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           #include <dlfcn.h>
           void *dlopen(const char *filename, int flag);
           char *dlerror(void);
           void *dlsym(void *handle, const char *symbol);
           int dlclose(void *handle);
           Link with -ldl.


           The four functions dlopen(), dlsym(),  dlclose(),  dlerror()  implement
           the interface to the dynamic linking loader.
           The  function  dlerror() returns a human-readable string describing the
           most recent error that occurred from  dlopen(),  dlsym()  or  dlclose()
           since  the  last  call to dlerror().  It returns NULL if no errors have
           occurred since initialization or since it was last called.
           The function dlopen() loads the dynamic library file named by the null-
           terminated  string  filename  and  returns  an  opaque "handle" for the
           dynamic library.  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 library 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 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-
           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  this  value  is  specified,  or  the  environment   variable
                  LD_BIND_NOW  is  set to a nonempty string, all undefined symbols
                  in the library 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 flag:
                  The  symbols  defined by this library will be made available for
                  symbol resolution of subsequently loaded libraries.
                  This is the converse of RTLD_GLOBAL, and the default if  neither
                  flag is specified.  Symbols defined in this library are not made
                  available  to  resolve   references   in   subsequently   loaded
           RTLD_NODELETE (since glibc 2.2)
                  Do  not  unload the library during dlclose().  Consequently, the
                  library's static variables are not reinitialized if the  library
                  is  reloaded  with  dlopen()  at a later time.  This flag is not
                  specified in POSIX.1-2001.
           RTLD_NOLOAD (since glibc 2.2)
                  Don't load the library.  This can be used to test if the library
                  is  already resident (dlopen() returns NULL if it is not, or the
                  library's handle if it is resident).  This flag can also be used
                  to  promote  the flags on a library that is already loaded.  For
                  example, a library that was previously  loaded  with  RTLD_LOCAL
                  can  be  reopened  with RTLD_NOLOAD | RTLD_GLOBAL.  This flag is
                  not specified in POSIX.1-2001.
           RTLD_DEEPBIND (since glibc 2.3.4)
                  Place the lookup scope of the symbols in this library  ahead  of
                  the global scope.  This means that a self-contained library will
                  use its own symbols in preference to  global  symbols  with  the
                  same  name contained in libraries that have already been loaded.
                  This flag is not specified in POSIX.1-2001.
           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  libraries  loaded  at  program
           startup, and then all shared libraries loaded by dlopen() with the flag
           External references in the library are resolved using the libraries  in
           that  library's  dependency  list  and  any  other libraries 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  refer-
           ences in a dynamically loaded library.
           where  that  symbol is loaded into memory.  If the symbol is not found,
           in the specified library or any of the libraries  that  were  automati-
           cally  loaded by dlopen() when that library was loaded, dlsym() returns
           NULL.  (The search performed by dlsym() is breadth  first  through  the
           dependency  tree  of  these  libraries.)  Since the value of the symbol
           could actually be NULL (so that a NULL return  from  dlsym()  need  not
           indicate  an  error),  the  correct way to test for an error is to call
           dlerror() to clear any old error conditions,  then  call  dlsym(),  and
           then call dlerror() again, saving its return value into a variable, and
           check whether this saved value is not NULL.
           There are two special pseudo-handles, RTLD_DEFAULT and RTLD_NEXT.   The
           former  will  find the first occurrence of the desired symbol using the
           default library search order.  The latter will find the next occurrence
           of  a  function  in  the  search order after the current library.  This
           allows one to provide a wrapper around a  function  in  another  shared
           The  function  dlclose()  decrements the reference count on the dynamic
           library handle handle.  If the reference count drops  to  zero  and  no
           other  loaded  libraries use symbols in it, then the dynamic library is
           The function dlclose() returns 0 on success, and nonzero on error.
       The obsolete symbols _init() and _fini()
           The linker recognizes special symbols _init and _fini.   If  a  dynamic
           library  exports  a  routine  named _init(), then that code is executed
           after the loading, before dlopen() returns.   If  the  dynamic  library
           exports  a  routine  named  _fini(),  then  that routine is called just
           before the library is unloaded.  In case  you  need  to  avoid  linking
           against  the system startup files, this can be done by using the gcc(1)
           -nostartfiles command-line option.
           Using these routines, or the gcc -nostartfiles or -nostdlib options, is
           not recommended.  Their use may result in undesired behavior, since the
           constructor/destructor routines will not be  executed  (unless  special
           measures are taken).
           Instead, libraries should export routines using the __attribute__((con-
           structor)) and __attribute__((destructor))  function  attributes.   See
           the  gcc info pages for information on these.  Constructor routines are
           executed before dlopen() returns, and destructor routines are  executed
           before dlclose() returns.
       Glibc extensions: dladdr() and dlvsym()
           Glibc adds two functions not described by POSIX, with prototypes
           #define _GNU_SOURCE         /* See feature_test_macros(7) */
           #include <dlfcn.h>
                                              overlaps addr */
                   void       *dli_saddr;  /* Exact address of symbol named
                                              in dli_sname */
               } Dl_info;
           If no symbol matching addr could be found, then dli_sname and dli_saddr
           are set to NULL.
           dladdr() returns 0 on error, and nonzero on success.
           The function dlvsym(), provided by glibc since version  2.1,  does  the
           same as dlsym() but takes a version string as an additional argument.


           POSIX.1-2001 describes dlclose(), dlerror(), dlopen(), and dlsym().


           The  symbols  RTLD_DEFAULT  and RTLD_NEXT are defined by <dlfcn.h> only
           when _GNU_SOURCE was defined before including it.
           Since glibc 2.2.3, atexit(3) can be used to register  an  exit  handler
           that is automatically called when a library is unloaded.
           The  dlopen  interface standard comes from SunOS.  That system also has
           dladdr(), but not dlvsym().


           Sometimes, the function pointers you pass to dladdr() may surprise you.
           On   some  architectures  (notably  i386  and  x86_64),  dli_fname  and
           dli_fbase may end up pointing back at the object from which you  called
           dladdr(),  even  if the function used as an argument should come from a
           dynamically linked library.
           The problem is that the function pointer will still be resolved at com-
           pile  time,  but merely point to the plt (Procedure Linkage Table) sec-
           tion of the original object (which dispatches the call after asking the
           dynamic  linker  to  resolve the symbol).  To work around this, you can
           try to compile the code to be position-independent: then, the  compiler
           cannot  prepare  the pointer at compile time anymore and today's gcc(1)
           will generate code that just loads the final symbol  address  from  the
           got (Global Offset Table) at run time before passing it to dladdr().


           Load the math library, and print the cosine of 2.0:
           #include <stdio.h>
           #include <stdlib.h>
           #include <dlfcn.h>
           main(int argc, char **argv)
               /* 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));
           If  this program were in a file named "foo.c", you would build the pro-
           gram with the following command:
               gcc -rdynamic -o foo foo.c -ldl
           Libraries exporting _init() and _fini() will want  to  be  compiled  as
           follows, using bar.c as the example name:
               gcc -shared -nostartfiles -o bar bar.c


           ld(1), ldd(1), dl_iterate_phdr(3), rtld-audit(7),, ldconfig(8)
  info pages, gcc info pages, ld info pages

    Linux 2014-01-08 DLOPEN(3)


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