldd(1),  pldd(1),  dl_iterate_phdr(3),  dladdr(3),  dlerror(3),
       dlinfo(3), dlsym(3), rtld-audit(7),, ldconfig(8)

       gcc info pages, ld info pages


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