NAME

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

LIBRARY

       Dynamic linking library (libdl, -ldl)

SYNOPSIS

       #include <dlfcn.h>

       void *dlsym(void *restrict handle, const char *restrict symbol);

       #define _GNU_SOURCE
       #include <dlfcn.h>

       void *dlvsym(void *restrict handle, const char *restrict symbol,
                    const char *restrict version);

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.)

       In unusual cases (see NOTES) the value of the symbol could actually be NULL.  Therefore, a
       NULL  return  from  dlsym() need not indicate an error.  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:

       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 │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

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

NOTES

       There  are  several scenarios when the address of a global symbol is NULL.  For example, a
       symbol can be placed at zero address by the linker, via a linker script or  with  --defsym
       command-line  option.   Undefined  weak symbols also have NULL value.  Finally, the symbol
       value may be the result of a GNU indirect function (IFUNC) resolver function that  returns
       NULL  as the resolved value.  In the latter case, dlsym() also returns NULL without error.
       However, in the former two cases, the behavior of  GNU  dynamic  linker  is  inconsistent:
       relocation  processing  succeeds  and  the  symbol can be observed to have NULL value, but
       dlsym() fails and dlerror() indicates a lookup error.

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

EXAMPLES

       See dlopen(3).

SEE ALSO

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