dlopen(S)

dlopen -- open a shared object

Synopsis

   #include <dlfcn.h>
   
   void *dlopen(const char file, int mode);

Description

dlopen is one of a family of routines that give the user direct access to the dynamic linking facilities. dlopen makes a shared object specified by file available to a running process. A shared object may specify other objects that it ``needs'' in order to execute properly. These dependencies are specified by DT_NEEDED entries in the .dynamic section of the original object. Each needed object may, in turn, specify other needed objects. All such objects are loaded along with the original object as a result of the call to dlopen.

A successful dlopen call returns to the process a handle the process may use on subsequent calls to dlsym and dlclose. This value should not be interpreted in any way by the process.

file is used to construct a pathname to the object file. If file contains a slash character, the file argument, itself, is used as the pathname. Otherwise, a series of directories is searched for file. First, any directories specified by a DT_RPATH entry in the .dynamic section of the original program object are searched. Then, any directories specified by the environment variable LD_LIBRARY_PATH are searched. Finally, the directory /usr/lib is searched.

If the value of file is ``0'', dlopen provides a handle on a ``global symbol object.'' This object provides access to the symbols from an ordered set of objects consisting of the original a.out, all of the objects that were loaded at program startup along with the a.out, and all objects loaded using a dlopen operation along with the RTLD_GLOBAL flag. As the latter set of objects can change during execution, the set identified by handle can also change dynamically.

Only a single copy of an object file is brought into the address space, even if dlopen is invoked multiple times in reference to the file, and even if different pathnames are used to reference the file.

When a shared object is brought into the address space of a process, it may 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. The mode parameter governs when these relocations take place and may have the following values:

RTLD_LAZY: Under this mode, only references to data symbols are relocated when the object is loaded. References to functions are not relocated until a given function is invoked for the first time. This mode should result in better performance, since a process may not reference all of the functions in any given shared object.
RTLD_NOW: Under this mode, all necessary relocations are performed when the object is first loaded. This may result in some wasted effort, if relocations are performed for functions that are never referenced, but is useful for applications that need to know as soon as an object is loaded that all symbols referenced during execution will be available.

Any object loaded by dlopen that requires relocations against global symbols can reference the symbols in the original a.out, any objects loaded at program startup, from the object itself as well as any other object included in the same dlopen invocation, and any objects that were loaded in any dlopen invocation that specified the RTLD_GLOBAL flag. To determine the scope of visibility for the symbols loaded with a dlopen invocation, the mode parameter should be bitwise or'ed with one of the following values:

RLTD_GLOBAL: The object's 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 loaded with RTLD_GLOBAL to be searched.
RTLD_LOCAL: The object's symbols are made available for relocation processing only to objects loaded in the same dlopen invocation.
RTLD_PRIVATE: The same as RTLD_LOCAL except that the object will be opened as a separate instance. In particular, any modifiable data owned by this object will be distinct from any other instance of the same object.

If none of RTLD_GLOBAL, RTLD_LOCAL, or RTLD_PRIVATE are specified, the default is RTLD_LOCAL.

If a file is specified in multiple dlopen invocations, mode is interpreted at each invocation. Note, however, that once RTLD_NOW has been specified, all relocations will have been completed, rendering any further RTLD_NOW operations redundant and any further RTLD_LAZY operations irrelevant. Similarly note that once RTLD_GLOBAL has been specified, the object will maintain the RTLD_GLOBAL status regardless of any previous or future specification of RTLD_LOCAL, so long as the object remains in the address space [see dlclose(S)].

Symbols introduced into a program through calls to dlopen may be used in relocation activities. Symbols so introduced may duplicate symbols already defined by the program or previous dlopen operations. To resolve the ambiguities such a situation might present, the resolution of a symbol reference to symbol definition is based on a symbol resolution order. Two such resolution orders are defined: load and dependency ordering. Load order establishes an ordering among symbol definitions using the temporal order in which the objects containing the definitions were loaded, such that the definition first loaded has priority over definitions added later. Load ordering is used in relocation processing. Dependency ordering uses a ``breadth-first'' order starting with a given object, then all of its dependencies, then any dependents of those, iterating until all dependencies are satisfied. With the exception of the global symbol object obtained via a dlopen operation on a file with value ``0'', dependency ordering is used by the dlsym function. Load ordering is used in dlsym operations upon the global symbol object.

When an object is first made accessible via dlopen, it and its dependent objects are added in dependency order. Once all the objects are added, relocations are performed using load order. Note that if an object or its dependencies had been loaded by a previous dlopen invocation or on startup, the load and dependency orders may yield different resolutions.

The symbols introduced by dlopen operations and available through dlsym are those which are ``exported'' as symbols of global scope by the object. For shared objects, such symbols will typically be those that were specified in (for example) C source code as having extern linkage. For a.outs, only a subset of externally visible symbols are typically exported: specifically those referenced by the shared objects with which the a.out is linked. The exact set of exported symbols for any shared object or the a.out can be controlled using the linker [see ld(C)].

Return values

If file cannot be found, cannot be opened for reading, is not a shared object, or if an error occurs during the process of loading file or relocating its symbolic references, dlopen returns NULL. More detailed diagnostic information is available through dlerror.

References

cc(C), dlclose(S), dlerror(S), dlsym(S), exec(S), ld(C), sh(C)

Notices

The environment variable LD_LIBRARY_PATH should contain a colon-separated list of directories in the same format as the PATH variable [see sh(C)]. LD_LIBRARY_PATH is ignored if the process' real user id is different from its effective user id or its real group id is different from its effective group id [see exec(S)] or if the process has acquired any privileges [see tfadmin(ADM)].