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

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

       #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 re-
       turns the address where that symbol is loaded into memory.  If the sym-
       bol  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.)

       In unusual cases (see NOTES) the value of the symbol could actually  be
       NULL.   Therefore,  a NULL return from dlsym() need not indicate an er-
       ror.  The correct way to distinguish an error from a symbol whose value
       is  NULL  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 de-
              fault 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 or-
              der 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  ob-
              ject (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.

       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 at-

       |Interface         | Attribute     | Value   |
       |dlsym(), dlvsym() | Thread safety | MT-Safe |
       POSIX.1-2001 describes dlsym().  The dlvsym() function is a GNU  exten-

       The  value  of  a  symbol returned by dlsym() will never be NULL if the
       shared object is the result of normal compilation, since a global  sym-
       bol  is never placed at the NULL address.  There are nevertheless cases
       where a lookup using dlsym() may return NULL as the value of a  symbol.
       For example, the symbol value may be the result of a GNU indirect func-
       tion (IFUNC) resolver function that returns NULL as the resolved value.

       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),

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Linux                             2019-03-06                          DLSYM(3)
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