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dlopen, dlmopen - gain access to an executable object file

#include <dlfcn.h>
#include <link.h>

void * dlopen(const char *pathname, int mode);

void * dlmopen(Lmid_t lmid, const char *pathname, int mode);

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

RTLD_NOW

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

RTLD_LOCAL

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 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 following values to affect the scope of symbol availability:

RTLD_GROUP

RTLD_PARENT

RTLD_WORLD

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

RTLD_NOLOAD

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

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

LM_ID_LDSO

LM_ID_NEWLM

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().

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.

See attributes(5) for descriptions of the following attributes:

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

ld(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

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.