DLOPEN(3C)DLOPEN(3C)NAME
dlopen, dlmopen - gain access to an executable object file
SYNOPSIS
#include <dlfcn.h>
#include <link.h>
void * dlopen(const char *pathname, int mode);
void * dlmopen(Lmid_t lmid, const char *pathname, int mode);
DESCRIPTION
The dlopen() function makes an executable object file available to a
running process. dlopen() returns to the process a handle that the
process can use on subsequent calls to dlsym(3C), dladdr(3C),
dlinfo(3C), and dlclose(3C). The value of this handle should not be
interpreted in any way by the process. The pathname argument is the
path name of the object to be opened. A path name containing an embed‐
ded '/' is interpreted as an absolute path or relative to the current
directory. Otherwise, the set of search paths currently in effect by
the runtime linker are used to locate the specified file. See NOTES.
The dlopen() function also loads any dependencies recorded within path‐
name. These dependencies are searched in the order in which the depen‐
dencies were loaded to locate any additional dependencies. This process
continues until all the dependencies of pathname are loaded. This
dependency tree is referred to as a group.
If the value of pathname is 0, dlopen() provides a handle on a set of
global symbol objects. These objects consist of the original program
image file, any dependencies loaded at program startup, and any objects
loaded using dlopen() with the RTLD_GLOBAL flag. Because the latter
set of objects can change during process execution, the set identified
by handle can also change dynamically.
The mode argument describes how dlopen() operates on pathname with
respect to the processing of reference relocations. The mode also
affects the scope of visibility of the symbols provided by pathname and
its dependencies. This visibility can affect how the resulting handle
is used.
When an object is loaded, the object can contain references to symbols
whose addresses are not known until the object is loaded. These refer‐
ences must be relocated before the symbols can be accessed. References
are categorized as either immediate or lazy. Immediate references are
typically references to data items used by the object code. Immediate
references include pointers to functions and calls to functions made
from position-dependent shared objects. Lazy references are typically
calls to global functions that are made from position-independent
shared objects. The mode argument governs when these references take
place. The mode argument can be one of the following values:
RTLD_LAZY
Only immediate symbol references are relocated when the
object is first loaded. Lazy references are not relocated
until a given function is called for the first time. This
value for mode should improve performance, since a process
might not require all lazy references in any given object.
This behavior mimics the normal loading of dependencies
during process initialization. See NOTES.
RTLD_NOW
All necessary relocations are performed when the object is
first loaded. This process might waste some processing if
relocations are performed for lazy references that are
never used. However, this mode ensures that when an object
is loaded, all symbols referenced during execution are
available. This behavior mimics the loading of dependen‐
cies when the environment variable LD_BIND_NOW is in
effect.
See the Linker and Libraries Guide for more information about symbol
references.
The visibility of symbols that are available for relocation can be
affected by mode. To specify the scope of visibility for symbols that
are loaded with a dlopen() call, mode should be a bitwise-inclusive OR
with one of the following values:
RTLD_GLOBAL
The object's global symbols are made available for the
relocation processing of any other object. In addition,
symbol lookup using dlopen(0, mode) and an associated
dlsym() allows objects that are loaded with RTLD_GLOBAL
to be searched.
RTLD_LOCAL
The object's globals symbols are only available for the
relocation processing of other objects that include the
same group.
The program image file and any objects loaded at program startup have
the mode RTLD_GLOBAL. The mode RTLD_LOCAL is the default mode for any
objects that are acquired with dlopen(). A local object can be a depen‐
dency of more then one group. Any object of mode RTLD_LOCAL that is
referenced as a dependency of an object of mode RTLD_GLOBAL is promoted
to RTLD_GLOBAL. In other words, the RTLD_LOCAL mode is ignored.
Any object loaded by dlopen() that requires relocations against global
symbols can reference the symbols in any RTLD_GLOBAL object. Objects of
this mode are at least the program image file and any objects loaded at
program startup. A loaded object can also reference symbols from
itself, and from any dependencies the object references. However, the
mode parameter can also be a bitwise-inclusive OR with one of the fol‐
lowing values to affect the scope of symbol availability:
RTLD_GROUP
Only symbols from the associated group are made avail‐
able for relocation. A group is established from the
defined object and all the dependencies of that object.
A group must be completely self-contained. All depen‐
dency relationships between the members of the group
must be sufficient to satisfy the relocation require‐
ments of each object that defines the group.
RTLD_PARENT
The symbols of the object initiating the dlopen() call
are made available to the objects obtained by dlopen().
This option is useful when hierarchical dlopen() fami‐
lies are created. Although the parent object can supply
symbols for the relocation of this object, the parent
object is not available to dlsym() through the returned
handle.
RTLD_WORLD
Only symbols from RTLD_GLOBAL objects are made available
for relocation.
