dlopen

DLOPEN(3)                  Linux Programmer's Manual                 DLOPEN(3)

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

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);

       Link with -ldl.

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  (rela-
       tive or absolute) pathname.  Otherwise, the dynamic linker searches for
       the object as follows (see ld.so(8) 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 di-
           rectories listed in the DT_RPATH tag are searched.

       o   If, at the time that the program was started, the environment vari-
           able  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 pro-
           grams.)

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

       o   The cache file  /etc/ld.so.cache  (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:

       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 immedi-
              ately 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 ob-
              ject 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(), 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  dy-
       namically  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  ta-
       ble,  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  ob-
       ject handle is returned.  The dynamic linker maintains reference counts
       for object handles, so a dynamically loaded shared object  is  not  de-
       allocated  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 refer-
       ence 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 ac-
       cepts 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 ap-
              plication's namespace).

       LM_ID_NEWLM
              Create a new namespace and load the shared object in that  name-
              space.   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  dynami-
       cally 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 re-
       location in another object.)

       All shared objects that were automatically loaded when dlopen() was in-
       voked 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).

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

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(7).

       +-------------------------------+---------------+---------+
       |Interface                      | Attribute     | Value   |
       +-------------------------------+---------------+---------+
       |dlopen(), dlmopen(), dlclose() | Thread safety | MT-Safe |
       +-------------------------------+---------------+---------+
CONFORMING TO
       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 exten-
       sions; the first two of these flags are also present on Solaris.

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 in-
       sufficient to isolate a loaded shared object except in  the  (uncommon)
       case  where  one  has explicit control over all shared object dependen-
       cies.

       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.  With-
       out 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  name-
       spaces.

       The glibc implementation supports a maximum of 16 namespaces.

   Initialization and finalization functions
       Shared  objects may export functions using the __attribute__((construc-
       tor)) and __attribute__((destructor)) function attributes.  Constructor
       functions  are  executed  before dlopen() returns, and destructor func-
       tions are executed before dlclose() returns.  A shared object  may  ex-
       port multiple constructors and destructors, and priorities can be asso-
       ciated with each function to determine the order in which they are exe-
       cuted.   See  the gcc info pages (under "Function attributes") for fur-
       ther 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 mul-
       tiple 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 dl-
       mopen() 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 name-
       space.

EXAMPLE
       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 <stdio.h>
       #include <stdlib.h>
       #include <dlfcn.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.1-2003 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);
               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

COLOPHON
       This page is part of release 5.05 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

Linux                             2019-08-02                         DLOPEN(3)
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