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
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
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 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.
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() .
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