The default modes for dlopen() are both RTLD_WORLD and RTLD_GROUP. If
an object is requires additional modes, the mode parameter can be the
bitwise-inclusive OR of the required modes together with the default
modes.
The following modes provide additional capabilities outside of reloca‐
tion processing:
RTLD_NODELETE
The specified object is tagged to prevent its deletion
from the address space as part of a dlclose().
RTLD_NOLOAD
The specified object is not loaded as part of the
dlopen(). However, a valid handle is returned if the
object already exists as part of the process address
space. Additional modes can be specified as a bitwise-
inclusive OR with the present mode of the object and
its dependencies. The RTLD_NOLOAD mode provides a
means of querying the presence or promoting the modes
of an existing dependency.
The default use of a handle with dlsym() allows a symbol search to
inspect all objects that are associated with the group of objects that
are loaded from dlopen(). The mode parameter can also be a bitwise-
inclusive OR with the following value to restrict this symbol search:
RTLD_FIRST
Use of this handle with dlsym(), restricts the symbol
search to the first object associated with the handle.
An object can be accessed from a process both with and without
RTLD_FIRST. Although the object will only be loaded once, two different
handles are created to provide for the different dlsym() requirements.
The dlmopen() function is identical to dlopen(), except that an identi‐
fying link-map ID (lmid) is provided. This link-map ID informs the
dynamic linking facilities upon which link-map list to load the object.
See the Linker and Libraries Guide for details about link-maps.
The lmid passed to dlmopen() identifies the link-map list on which the
object is loaded. This parameter can be any valid Lmid_t returned by
dlinfo() or one of the following special values:
LM_ID_BASE
Load the object on the applications link-map list.
LM_ID_LDSO
Load the object on the dynamic linkers (ld.so.1) link-
map list.
LM_ID_NEWLM
Cause the object to create a new link-map list as part
of loading. Objects that are opened on a new link-map
list must express all of their dependencies.
RETURN VALUES
The dlopen() function returns NULL if pathname cannot be found, cannot
be opened for reading, or is not a shared object or a relocatable
object. dlopen() also returns NULL if an error occurs during the
process of loading pathname or relocating its symbolic references. See
NOTES. Additional diagnostic information is available through dler‐
ror().
USAGE
The dlopen() and dlmopen() functions are members of a family of func‐
tions that give the user direct access to the dynamic linking facili‐
ties. This family of functions is available only to dynamically-linked
processes. See the Linker and Libraries Guide.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Standard │
├────────────────────┼─────────────────┤
│MT−Level │ MT−Safe │
└────────────────────┴─────────────────┘
SEE ALSOld(1), ld.so.1(1), dladdr(3C), dlclose(3C), dldump(3C), dlerror(3C),
dlinfo(3C), dlsym(3C), attributes(5), standards(5)
Linker and Libraries Guide
NOTES
If pathname has dependencies on other objects, these objects are auto‐
matically loaded by dlopen(). The directory search path used to find
pathname and any dependencies can be affected by setting the environ‐
ment variable LD_LIBRARY_PATH. Any LD_LIBRARY_PATH variable is analyzed
once at process startup. The search path can also be affected from a
runpath setting within the object from which the call to dlopen() orig‐
inates. These search rules will only be applied to path names that do
not contain an embedded '/'. Objects whose names resolve to the same
absolute path name or relative path name can be opened any number of
times using dlopen(). However, the object that is referenced will only
be loaded once into the address space of the current process.
When loading shared objects, the application should open a specific
version of the shared object. Do not rely on the version of the shared
object pointed to by the symbolic link.
When building objects to be loaded on a new link-map list, some precau‐
tions need to be taken. In general, all dependencies must be included
when building an object. Also, include /usr/lib/libmapmalloc.so.1
before /lib/libc.so.1 when building an object.
When an object is loaded on a new link-map list, the object is isolated
from the main running program. Certain global resources are only usable
from one link-map list. A few examples are the sbrk() based malloc(),
libthread(), and the signal vectors. Care must be taken not to use any
of these resources other than from the primary link-map list. These
issues are discussed in further detail in the Linker and Libraries
Guide.
Some symbols defined in dynamic executables or shared objects can not
be available to the runtime linker. The symbol table created by ld for
use by the runtime linker might contain only a subset of the symbols
that are defined in the object.
As part of loading a new object, initialization code within the object
is called before the dlopen() returns. This initialization is user
code, and as such, can produce errors that can not be caught by
dlopen(). For example, an object loaded using RTLD_LAZY that attempts
to call a function that can not be located results in process termina‐
tion. Erroneous programming practices within the initialization code
can also result in process termination. The runtime linkers debugging
facility can offer help identifying these types of error. See the
LD_DEBUG environment variable of ld.so.1(1).
Loading relocatable objects is an expensive operation that requires
converting the relocatable object into a shared object memory image.
This capability may be useful in a debugging environment, but is not
recommended for production software.
Feb 4, 2005 DLOPEN(3C)
