DLOPEN(3P) POSIX Programmer's Manual DLOPEN(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the correspond-
ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
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 refer-
enced 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 han-
dle 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 refer-
ence 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 per-
formed 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 sym-
bols already defined by the program or previous dlopen() operations. To resolve the ambiguities such a situation might present, the resolu-
tion 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 defini-
tions 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 implementa-
tion.
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 Technol-
ogy -- 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 .
IEEE
/The Open Group 2003 DLOPEN(3P)