dlopen(3) - Linux manual page (original) (raw)
dlopen(3) Library Functions Manual dlopen(3)
NAME top
dlclose, dlopen, dlmopen - open and close a shared objectLIBRARY top
Dynamic linking library (_libdl_, _-ldl_)SYNOPSIS top
**#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 top
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 (relative or absolute) pathname. Otherwise, the
dynamic linker searches for the object as follows (see [ld.so(8)](../man8/ld.so.8.html)
for further details):
• (ELF only) If the calling object (i.e., the shared library or
executable 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
variable **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)](../man8/ldconfig.8.html)) 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 immediately 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 object 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)](../man3/dlsym.3.html), 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)](../man1/ld.1.html) can also be used to resolve
references in a dynamically 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 table, or because [ld(1)](../man1/ld.1.html)
noted a dependency on a symbol in another object during static
linking.
If the same shared object is opened again with **dlopen**(), the same
object handle is returned. The dynamic linker maintains reference
counts for object handles, so a dynamically loaded shared object
is not deallocated 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 reference 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
accepts 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 _Lmidt_ 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)](../man3/dlinfo.3.html) **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
application's namespace).
**LM_ID_NEWLM**
Create a new namespace and load the shared object in that
namespace. 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 dynamically 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 relocation in another object.)
All shared objects that were automatically loaded when **dlopen**()
was invoked 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 top
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)](../man3/dlerror.3.html).ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ **Interface** │ **Attribute** │ **Value** │
├──────────────────────────────────────┼───────────────┼─────────┤
│ **dlopen**(), **dlmopen**(), **dlclose**() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘STANDARDS top
**dlopen**()
**dlclose**()
POSIX.1-2008.
**dlmopen**()
**RTLD_NOLOAD**
**RTLD_NODELETE**
GNU.
**RTLD_DEEPBIND**
Solaris.HISTORY top
**dlopen**()
**dlclose**()
glibc 2.0. POSIX.1-2001.
**dlmopen**()
glibc 2.3.4.NOTES top
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 insufficient to isolate a loaded shared object
except in the (uncommon) case where one has explicit control over
all shared object dependencies.
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. Without 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 namespaces.
The glibc implementation supports a maximum of 16 namespaces.Initialization and finalization functions Shared objects may export functions using the attribute((constructor)) and attribute((destructor)) function attributes. Constructor functions are executed before dlopen() returns, and destructor functions are executed before dlclose() returns. A shared object may export multiple constructors and destructors, and priorities can be associated with each function to determine the order in which they are executed. See the gcc info pages (under "Function attributes") for further 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)](../man1/gcc.1.html) _-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 multiple initialization and finalization
functions to be defined.
Since glibc 2.2.3, [atexit(3)](../man3/atexit.3.html) 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 top
As at glibc 2.24, specifying the **RTLD_GLOBAL** flag when calling
**dlmopen**() 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 namespace.EXAMPLES top
The program below loads the (glibc) math library, looks up the
address of the [cos(3)](../man3/cos.3.html) 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**
-0.416147Program 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;
typeof(double (double)) *cosine;
char *error;
handle = dlopen(LIBM_SO, RTLD_LAZY);
if (!handle) {
fprintf(stderr, "%s\n", dlerror());
exit(EXIT_FAILURE);
}
dlerror(); /* Clear any existing error */
cosine = (typeof(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 top
[ld(1)](../man1/ld.1.html), [ldd(1)](../man1/ldd.1.html), [pldd(1)](../man1/pldd.1.html), [dl_iterate_phdr(3)](../man3/dl%5Fiterate%5Fphdr.3.html), [dladdr(3)](../man3/dladdr.3.html), [dlerror(3)](../man3/dlerror.3.html),
[dlinfo(3)](../man3/dlinfo.3.html), [dlsym(3)](../man3/dlsym.3.html), [rtld-audit(7)](../man7/rtld-audit.7.html), [ld.so(8)](../man8/ld.so.8.html), [ldconfig(8)](../man8/ldconfig.8.html)
gcc info pages, ld info pagesCOLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2025-01-05 dlopen(3)
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