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NAME
dlopen - gain access to an executable object file
SYNOPSIS
#include <dlfcn.h> void *dlopen(const char *file, int mode);
DESCRIPTION
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:
RTLD_LAZY
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.
RTLD_NOW
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:
RTLD_GLOBAL
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.
RTLD_LOCAL
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 an implementation-defined default behavior
shall 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 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.
RETURN VALUE
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() .
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
dlclose() , dlerror() , dlsym() , the Base Definitions volume of IEEE Std 1003.1-2001, <dlfcn.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .