Node:Foreign Resources, Next:Conversion Declarations, Previous:Calling C, Up:Calling C
A foreign resource is a set of C functions, defined in one or more files, installed as an atomic operation. The name of a foreign resource, the resource name, is an atom, which should uniquely identify the resource. Thus, two foreign resources with the same name cannot be installed at the same time.
It is recommended that a resource name be all lowercase, starting with
a to z followed by a sequence consisting of a to
z, underscore (_), and digits.
The resource name is used to construct the file name containing the
foreign resource.
For each foreign resource, a foreign_resource/2 fact is used to
declare the interfaced functions. For each of these functions, a
foreign/[2,3] fact is used to specify conversions between predicate
arguments and C-types. These conversion declarations are used for
creating the necessary interface between Prolog and C.
The functions making up the foreign resource, the automatically
generated glue code, and any libraries, are compiled and linked, using
the program splfr (see The Foreign Resource Linker),
to form a
linked foreign resource. A linked foreign resource exists in two
different flavors, static and dynamic. A static resource is
simply a relocatable object file containing the foreign code. A dynamic
resource is a shared library (.so under most UNIX dialects,
.dll under Windows) which is loaded into the Prolog executable at
runtime.
Foreign resources can be linked into the Prolog executable either when the executable is built (pre-linked), or at runtime. Pre-linking can be done using static or dynamic resources. Runtime-linking can only be done using dynamic resources. Dynamic resources can also be unlinked.
In all cases, the declared predicates are installed by the built-in
predicate load_foreign_resource/1. The resource name of a linked
foreign resource is derived from its filename by deleting any leading
path and the suffix. If the resource was pre-linked, only the predicate
names are bound; otherwise, runtime-linking is attempted (using
dlopen(), LoadLibrary(), or similar).
See Overview, for more information.