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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 _GNU_SOURCE feature test macro must be defined in order  to	obtain
       the definitions of RTLD_DEFAULT and RTLD_NEXT from <dlfcn.h>.

       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 4.14 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				  2017-09-15			      DLSYM(3)
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