12 Foreign Language Interface
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12.3 Interface Data Types

12.3.1 Type term_t: a reference to a Prolog term

The principal data type is term_t. Type term_t is what Quintus calls QP_term_ref. This name indicates better what the type represents: it is a handle for a term rather than the term itself. Terms can only be represented and manipulated using this type, as this is the only safe way to ensure the Prolog kernel is aware of all terms referenced by foreign code and thus allows the kernel to perform garbage collection and/or stack-shifts while foreign code is active, for example during a callback from C.

A term reference is a C uintptr_t, representing the offset of a variable on the Prolog environment stack. A foreign function is passed term references for the predicate arguments, one for each argument. If references for intermediate results are needed, such references may be created using PL_new_term_ref() or PL_new_term_refs(). These references normally live till the foreign function returns control back to Prolog. Their scope can be explicitly limited using PL_open_foreign_frame() and PL_close_foreign_frame()/PL_discard_foreign_frame().

A term_t always refers to a valid Prolog term (variable, atom, integer, float or compound term). A term lives either until backtracking takes us back to a point before the term was created, the garbage collector has collected the term, or the term was created after a PL_open_foreign_frame() and PL_discard_foreign_frame() has been called.

The foreign interface functions can either read, unify or write to term references. In this document we use the following notation for arguments of type term_t:

term_t +tAccessed in read-mode. The‘+’indicates the argument is‘input’.
term_t -tAccessed in write-mode.
term_t ?tAccessed in unify-mode.

WARNING Term references that are accessed in‘write’(-) mode will refer to an invalid term if the term is allocated on the global stack and backtracking takes us back to a point before the term was written.214This could have been avoided by trailing term references when data is written to them. This seriously hurts performance in some scenarios though. If this is desired, use PL_put_variable() followed by one of the PL_unify_*() functions. Compound terms, dicts, large integers, rational numbers, floats and strings are all allocated on the global stack. Below is a typical scenario where this may happen. The first solution writes a term extracted from the solution into a. After the system backtracks due to PL_next_solution(), a becomes a reference to a term that no longer exists. 

term_t a = PL_new_term_ref();
...
query = PL_open_query(...);
while(PL_next_solution(query))
{ PL_get_arg(i, ..., a);
}
PL_close_query(query);

There are two solutions to this problem. One is to scope the term reference using PL_open_foreign_frame() and PL_close_foreign_frame() and makes sure it goes out of scope before backtracking happens. The other is to clear the term reference using PL_put_variable() before backtracking.

Term references are obtained in any of the following ways:

Term references can safely be copied to other C variables of type term_t, but all copies will always refer to the same term.

term_t PL_new_term_ref()
Return a fresh reference to a term. The reference is allocated on the local stack. Allocating a term reference may trigger a stack-shift on machines that cannot use sparse memory management for allocation of the Prolog stacks. The returned reference describes a variable. Raise a resource exception and returns (term_t)0 on failure.
term_t PL_new_term_refs(size_t n)
Return n new term references. The first term reference is returned. The others are t+1, t+2, etc. Raise a resource exception and returns (term_t)0 on failure. There are two reasons for using this function. PL_open_query() and PL_cons_functor() expect the arguments as a set of consecutive term references, and very time-critical code requiring a number of term references can be written as: 
pl_mypredicate(term_t a0, term_t a1)
{ term_t t0 = PL_new_term_refs(2);
  term_t t1 = t0+1;

  ...
}
term_t PL_copy_term_ref(term_t from)
Create a new term reference and make it point initially to the same term as from. This function is commonly used to copy a predicate argument to a term reference that may be written. Raise a resource exception and returns (term_t)0 on failure. An example of its use is given below, in the sample code pl_write_atoms().
void PL_free_term_ref(term_t t)
Release a specific term reference. Normally all term references in a scope are discarded together or all term references created after a specific one are reclaimed using PL_reset_term_refs(). This function shrinks the current foreign frame if t is the last one in the frame. Else it marks t for reuse by PL_new_term_ref().
void PL_reset_term_refs(term_t after)
Destroy all term references that have been created after after, including after itself. Any reference to the invalidated term references after this call results in undefined behaviour.

Note that returning from the foreign context to Prolog will reclaim all references used in the foreign context. This call is only necessary if references are created inside a loop that never exits back to Prolog. See also PL_open_foreign_frame(), PL_close_foreign_frame() and PL_discard_foreign_frame().

12.3.1.1 Interaction with the garbage collector and stack-shifter

Prolog implements two mechanisms for avoiding stack overflow: garbage collection and stack expansion. On machines that allow for it, Prolog will use virtual memory management to detect stack overflow and expand the runtime stacks. On other machines Prolog will reallocate the stacks and update all pointers to them. To do so, Prolog needs to know which data is referenced by C code. As all Prolog data known by C is referenced through term references (term_t), Prolog has all the information necessary to perform its memory management without special precautions from the C programmer.

