dlopen(3) Library Functions Manual dlopen(3)
NAME
dlclose, dlopen, dlmopen - open and close a shared object
LIBRARY
Dynamic linking library (libdl, -ldl)
SYNOPSIS
#include <dlfcn.h>
void *dlopen(const char *filename, int flags);
int dlclose(void *handle);
#define _GNU_SOURCE
#include <dlfcn.h>
void *dlmopen(Lmid_t lmid, const char *filename, int flags);
DESCRIPTION
dlopen()
The function dlopen() loads the dynamic shared object (shared library)
file named by the null-terminated string filename and returns an opaque
"handle" for the loaded object. This handle is employed with other
functions in the dlopen API, such as dlsym(3), dladdr(3), dlinfo(3),
and dlclose().
If filename is NULL, then the returned handle is for the main program.
If filename contains a slash ("/"), then it is interpreted as a (rela-
tive or absolute) pathname. Otherwise, the dynamic linker searches for
the object as follows (see ld.so(8) for further details):
• (ELF only) If the calling object (i.e., the shared library or exe-
cutable from which dlopen() is called) contains a DT_RPATH tag, and
does not contain a DT_RUNPATH tag, then the directories listed in
the DT_RPATH tag are searched.
• If, at the time that the program was started, the environment vari-
able LD_LIBRARY_PATH was defined to contain a colon-separated list
of directories, then these are searched. (As a security measure,
this variable is ignored for set-user-ID and set-group-ID programs.)
• (ELF only) If the calling object contains a DT_RUNPATH tag, then the
directories listed in that tag are searched.
• The cache file /etc/ld.so.cache (maintained by ldconfig(8)) is
checked to see whether it contains an entry for filename.
• The directories /lib and /usr/lib are searched (in that order).
If the object specified by filename has dependencies on other shared
objects, then these are also automatically loaded by the dynamic linker
using the same rules. (This process may occur recursively, if those
objects in turn have dependencies, and so on.)
One of the following two values must be included in flags:
RTLD_LAZY
Perform lazy binding. Resolve symbols only as the code that
references them is executed. If the symbol is never referenced,
then it is never resolved. (Lazy binding is performed only for
function references; references to variables are always immedi-
ately bound when the shared object is loaded.) Since glibc
2.1.1, this flag is overridden by the effect of the LD_BIND_NOW
environment variable.
RTLD_NOW
If this value is specified, or the environment variable
LD_BIND_NOW is set to a nonempty string, all undefined symbols
in the shared object are resolved before dlopen() returns. If
this cannot be done, an error is returned.
Zero or more of the following values may also be ORed in flags:
RTLD_GLOBAL
The symbols defined by this shared object will be made available
for symbol resolution of subsequently loaded shared objects.
RTLD_LOCAL
This is the converse of RTLD_GLOBAL, and the default if neither
flag is specified. Symbols defined in this shared object are
not made available to resolve references in subsequently loaded
shared objects.
RTLD_NODELETE (since glibc 2.2)
Do not unload the shared object during dlclose(). Consequently,
the object's static and global variables are not reinitialized
if the object is reloaded with dlopen() at a later time.
RTLD_NOLOAD (since glibc 2.2)
Don't load the shared object. This can be used to test if the
object is already resident (dlopen() returns NULL if it is not,
or the object's handle if it is resident). This flag can also
be used to promote the flags on a shared object that is already
loaded. For example, a shared object that was previously loaded
with RTLD_LOCAL can be reopened with RTLD_NOLOAD | RTLD_GLOBAL.
RTLD_DEEPBIND (since glibc 2.3.4)
Place the lookup scope of the symbols in this shared object
ahead of the global scope. This means that a self-contained ob-
ject will use its own symbols in preference to global symbols
with the same name contained in objects that have already been
loaded.
If filename is NULL, then the returned handle is for the main program.
When given to dlsym(3), this handle causes a search for a symbol in the
main program, followed by all shared objects loaded at program startup,
and then all shared objects loaded by dlopen() with the flag
RTLD_GLOBAL.
Symbol references in the shared object are resolved using (in order):
symbols in the link map of objects loaded for the main program and its
dependencies; symbols in shared objects (and their dependencies) that
were previously opened with dlopen() using the RTLD_GLOBAL flag; and
definitions in the shared object itself (and any dependencies that were
loaded for that object).
