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).

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                                     │ Attribute     │ Value   │
       ├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │dlsym(), dlvsym()                                                             │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       dlsym()
              POSIX.1-2008.

       dlvsym()
              GNU.

HISTORY

       dlsym()
              glibc 2.0.  POSIX.1-2001.

       dlvsym()
              glibc 2.1.

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)