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 corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAMEdlopen — open a symbol table handle
void *dlopen(const char *file, int mode);
The dlopen() function shall make the symbols (function identifiers and
data object identifiers) in the executable object file specified by
file available to the calling program.
The class of executable object files eligible for this operation and
the manner of their construction are implementation-defined, though
typically such files are shared libraries or programs.
Implementations may permit the construction of embedded dependencies in
executable object files. In such cases, a dlopen() operation shall load
those dependencies in addition to the executable object file specified
by file. Implementations may also impose specific constraints on the
construction of programs that can employ dlopen() and its related ser‐
A successful dlopen() shall return a symbol table handle which the
caller may use on subsequent calls to dlsym() and dlclose().
The value of this symbol table handle should not be interpreted in any
way by the caller.
The file argument is used to construct a pathname to the executable
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 imple‐
mentation-defined manner to yield a pathname.
If file is a null pointer, dlopen() shall return a global symbol table
handle for the currently running process image. This symbol table han‐
dle shall provide access to the symbols from an ordered set of exe‐
cutable object files consisting of the original program image file, any
executable object files loaded at program start-up as specified by that
process file (for example, shared libraries), and the set of executable
object files loaded using dlopen() operations with the RTLD_GLOBAL
flag. As the latter set of executable object files can change during
execution, the set of symbols made available by this symbol table han‐
dle can also change dynamically.
Only a single copy of an executable object file shall be brought into
the address space, even if dlopen() is invoked multiple times in refer‐
ence to the executable object file, and even if different pathnames are
used to reference the executable object 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 executable object file is
brought into the address space of a process, it may contain references
to symbols whose addresses are not known until the executable object
file 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 since a process might not
reference all of the symbols in an executable object file.
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 exe‐
cutable object file is first loaded. This may waste some
processing if relocations are performed for symbols that
are never referenced. This behavior may be useful for
applications that need to know that all symbols referenced
during execution will be available before dlopen() returns.
Any executable object file loaded by dlopen() that requires relocations
against global symbols can reference the symbols in the original
process image file, any executable object files loaded at program
start-up, from the initial process image itself, from any other exe‐
cutable object file included in the same dlopen() invocation, and any
executable object files that were loaded in any dlopen() invocation and
which specified the RTLD_GLOBAL flag. To determine the scope of visi‐
bility for the symbols loaded with a dlopen() invocation, the mode
parameter should be a bitwise-inclusive OR with one of the following
RTLD_GLOBAL The executable object file's symbols shall be made avail‐
able for relocation processing of any other executable
object file. In addition, symbol lookup using
dlopen(NULL,mode) and an associated dlsym() allows exe‐
cutable object files loaded with this mode to be searched.
RTLD_LOCAL The executable object file's symbols shall not be made
available for relocation processing of any other executable
If neither RTLD_GLOBAL nor RTLD_LOCAL is specified, the default behav‐
ior is unspecified.
If an executable object file is specified in multiple dlopen() invoca‐
tions, mode is interpreted at each invocation.
If RTLD_NOW has been specified, all relocations shall have been com‐
pleted rendering further RTLD_NOW operations redundant and any further
RTLD_LAZY operations irrelevant.
If RTLD_GLOBAL has been specified, the executable object file shall
maintain the RTLD_GLOBAL status regardless of any previous or future
specification of RTLD_LOCAL, as long as the executable object file
remains in the address space (see dlclose()).
Symbols introduced into the process image 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 resolu‐
tion of a symbol reference to symbol definition is based on a symbol
resolution order. Two such resolution orders are defined: load order
and dependency order. Load order establishes an ordering among symbol
definitions, such that the first definition loaded (including defini‐
tions from the process image file and any dependent executable object
files loaded with it) has priority over executable object files added
later (by dlopen()). Load ordering is used in relocation processing.
Dependency ordering uses a breadth-first order starting with a given
executable object file, then all of its dependencies, then any depen‐
dents of those, iterating until all dependencies are satisfied. With
the exception of the global symbol table handle obtained via a dlopen()
operation with a null pointer as the file argument, dependency ordering
is used by the dlsym() function. Load ordering is used in dlsym() oper‐
ations upon the global symbol table handle.
When an executable object file is first made accessible via dlopen(),
it and its dependent executable object files are added in dependency
order. Once all the executable object files are added, relocations are
performed using load order. Note that if an executable object file 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 identifiers of
global scope by the executable object file. Typically, such identifiers
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 an executable object file is
Upon successful completion, dlopen() shall return a symbol table han‐
dle. If file cannot be found, cannot be opened for reading, is not of
an appropriate executable object file format for processing by
dlopen(), or if an error occurs during the process of loading file or
relocating its symbolic references, dlopen() shall return a null
pointer. More detailed diagnostic information shall be available
No errors are defined.
The following sections are informative.
Refer to dlsym().
SEE ALSOdlclose(), dlerror(), dlsym()
The Base Definitions volume of POSIX.1‐2008, <dlfcn.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
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is the referee document. The original Standard can be obtained online
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