The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


dlopen - gain access to an executable object file


#include <dlfcn.h>

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


dlopen() makes an executable object file specified by file available to the calling program. The class of files eligible for this operation and the manner of their construction are specified by the implementation, though typically such files are executable objects such as shared libraries, relocatable files or programs. Note that some implementations permit the construction of dependencies between such objects that are embedded within files. In such cases, a dlopen() operation will load such dependencies in addition to the object referenced by file. Implementations may also impose specific constraints on the construction of programs that can employ dlopen() and its related services.

A successful dlopen() returns a handle which the caller may use on subsequent calls to dlsym() and dlclose(). The value of this handle should not be interpreted in any way by the caller.

file is used to construct a pathname to the object file. If file contains a slash character, the file argument is used as the pathname for the file. Otherwise, file is used in an implementation-dependent manner to yield a pathname.

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 program image file, together with any objects loaded at program startup as specified by that process image file (for example, shared libraries), and the set of objects loaded using a dlopen() operation together 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.

The mode parameter describes how dlopen() will operate upon file with respect to the processing of relocations and the scope of visibility of the symbols provided within file. When an 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:

Relocations are performed at an implementation-dependent time, ranging from the time of the dlopen() call until the first reference to a given symbol occurs. Specifying RTLD_LAZY should improve performance on implementations supporting dynamic symbol binding as a process may not reference all of the functions in any given object. And, for systems supporting dynamic symbol resolution for normal process execution, this behaviour mimics the normal handling of process execution.
All necessary relocations are performed when the object is first loaded. This may waste some processing if relocations are performed for functions that are never referenced. This behaviour may be 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 process image file, 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 and which 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:

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 this mode to be searched.
The object's symbols are not made available for the relocation processing of any other object.

If neither RTLD_GLOBAL nor RTLD_LOCAL are specified, then an implementation-specified default behaviour will be applied.

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 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()).

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 or dependency ordering. Load order establishes an ordering among symbol definitions, such that the definition first loaded (including definitions from the image file and any dependent objects loaded with it) has priority over objects added later (via dlopen()). 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 of 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 previously loaded, the load and dependency orders may yield different resolutions.

The symbols introduced by dlopen() operations, and available through dlsym() are at a minimum those which are exported as symbols of global scope by the object. Typically such symbols will be those that were specified in (for example) C source code as having extern linkage. The precise manner in which an implementation constructs the set of exported symbols for a dlopen() object is specified by that implementation.


If file cannot be found, cannot be opened for reading, is not of an appropriate object format for processing by dlopen(), or if an error occurs during the process of loading file or relocating its symbolic references, dlopen() will return NULL. More detailed diagnostic information will be available through dlerror().


No errors are defined.








dlclose(), dlerror(), dlsym().

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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