SWI-Prolog atoms as well as strings can represent arbitrary binary data of arbitrary length. This facility is attractive for storing foreign data such as images in an atom. An atom is a unique handle to this data and the atom garbage collector is able to destroy atoms that are no longer referenced by the Prolog engine. This property of atoms makes them attractive as a handle to foreign resources, such as Java atoms, Microsoft's COM objects, etc., providing safe combined garbage collection.
To exploit these features safely and in an organised manner, the SWI-Prolog foreign interface allows creating‘atoms' with additional type information. The type is represented by a structure holding C function pointers that tell Prolog how to handle releasing the atom, writing it, sorting it, etc. Two atoms created with different types can represent the same sequence of bytes. Atoms are first ordered on the rank number of the type and then on the result of the compare() function. Rank numbers are assigned when the type is registered. This implies that the results of inequality comparisons between blobs of different types is undefined and can change if the program is run twice (the ordering within a blob type will not change, of course).
While the blob is alive, neither its handle nor the location of the
contents (see PL_blob_data())
change. If the blob's type has the
PL_BLOB_UNIQUE feature, the content of the blob must remain
unmodified. Blobs are only reclaimed by the atom garbage
collector. In this case the atom garbage collector calls the
release(f)unction
associated with the blob type and reclaims the memory allocated for the
content unless this is owned by the creator of the blob indicated by the PL_BLOB_NOCOPY
flag. After an
atom_t value is reclaimed by the atom garbage collector,
the value may be reused for allocating a new blob or atom.
If foreign code stores the atom_t handle in some
permanent location it must make sure the handle is registered
to prevent it from being garbage collected. If the handle is obtained
from a
term_t object it is not registered because it is
protected by the term_t object. This applies to e.g.,
PL_get_atom().
Functions that create a handle from data such as
PL_new_atom()
return a registered handle to prevent the asynchronous atom garbage
collector from reclaiming it immediately. Note that many of the API
functions create an atom or blob handle and use this to fill a
term_t object, e.g., PL_unify_blob(), PL_unify_chars(),
etc. In this scenario the handle is protected by the term_t
object. Registering and unregistering atom_t handles is
done by
PL_register_atom()
and PL_unregister_atom().
Note that during program shutdown using PL_cleanup(), all atoms and blobs are reclaimed as described above. These objects are reclaimed regardless of their registration count. The order in which the atoms or blobs are reclaimed under PL_cleanup() is undefined. However, when these objects are reclaimed using garbage_collect_atoms/0, registration counts are taken into account.
The type PL_blob_t represents a structure with the
layout displayed below. The structure contains additional fields at the
... for internal bookkeeping as well as future extensions.
typedef struct PL_blob_t
{ uintptr_t magic; /* PL_BLOB_MAGIC */
uintptr_t flags; /* Bitwise or of PL_BLOB_* */
const char * name; /* name of the type */
int (*release)(atom_t a);
int (*compare)(atom_t a, atom_t b);
int (*write)(IOSTREAM *s, atom_t a, int flags);
void (*acquire)(atom_t a);
int (*save)(atom_t a, IOSTREAM *s);
atom_t (*load)(IOSTREAM *s);
...
} PL_blob_t;
For each type, exactly one such structure should be allocated. Its
first field must be initialised to PL_BLOB_MAGIC. The
flags is a bitwise or of the following constants:
PL_BLOB_NOCOPY is also specified, in which case the
pointers are compared.PL_BLOB_UNIQUE
is also specified, uniqueness is determined by comparing the pointer
rather than the data pointed at. Using
PL_BLOB_UNIQUE| can
be used to make a blob reference an arbitrary pointer where the pointer
data may be reclaimed in the release() handler.PL_BLOB_TEXT is also set, then the text is made up of
pl_wchar_t items and the blob's lenght is the number of
bytes (that is, the number of characters times sizeof(pl_wchar_t)).
As PL_BLOB_TEXT, this flag should not be set in
user-defined blobs.
The name field represents the type name as available to
Prolog. See also current_blob/2.
