- An atom in Prolog's internal representation. Atoms are pointers to an
opaque structure. They are a unique representation for represented text,
which implies that atom A represents the same text as atom B
if and only if A and B are the same pointer.
Atoms are the central representation for textual constants in Prolog. The transformation of a character string C to an atom implies a hash-table lookup. If the same atom is needed often, it is advised to store its reference in a global variable to avoid repeated lookup.
- 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.
- Handle to a Prolog predicate. Predicate handles live forever (although they can lose their definition).
- Query identifier. Used by PL_open_query(), PL_next_solution() and PL_close_query() to handle backtracking from C.
- Frame identifier. Used by PL_open_foreign_frame() and PL_close_foreign_frame().
- A module is a unique handle to a Prolog module. Modules are used only to call predicates in a specific module.
- Return type for a C function implementing a Prolog predicate.
- Passed as additional argument to non-deterministic foreign functions. See PL_retry*() and PL_foreign_context*().
- Type for the install() and uninstall() functions of shared or dynamic link libraries. See section 12.2.3.
- 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_tis defined in the
stdint.hstandard 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
FALSEon 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.
As of SWI-Prolog 7.3.12, the arity of terms has changed from
size_t. To deal with this transition, all affecting
functions have two versions, where the old name exchanges the arity as
and a new function with name *_sz() exchanges the arity as
size_t. If the C macro PL_ARITY_AS_SIZE is defined before
SWI-Prolog.h, macros are put in place that map the
old names to the new functions. Without precautions, the old code is
compatible, but the following warning is printed when compiling:
#warning "Term arity has changed from int to size_t." #warning "Please update your code and use #define PL_ARITY_AS_SIZE 1."
To make the code compile silently again, include
as below and change the types you use to represent arity from
size_t. Please be aware that
#define PL_ARITY_AS_SIZE #include <SWI-Prolog.h>