Provided by: manpages-dev_4.04-2_all bug


       dlsym,  dlvsym  -  obtain  address  of  a  symbol in a shared object or


       #include <dlfcn.h>

       void *dlsym(void *handle, const char *symbol);

       #define _GNU_SOURCE
       #include <dlfcn.h>

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

       Link with -ldl.


       The function dlsym() takes a "handle" of a dynamic loaded shared object
       returned  by  dlopen(3)  along  with a null-terminated symbol name, and
       returns the address where that symbol is loaded into  memory.   If  the
       symbol  is  not  found,  in  the  specified object or any of the shared
       objects that were automatically loaded by dlopen(3)  when  that  object
       was  loaded, dlsym() returns NULL.  (The search performed by dlsym() is
       breadth first through the dependency tree of these shared objects.)

       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(3)  to  clear  any  old  error
       conditions,  then  call dlsym(), and then call dlerror(3) again, saving
       its return value into a variable, and check whether this saved value is
       not NULL.

       There are two special pseudo-handles that may be specified in handle:

              Find  the  first  occurrence  of  the  desired  symbol using the
              default shared object search order.   The  search  will  include
              global  symbols  in the executable and its dependencies, as well
              as symbols in shared objects that were dynamically  loaded  with
              the RTLD_GLOBAL flag.

              Find  the  next  occurrence  of the desired symbol in the search
              order after the current object.  This allows one  to  provide  a
              wrapper around a function in another shared object, so that, for
              example, the definition of a  function  in  a  preloaded  shared
              object  (see  LD_PRELOAD  in  can find and invoke the
              "real" function provided in another shared object (or  for  that
              matter,  the  "next"  definition  of the function in cases where
              there are multiple layers of preloading).

              The function dlvsym() does the  same  as  dlsym()  but  takes  a
              version string as an additional argument.


       On  success, these functions return the address associated with symbol.
       On failure, they return NULL; the cause of the error can  be  diagnosed
       using dlerror(3).


       dlsym()  is present in glibc 2.0 and later.  dlvsym() first appeared in
       glibc 2.1.


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

       │InterfaceAttributeValue   │
       │dlsym(), dlvsym() │ Thread safety │ MT-Safe │


       POSIX.1-2001  describes  dlsym().   The  dlvsym()  function  is  a  GNU


       The dlsym() function is part of the dlopen  API,  derived  from  SunOS.
       That system does not have dlvsym().


       See dlopen(3).


       dl_iterate_phdr(3),   dladdr(3),   dlerror(3),   dlinfo(3),  dlopen(3),


       This page is part of release 4.04 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