dlsym, dlvsym - obtain address of a symbol in a shared object or executable
Dynamic linking library (
libdl,
-ldl)
#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);
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.
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
attributes(7).
Interface |
Attribute |
Value |
dlsym (), dlvsym () |
Thread safety |
MT-Safe |
POSIX.1-2001 describes
dlsym(). The
dlvsym() function is a GNU
extension.
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.
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),
ld.so(8)