| DLSYM(3) | Library Functions Manual | DLSYM(3) |
dlsym — get
address of a symbol
#include
<dlfcn.h>
void*
dlsym(void*
handle, const char*
symbol);
dlsym()
returns the address of the code or data location specified by the
null-terminated character string symbol. Which
libraries and bundles are searched depends on the
handle parameter.
If
dlsym() is
called with a handle, returned by
dlopen()
then only that image and any libraries it depends on are searched for
symbol.
If
dlsym() is
called with the special handle
RTLD_DEFAULT, then all mach-o images in the process
(except those loaded with dlopen(xxx, RTLD_LOCAL)) are searched in the order
they were loaded. This can be a costly search and should be avoided.
If
dlsym() is
called with the special handle
RTLD_NEXT, then dyld searches for the symbol in the
dylibs the calling image linked against when built. It is usually used when
you intentionally have multiply defined symbol across images and want to
find the "next" definition. It searches other images for the
definition that the caller would be using if it did not have a definition.
The exact search algorithm depends on whether the caller's image was linked
-flat_namespace or -twolevel_namespace. For flat linked images, the search
starts in the load ordered list of all images, in the image right after the
caller's image. For two-level images, the search simulates how the static
linker would have searched for the symbol when linking the caller's
image.
If
dlsym() is
called with the special handle
RTLD_SELF, then the search for the symbol starts
with the image that called dlsym(). If it is not
found, the search continues as if RTLD_NEXT was used.
If
dlsym() is
called with the special handle
RTLD_MAIN_ONLY, then it only searches for
symbol in the main executable.
The dlsym() function returns a null
pointer if the symbol cannot be found, and sets an error condition which may
be queried with dlerror().
The symbol name passed to
dlsym()
is the name used in C source code. For example to find the address of
function foo(), you would pass "foo" as the symbol name. This is
unlike the older dyld APIs which required a leading underscore. If you are
looking up a C++ symbol, you need to use the mangled C++ symbol name.
| August 28, 2008 |