Any global symbols in the executable that were placed into its dynamic
symbol table by ld(1) can also be used to resolve references in a dy-
namically loaded shared object. Symbols may be placed in the dynamic
symbol table either because the executable was linked with the flag
"-rdynamic" (or, synonymously, "--export-dynamic"), which causes all of
the executable's global symbols to be placed in the dynamic symbol ta-
ble, or because ld(1) noted a dependency on a symbol in another object
during static linking.
If the same shared object is opened again with dlopen(), the same ob-
ject handle is returned. The dynamic linker maintains reference counts
for object handles, so a dynamically loaded shared object is not de-
allocated until dlclose() has been called on it as many times as
dlopen() has succeeded on it. Constructors (see below) are called only
when the object is actually loaded into memory (i.e., when the refer-
ence count increases to 1).
A subsequent dlopen() call that loads the same shared object with
RTLD_NOW may force symbol resolution for a shared object earlier loaded
with RTLD_LAZY. Similarly, an object that was previously opened with
RTLD_LOCAL can be promoted to RTLD_GLOBAL in a subsequent dlopen().
If dlopen() fails for any reason, it returns NULL.
dlmopen()
This function performs the same task as dlopen()—the filename and flags
arguments, as well as the return value, are the same, except for the
differences noted below.
The dlmopen() function differs from dlopen() primarily in that it ac-
cepts an additional argument, lmid, that specifies the link-map list
(also referred to as a namespace) in which the shared object should be
loaded. (By comparison, dlopen() adds the dynamically loaded shared
object to the same namespace as the shared object from which the
dlopen() call is made.) The Lmid_t type is an opaque handle that
refers to a namespace.
The lmid argument is either the ID of an existing namespace (which can
be obtained using the dlinfo(3) RTLD_DI_LMID request) or one of the
following special values:
LM_ID_BASE
Load the shared object in the initial namespace (i.e., the ap-
plication's namespace).
LM_ID_NEWLM
Create a new namespace and load the shared object in that name-
space. The object must have been correctly linked to reference
all of the other shared objects that it requires, since the new
namespace is initially empty.
If filename is NULL, then the only permitted value for lmid is
LM_ID_BASE.
dlclose()
The function dlclose() decrements the reference count on the dynami-
cally loaded shared object referred to by handle.
If the object's reference count drops to zero and no symbols in this
object are required by other objects, then the object is unloaded after
first calling any destructors defined for the object. (Symbols in this
object might be required in another object because this object was
opened with the RTLD_GLOBAL flag and one of its symbols satisfied a re-
location in another object.)
All shared objects that were automatically loaded when dlopen() was in-
voked on the object referred to by handle are recursively closed in the
same manner.
A successful return from dlclose() does not guarantee that the symbols
associated with handle are removed from the caller's address space. In
addition to references resulting from explicit dlopen() calls, a shared
object may have been implicitly loaded (and reference counted) because
of dependencies in other shared objects. Only when all references have
been released can the shared object be removed from the address space.
RETURN VALUE
On success, dlopen() and dlmopen() return a non-NULL handle for the
loaded object. On error (file could not be found, was not readable,
had the wrong format, or caused errors during loading), these functions
return NULL.
On success, dlclose() returns 0; on error, it returns a nonzero value.
Errors from these functions can be diagnosed using dlerror(3).
VERSIONS
dlopen() and dlclose() are present in glibc 2.0 and later. dlmopen()
first appeared in glibc 2.3.4.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│dlopen(), dlmopen(), dlclose() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2001 describes dlclose() and dlopen(). The dlmopen() function
is a GNU extension.
The RTLD_NOLOAD, RTLD_NODELETE, and RTLD_DEEPBIND flags are GNU exten-
sions; the first two of these flags are also present on Solaris.
NOTES
dlmopen() and namespaces
A link-map list defines an isolated namespace for the resolution of
symbols by the dynamic linker. Within a namespace, dependent shared
objects are implicitly loaded according to the usual rules, and symbol
references are likewise resolved according to the usual rules, but such
resolution is confined to the definitions provided by the objects that
have been (explicitly and implicitly) loaded into the namespace.
The dlmopen() function permits object-load isolation—the ability to
load a shared object in a new namespace without exposing the rest of
the application to the symbols made available by the new object. Note
that the use of the RTLD_LOCAL flag is not sufficient for this purpose,
since it prevents a shared object's symbols from being available to any
other shared object. In some cases, we may want to make the symbols
provided by a dynamically loaded shared object available to (a subset
of) other shared objects without exposing those symbols to the entire
application. This can be achieved by using a separate namespace and
the RTLD_GLOBAL flag.
