The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


dlopen - gain access to an executable object file


[XSI] [Option Start] #include <dlfcn.h>

void *dlopen(const char *
file, int mode); [Option End]


The dlopen() function shall make 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 implementation-defined, 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 shall 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() shall return 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.

The file argument 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-defined manner to yield a pathname.

If the value of file is 0, dlopen() shall provide a handle on a global symbol object. This object shall provide access to the symbols from an ordered set of objects consisting of the original program image file, together with any objects loaded at program start-up 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() shall 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 shall be relocated before the symbols can be accessed. The mode parameter governs when these relocations take place and may have the following values:

Relocations shall be performed at an implementation-defined 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 behavior mimics the normal handling of process execution.
All necessary relocations shall be performed when the object is first loaded. This may waste some processing if relocations are performed for functions that are never referenced. This behavior may be useful for applications that need to know as soon as an object is loaded that all symbols referenced during execution are 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 start-up, 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 a bitwise-inclusive OR with one of the following values:

The object's symbols shall be 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 shall not be made available for the relocation processing of any other object.

If neither RTLD_GLOBAL nor RTLD_LOCAL are specified, then the default behavior is unspecified.

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 shall 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 shall maintain the RTLD_GLOBAL status regardless of any previous or future specification of RTLD_LOCAL, as 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 shall 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() shall return NULL. More detailed diagnostic information shall be available through dlerror().


No errors are defined.

The following sections are informative.










dlclose(), dlerror(), dlsym(), the Base Definitions volume of IEEE Std 1003.1-2001, <dlfcn.h>


First released in Issue 5.

Issue 6

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/21 is applied, changing the default behavior in the DESCRIPTION when neither RTLD_GLOBAL nor RTLD_LOCAL are specified from implementation-defined to unspecified.

End of informative text.

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