huschi.net

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 4.15 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)