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NAME

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

LIBRARY

       Dynamic linking library (libdl, -ldl)

SYNOPSIS

       #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);

DESCRIPTION

   dlopen()
       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 ld.so(8) for further details):

       •  (ELF  only)  If  the  calling  object  (i.e.,  the shared library or executable from which dlopen() is
          called) contains a DT_RPATH tag, and does not contain a DT_RUNPATH tag, then the directories listed in
          the DT_RPATH tag are searched.

       •  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 programs.)

       •  (ELF  only)  If  the calling object contains a DT_RUNPATH tag, then the directories listed in that tag
          are searched.

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

       •  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:

       RTLD_LAZY
              Perform  lazy binding.  Resolve symbols only 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.

       RTLD_NOW
              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:

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

       RTLD_LOCAL
              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  and  global
              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 loaded.

       If  filename  is  NULL,  then  the returned handle is for the main program.  When given to dlsym(3), 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 RTLD_GLOBAL.

       Symbol  references in the shared object are resolved using (in order): symbols in the link map of objects
       loaded for the main program and its dependencies; symbols in shared objects (and their dependencies) that
       were  previously  opened  with  dlopen() using the RTLD_GLOBAL flag; and definitions in the shared object
       itself (and any dependencies that were loaded for that object).

       Any global symbols in the executable that were placed into its dynamic symbol table by ld(1) can also  be
       used  to  resolve references in a dynamically loaded shared object.  Symbols may be placed in the dynamic
       symbol table either because the executable was  linked  with  the  flag  "-rdynamic"  (or,  synonymously,
       "--export-dynamic"),  which  causes  all  of  the executable's global symbols to be placed in the dynamic
       symbol table, or because ld(1) noted a dependency on a symbol in another object during static linking.

       If the same shared object is opened 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.
       Constructors  (see  below) are called only when the object is actually loaded into memory (i.e., when the
       reference count increases to 1).

       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.  Similarly, an object that was previously opened with
       RTLD_LOCAL can be promoted to RTLD_GLOBAL in a subsequent dlopen().

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

   dlmopen()
       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:

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

       LM_ID_NEWLM
              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 LM_ID_BASE.

   dlclose()
       The function dlclose() decrements the reference count on the dynamically loaded shared object referred to
       by handle.

       If  the  object's  reference  count  drops  to  zero  and no symbols in this object are required by other
       objects, then the object is unloaded  after  first  calling  any  destructors  defined  for  the  object.
       (Symbols  in  this  object  might  be  required in another object because this object was opened with the
       RTLD_GLOBAL flag and one of its symbols satisfied a relocation in another object.)

       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.

RETURN VALUE

       On  success, dlopen() and dlmopen() return a non-NULL handle for the loaded object.  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).

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │dlopen(), dlmopen(), dlclose()                                                │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       dlopen()
       dlclose()
              POSIX.1-2008.

       dlmopen()
       RTLD_NOLOAD
       RTLD_NODELETE
              GNU.

       RTLD_DEEPBIND
              Solaris.

HISTORY

       dlopen()
       dlclose()
              glibc 2.0.  POSIX.1-2001.

       dlmopen()
              glibc 2.3.4.

NOTES

   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
       dependencies.

       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 namespaces.

       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.

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

BUGS

       As  at  glibc  2.24,  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 namespace.

EXAMPLES

       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
           -0.416147

   Program source

       #include <dlfcn.h>
       #include <stdio.h>
       #include <stdlib.h>

       #include <gnu/lib-names.h>  /* Defines LIBM_SO (which will be a
                                      string such as "libm.so.6") */
       int
       main(void)
       {
           void *handle;
           double (*cosine)(double);
           char *error;

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

           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.1-2001 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 1 to POSIX.1-2008 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);
               exit(EXIT_FAILURE);
           }

           printf("%f\n", (*cosine)(2.0));
           dlclose(handle);
           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

       gcc info pages, ld info pages