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

NAME

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

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 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  sym-
	      bols  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 diag-
       nosed 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 extension.

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)