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



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

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	condi-
       tions,  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 exten-
       sion.

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
       This page is part of release 4.10 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				  2015-08-08			      DLSYM(3)