DLOPEN(3C) Standard C Library Functions 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 embedded '/' 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
pathname. These dependencies are searched in the order in which the
dependencies 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
references 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 dependencies
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 dependency of
more than 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 following
values to affect the scope of symbol availability:

RTLD_GROUP
Only symbols from the associated group are made available
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 dependency
relationships between the members of the group must be
sufficient to satisfy the relocation requirements 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() families 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 relocation
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
identifying 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 dlerror().

USAGE

The dlopen() and dlmopen() functions are members of a family of functions
that give the user direct access to the dynamic linking facilities. This
family of functions is available only to dynamically-linked processes.
See the Linker and Libraries Guide.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO

ld(1), ld.so.1(1), dladdr(3C), dlclose(3C), dldump(3C), dlerror(3C),
dlinfo(3C), dlsym(3C), attributes(7), standards(7)


Linker and Libraries Guide

NOTES

If pathname has dependencies on other objects, these objects are
automatically loaded by dlopen(). The directory search path used to find
pathname and any dependencies can be affected by setting the environment
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() originates.
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
precautions 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 termination.
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.

May 16, 2020 DLOPEN(3C)