dlopen(3)



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

NAME
       dladdr, dlclose, dlerror, dlopen, dlsym, dlvsym - programming interface
       to dynamic linking loader

SYNOPSIS
       #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.

DESCRIPTION
       The four functions dlopen(), dlsym(),  dlclose(),  dlerror()  implement
       the interface to the dynamic linking loader.

   dlerror()
       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.

   dlopen()
       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 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
           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-
           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  library has dependencies on other shared libraries, then these
       are also automatically loaded by the  dynamic  linker  using  the  same
       rules.  (This process may occur recursively, if those libraries in turn
       have dependencies, and so on.)

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

       RTLD_LAZY
              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 immedi-
              ately bound when the library is loaded.)

       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 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:

       RTLD_GLOBAL
              The  symbols  defined by this library will be made available for
              symbol resolution of subsequently loaded libraries.

       RTLD_LOCAL
              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
              libraries.

       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
       RTLD_GLOBAL.

       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.

       If  the  same  library  is loaded again with dlopen(), the same library
       handle is returned.  The dl  library  maintains  reference  counts  for
       library  handles,  so  a  dynamic  library  is  not  deallocated  until
       dlclose() has been called on it as many times as dlopen() has succeeded
       on  it.   The  _init() routine, if present, is called only once.  But a
       subsequent call with RTLD_NOW may force symbol resolution for a library
       earlier loaded with RTLD_LAZY.

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

   dlsym()
       The  function dlsym() takes a "handle" of a dynamic library returned by
       dlopen() and the null-terminated symbol  name,  returning  the  address
       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
       library.

   dlclose()
       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
       unloaded.

       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>

       int dladdr(void *addr, Dl_info *info);

       void *dlvsym(void *handle, char *symbol, char *version);

       The  function  dladdr()  takes  a function pointer and tries to resolve
       name and file where it  is  located.   Information  is  stored  in  the
       Dl_info structure:

           typedef struct {
               const char *dli_fname;  /* Pathname of shared object that
                                          contains address */
               void       *dli_fbase;  /* Address at which shared object
                                          is loaded */
               const char *dli_sname;  /* Name of symbol whose definition
                                          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.

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

NOTES
       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.

   History
       The  dlopen  interface standard comes from SunOS.  That system also has
       dladdr(), but not dlvsym().

BUGS
       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().

EXAMPLE
       Load the math library, and print the cosine of 2.0:

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

       int
       main(int argc, char **argv)
       {
           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);
       }

       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

SEE ALSO
       ld(1),  ldd(1),  pldd(1),  dl_iterate_phdr(3), rtld-audit(7), ld.so(8),
       ldconfig(8)

       ld.so info pages, gcc info pages, ld info pages

COLOPHON
       This page is part of release 3.74 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2014-10-02                         DLOPEN(3)

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