The dlmopen() function also can be used to provide better isolation
than the RTLD_LOCAL flag. In particular, shared objects loaded with
RTLD_LOCAL may be promoted to RTLD_GLOBAL if they are dependencies of
another shared object loaded with RTLD_GLOBAL. Thus, RTLD_LOCAL is in-
sufficient to isolate a loaded shared object except in the (uncommon)
case where one has explicit control over all shared object dependen-
cies.
Possible uses of dlmopen() are plugins where the author of the plugin-
loading framework can't trust the plugin authors and does not wish any
undefined symbols from the plugin framework to be resolved to plugin
symbols. Another use is to load the same object more than once. With-
out the use of dlmopen(), this would require the creation of distinct
copies of the shared object file. Using dlmopen(), this can be
achieved by loading the same shared object file into different name-
spaces.
The glibc implementation supports a maximum of 16 namespaces.
Initialization and finalization functions
Shared objects may export functions using the __attribute__((construc-
tor)) and __attribute__((destructor)) function attributes. Constructor
functions are executed before dlopen() returns, and destructor func-
tions are executed before dlclose() returns. A shared object may ex-
port multiple constructors and destructors, and priorities can be asso-
ciated with each function to determine the order in which they are exe-
cuted. See the gcc info pages (under "Function attributes") for fur-
ther information.
An older method of (partially) achieving the same result is via the use
of two special symbols recognized by the linker: _init and _fini. If a
dynamically loaded shared object exports a routine named _init(), then
that code is executed after loading a shared object, before dlopen()
returns. If the shared object exports a routine named _fini(), then
that routine is called just before the object is unloaded. In this
case, one must avoid linking against the system startup files, which
contain default versions of these files; this can be done by using the
gcc(1) -nostartfiles command-line option.
Use of _init and _fini is now deprecated in favor of the aforementioned
constructors and destructors, which among other advantages, permit mul-
tiple initialization and finalization functions to be defined.
Since glibc 2.2.3, atexit(3) can be used to register an exit handler
that is automatically called when a shared object is unloaded.
History
These functions are part of the dlopen API, derived from SunOS.
BUGS
As at glibc 2.24, specifying the RTLD_GLOBAL flag when calling dl-
mopen() generates an error. Furthermore, specifying RTLD_GLOBAL when
calling dlopen() results in a program crash (SIGSEGV) if the call is
made from any object loaded in a namespace other than the initial name-
space.
EXAMPLES
The program below loads the (glibc) math library, looks up the address
of the cos(3) function, and prints the cosine of 2.0. The following is
an example of building and running the program:
$ cc dlopen_demo.c -ldl
$ ./a.out
-0.416147
Program source
#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>
#include <gnu/lib-names.h> /* Defines LIBM_SO (which will be a
string such as "libm.so.6") */
int
main(void)
{
void *handle;
double (*cosine)(double);
char *error;
handle = dlopen(LIBM_SO, RTLD_LAZY);
if (!handle) {
fprintf(stderr, "%s\n", dlerror());
exit(EXIT_FAILURE);
}
dlerror(); /* Clear any existing error */
cosine = (double (*)(double)) dlsym(handle, "cos");
/* According to the ISO C standard, casting between function
pointers and 'void *', as done above, produces undefined results.
POSIX.1-2001 and POSIX.1-2008 accepted this state of affairs and
proposed the following workaround:
*(void **) (&cosine) = dlsym(handle, "cos");
This (clumsy) cast conforms with the ISO C standard and will
avoid any compiler warnings.
The 2013 Technical Corrigendum 1 to POSIX.1-2008 improved matters
by requiring that conforming implementations support casting
'void *' to a function pointer. Nevertheless, some compilers
(e.g., gcc with the '-pedantic' option) may complain about the
cast used in this program. */
error = dlerror();
if (error != NULL) {
fprintf(stderr, "%s\n", error);
exit(EXIT_FAILURE);
}
printf("%f\n", (*cosine)(2.0));
dlclose(handle);
exit(EXIT_SUCCESS);
}
SEE ALSO
ld(1), ldd(1), pldd(1), dl_iterate_phdr(3), dladdr(3), dlerror(3),
dlinfo(3), dlsym(3), rtld-audit(7), ld.so(8), ldconfig(8)
gcc info pages, ld info pages
Linux man-pages 6.03 2023-02-05 dlopen(3)
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