12.3.2 Other foreign interface types

atom_t
The type atom_t actually represents a blob (see section 12.4.10). Blobs are the super type of Prolog atoms, where atoms are blobs that represent textual content. Textual content is also represented by Prolog string (see section 5.2), which makes the general notion of string in Prolog ambiguous. The core idea behind blobs/atoms is to represent arbitrary content using a unique handle, such that comparing the handles is enough to prove equivalence of the contents; i.e., given two different atom handles we know they represent different texts. This uniqueness feature allows the core engine to reason about atom equality and inequality without considering their content. Blobs without the PL_BLOB_UNIQUE feature are also tested for uniqueness without considering their content. Each time an atom or a PL_BLOB_UNIQUE blob is created, it must be looked up in the atom table; if a blob without PL_BLOB_UNIQUE is created, no lookup is done. Strings (section 5.2) and blobs without the PL_BLOB_UNIQUE feature do not have this uniqueness property - to test for equality, the contents of the strings or blobs must be compared. For both atoms and strings, comparisons for ordering (e.g., used by sort/2 or @</2) must use the contents; in the case of blobs, compare() can be specified in the PL_blob_t structure to override the default bitwise comparison.

Because atoms are often used to represent (parts of) arbitrary input, intermediate results, and output of data processed by Prolog, it is necessary that atoms be subject to garbage collection (see garbage_collect_atoms/0). The garbage collection makes atoms ideal handles for arbitrary data structures, which are generalized as blobs. Blobs provide safe access to many internal Prolog data structures such as streams, clause references, etc.

functor_t
A functor is the internal representation of a name/arity pair. They are used to find the name and arity of a compound term as well as to construct new compound terms. Like atoms they live for the whole Prolog session and are unique.
predicate_t
Handle to a Prolog predicate. Predicate handles live forever (although they can lose their definition).
qid_t
Query identifier. Used by PL_open_query(), PL_next_solution(), PL_cut_query(), and PL_close_query() to handle calling Prolog from C.
fid_t
Frame identifier. Used by PL_open_foreign_frame() and PL_close_foreign_frame().
module_t
A module is a unique handle to a Prolog module. Modules are used only to call predicates in a specific module.
foreign_t
Return type for a C function implementing a Prolog predicate.
control_t
Passed as additional argument to non-deterministic foreign functions. See PL_retry*() and PL_foreign_context*().
install_t
Type for the install() and uninstall() functions of shared or dynamic link libraries. See section 12.2.3.
int64_t
Actually part of the C99 standard rather than Prolog. As of version 5.5.6, Prolog integers are 64-bit on all hardware. The C99 type int64_t is defined in the stdint.h standard header and provides platform-independent 64-bit integers. Portable code accessing Prolog should use this type to exchange integer values. Please note that PL_get_long() can return FALSE on Prolog integers that cannot be represented as a C long. Robust code should not assume any of the integer fetching functions to succeed, even if the Prolog term is known to be an integer.

12.3.2.1 PL_ARITY_AS_SIZE

As of SWI-Prolog 7.3.12, the arity of terms has changed from int to size_t. To deal with this transition, all affecting functions have two versions, where the old name exchanges the arity as int and a new function with name *_sz() exchanges the arity as size_t. Up to 8.1.28, the default was to use the old int functions. As of 8.1.29/8.2.x, the default is to use size_t and the old behaviour can be restored by defining PL_ARITY_AS_SIZE to 0 (zero). This makes old code compatible, but the following warning is printed when compiling: 

#warning "Term arity has changed from int to size_t."
#warning "Please update your code or use #define PL_ARITY_AS_SIZE 0."

To make the code compile silently again, change the types you use to represent arity from int to size_t. Please be aware that size_t is unsigned. At some point representing arity as int will be dropped completely.

12.3.2.2 Notes on C API bool return values

Most of the SWI-Prolog C-API consists of C functions that return a Boolean result. Up to version 9.3.10, these functions are defined to return int. Later versions define these functions to return the bool. This type is provided by the standard header stdbool.h and will be supported as a native type starting with the C23 standard, which introduces the keywords false, true and bool. SWI-Prolog.h defines the constants FALSE and TRUE. These constants are consistent with false, and true and may be used interchangeably. Future versions will deprecate FALSE and TRUE. As of version 9.3.11 SWI-Prolog.h includes stdbool.h and thus provides the standard names.

The Boolean result true indicates success, while false may indicate an error or logical failure. Which of the two happened can be examined by calling PL_exception(0), which returns a term_t of value 0 if there was a logical failure. Otherwise the returned term reference is a handle to the Prolog exception. Typically there is no need to test whether or not there has been an exception. Instead, the implementation of a foreign predicate can often simply return false in case some API returned false. Prolog will map this to logical failure or raise the pending exception. The C API defines several groups of bool functions that behave consistently. Note that errors which as the Prolog term handle (term_t) not being a valid is not reported through the API. If this is detected PL_api_error() is called, which aborts the process with a diagnostic message. If not detected, such errors lead to undefined behaviour (read: arbitrary crashes or wrong behaviour now or later).

PL_is_*()
These are type checking functions. They have no side effects and no error conditions. Returning false implies the argument is not of the tested type.
PL_get_*()
This group extracts C value from a Prolog term. If the term is not of the expected type or the C value cannot represent the value the function returns false. No exception is raised.
PL_get_*_ex()
This group is similar to PL_get_*(), but raises a Prolog exception. The exception is either an instantiation_error in case the term is unbound but should not be, a type_error in case the term is of the wrong type or a representation_error in case the C type cannot represent the Prolog value (e.g., a C int while the Prolog integer is out of reach for this type).
PL_put_*()
This group converts C data to a Prolog term. Such a function returning false always raises a resource_error, indicating that Prolog does not have sufficient resources to store the result.
PL_unify_*()
This group unifies a Prolog term to a converted C value. Here, the failure can be logical if the unification failed because the term was already bound to some other value or the failure may be the result of a resource error as with the PL_put_*() group.

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