dlsym(3)

NAME

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

SYNOPSIS

   #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.

DESCRIPTION

   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:

   RTLD_DEFAULT
          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.

   RTLD_NEXT
          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  ld.so(8))  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.

RETURN VALUE

   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).

VERSIONS

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

ATTRIBUTES

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

   
   Interface          Attribute      Value   
   
   dlsym(), dlvsym()  Thread safety  MT-Safe 
   

CONFORMING TO

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

NOTES

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

EXAMPLE

   See dlopen(3).

SEE ALSO

   dl_iterate_phdr(3),   dladdr(3),   dlerror(3),   dlinfo(3),  dlopen(3),
   ld.so(8)

COLOPHON

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   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/.

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