Ubuntu Manpage: dlopen - gain access to an executable object file

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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.

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 .