The other fields are function pointers that must be initialised to
proper functions or NULL to get the default behaviour of
built-in atoms. Below are the defined member functions:
atom_t handle into the content of the blob. Given a pointer
to the content, we can now use PL_unify_atom()
to bind a Prolog term with a reference to the pointed to object. If the
content of the blob can be modified (PL_BLOB_UNIQUE is not
present) this is the only way to get access to the atom_t
handle that belongs to this blob. If
PL_BLOB_UNIQUE is provided and respected, PL_unify_blob()
given the same pointer and length will produce the same atom_t
handle.FALSE, the atom garbage collector will not reclaim
the atom. The release(f)unction
is called when the atom is reclaimed by the atom garbage collector. For
critical resources such as file handles or significant memory resources
it may be desirable to have an explicit call to dispose (most of) the
resources. For example,
close/1
reclaims the file handle and most of the resources associated with a
stream, leaving only a tiny bit of content to the garbage collector. See
also setup_call_cleanup/3.
static int
compare_my_blob(atom_t a, atom_t b)
{ const struct my_blob_data *blob_a = PL_blob_data(a, NULL, NULL);
const struct my_blob_data *blob_b = PL_blob_data(b, NULL, NULL);
return (blob_a > blob_b) ? 1 : (blob_a < blob_b) ? -1 : 0;
}
PL_WRT_* flags defined
in
SWI-Prolog.h. This prototype is available if the
SWI-Stream.h is included before
SWI-Prolog.h. This function can retrieve the data of the
blob using PL_blob_data().
If this function is not provided, write/1
emits the content of the blob for blobs of type PL_BLOB_TEXT
or a string of the format <#hex data>
for binary blobs.
If a blob type is registered from a loadable object (shared object or DLL) the blob type must be deregistered before the object may be released.
NULL), the default implementation saves and
restores the blob as if it is an array of bytes which may contain null (’
0') bytes.
SWI-Stream.h defines a number of PL_qlf_put_*()
functions that write data in a machine-independent form that can be read
by the corresponding PL_qlf_get_*() functions.
If the “save'' function encounters an error, it should call
PL_warning(),
raise an exception (see PL_raise_exception()),
and return FALSE.221Details
are subject to change. Note that failure to save/restore a
blob makes it impossible to compile a file that contains such a blob
using qcompile/2
as well as creating a
saved state from a program that contains such a blob
impossible. Here, contains means that the blob appears in a
clause or directive.
NULL, the
default implementation assumes that the blob was written by the default “save''
- that is, as an array of bytes
SWI-Stream.h defines a number of PL_qlf_get_*()
functions that read data in a machine-independent form, as written by
the by the corresponding PL_qlf_put_*() functions.
The atom that the “load'' function returns can be created using PL_new_blob().
unregistered, avoiding further
reference to the type structure, functions referred by it, as well as
the data. This function returns TRUE if no blobs of this
type existed and FALSE otherwise. PL_unregister_blob_type()
is intended for the uninstall() hook of foreign modules, avoiding
further references to the module.
The blob access functions are similar to the atom accessing functions. Blobs being atoms, the atom functions operate on blobs and vice versa. For clarity and possible future compatibility issues, however, it is not advised to rely on this.
PL_BLOB_UNIQUE set, search
the blob database for a blob of the same type with the same content. If
found, unify t with the existing handle.PL_BLOB_UNIQUE is not set, create a new
blob handle. If PL_BLOB_NOCOPY is set, associate it to the
given memory; else, copy the memory to a new area owned by the blob.
Call the acquire()
function of the type.It is possible that a blob referencing critial resources is created after which the unification fails. Typically these resources are eventually reclaimed because the new blob is not referenced and reclaimed by the atom garbage collector. As described with the release(f)unction, it can be desirable to reclaim the critical resources after the failing PL_unify_blob() call.
FALSE) or the blob is a
reference to an existing blob (TRUE). Reporting
new/existing can be used to deal with external objects having their own
reference counts. If the return is TRUE this reference
count must be incremented, and it must be decremented on blob
destruction callback. See also
PL_put_atom_nchars().TRUE. Otherwise return FALSE. Each result
pointer may be NULL, in which case the requested
information is ignored.
Get the data and type associated to a blob. This function is mainly
used from the callback functions described in section
12.4.9.1. Note that if the release() hook is called from PL_cleanup(),
blobs are releases regardless of whether or not they are referenced and
the order in which blobs are released is undefined (the order depends on
the ordering in the atom hash table). PL_blob_data()
may be called safely on a blob that has already been released. If this
happens during PL_cleanup()
the return value is guaranteed to be NULL. During normal
execution it may return the content of a newly allocated blob that
reused the released handle.