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The Foreign Code Interface

YAP provides the user with three facilities for writing predicates in a language other than Prolog. Under Unix systems, most language implementations were linkable to C, and the first interface exported the YAP machinery to the C language. YAP also implements most of the SWI-Prolog foreign language interface. This gives portability with a number of SWI-Prolog packages and avoids garnage collection by using @ref slotInterface. Last, a new C++ based interface is being designed to work with the swig (www.swig.orgv) interface compiler.

The @subpage c-interface
The @ref swi-c-interface emulates Jan Wielemaker's SWI foreign language interface.
The @ref yap-cplus-interface is desiged to interface with the SWIG package by using Object-Oriented concepts
The @ref LoadForeign handles the setup of foreign files

@page c-interface YAP original C-interface

Before describing in full detail how to interface to C code, we will examine a brief example.

Assume the user requires a predicate my_process_id(Id) which succeeds when Id unifies with the number of the process under which YAP is running.

In this case we will create a my_process.c file containing the C-code described below.

#include "YAP/YapInterface.h"

static int my_process_id(void)
{
     YAP_Term pid = YAP_MkIntTerm(getpid());
     YAP_Term out = YAP_ARG1;
     return(YAP_Unify(out,pid));
}

void init_my_predicates()
{
     YAP_UserCPredicate("my_process_id",my_process_id,1);
}

The commands to compile the above file depend on the operating system.

/** *

Using the compiler:

Under Linux you should use:

      gcc -c -shared -fPIC my_process.c
      ld -shared -o my_process.so my_process.o

Under WIN32 in a MINGW/CYGWIN environment, using the standard installation path you should use:

      gcc -mno-cygwin  -I "c:/Yap/include" -c my_process.c
      gcc -mno-cygwin "c:/Yap/bin/yap.dll" --shared -o my_process.dll
my_process.o

Under WIN32 in a pure CYGWIN environment, using the standard installation path, you should use:

      gcc -I/usr/local -c my_process.c
      gcc -shared -o my_process.dll my_process.o /usr/local/bin/yap.dll

And could be loaded, under YAP, by executing the following Prolog goal

      load_foreign_files(['my_process'],[],init_my_predicates).

Note that since YAP4.3.3 you should not give the suffix for object files. YAP will deduce the correct suffix from the operating system it is running under.

After loading that file the following Prolog goal

       my_process_id(N)

would unify N with the number of the process under which YAP is running.

Having presented a full example, we will now examine in more detail the contents of the C source code file presented above.

The include statement is used to make available to the C source code the macros for the handling of Prolog terms and also some YAP public definitions.

The function my_process_id is the implementation, in C, of the desired predicate. Note that it returns an integer denoting the success of failure of the goal and also that it has no arguments even though the predicate being defined has one. In fact the arguments of a Prolog predicate written in C are accessed through macros, defined in the include file, with names YAP_ARG1, YAP_ARG2, ..., YAP_ARG16 or with YAP_A( N)

where N is the argument number (starting with 1). In the present case the function uses just one local variable of type YAP_Term, the type used for holding YAP terms, where the integer returned by the standard unix function getpid() is stored as an integer term (the conversion is done by YAP_MkIntTerm(Int)). Then it calls the pre-defined routine YAP_Unify(YAP_Term, YAP_Term) which in turn returns an integer denoting success or failure of the unification.

The role of the procedure init_my_predicates is to make known to YAP, by calling YAP_UserCPredicate(), the predicates being defined in the file. This is in fact why, in the example above, init_my_predicates() was passed as the third argument to load_foreign_files/3.

The rest of this appendix describes exhaustively how to interface C to YAP.

Terms

This section provides information about the primitives available to the C programmer for manipulating Prolog terms.

Several C typedefs are included in the header file yap/YAPInterface.h to describe, in a portable way, the C representation of Prolog terms. The user should write is programs using this macros to ensure portability of code across different versions of YAP.

The more important typedef is YAP_Term which is used to denote the type of a Prolog term.

Terms, from a point of view of the C-programmer, can be classified as follows

uninstantiated variables
instantiated variables
integers
floating-point numbers
database references
atoms
pairs (lists)
compound terms

The primitive

YAP_Bool YAP_IsVarTerm(YAP_Term t)

returns true iff its argument is an uninstantiated variable. Conversely the primitive

YAP_Bool YAP_NonVarTerm(YAP_Term _t_)
returns true iff its argument is not a variable.

The user can create a new uninstantiated variable using the primitive

YAP_Term YAP_MkVarTerm()

The following primitives can be used to discriminate among the different types of non-variable terms:

YAP_Bool YAP_IsIntTerm(YAP_Term _t_)
YAP_Bool YAP_IsFloatTerm(YAP_Term _t_)
YAP_Bool YAP_IsDbRefTerm(YAP_Term _t_)
YAP_Bool YAP_IsAtomTerm(YAP_Term _t_)
YAP_Bool YAP_IsPairTerm(YAP_Term _t_)
YAP_Bool YAP_IsApplTerm(YAP_Term _t_)
YAP_Bool YAP_IsCompoundTerm(YAP_Term _t_)

The next primitive gives the type of a Prolog term:

YAP_tag_t YAP_TagOfTerm(YAP_Term _t_)

The set of possible values is an enumerated type, with the following values:

`YAP_TAG_ATT`: an attributed variable
`YAP_TAG_UNBOUND`: an unbound variable
`YAP_TAG_REF`: a reference to a term
`YAP_TAG_PAIR`: a list
`YAP_TAG_ATOM`: an atom
`YAP_TAG_INT`: a small integer
`YAP_TAG_LONG_INT`: a word sized integer
`YAP_TAG_BIG_INT`: a very large integer
`YAP_TAG_RATIONAL`: a rational number
`YAP_TAG_FLOAT`: a floating point number
`YAP_TAG_OPAQUE`: an opaque term
`YAP_TAG_APPL`: a compound term

Next, we mention the primitives that allow one to destruct and construct terms. All the above primitives ensure that their result is a dereferenced, i.e. that it is not a pointer to another term.

The following primitives are provided for creating an integer term from an integer and to access the value of an integer term.

YAP_Term YAP_MkIntTerm(YAP_Int _i_)
YAP_Int YAP_IntOfTerm(YAP_Term _t_)

where `YAP_Int` is a typedef for the C integer type appropriate for the machine or compiler in question (normally a long integer). The size of the allowed integers is implementation dependent but is always greater or equal to 24 bits: usually 32 bits on 32 bit machines, and 64 on 64 bit machines.

The two following primitives play a similar role for floating-point terms

YAP_Term YAP_MkFloatTerm(YAP_flt _double_)
YAP_flt YAP_FloatOfTerm(YAP_Term _t_)

where `flt` is a typedef for the appropriate C floating point type, nowadays a `double`

The following primitives are provided for verifying whether a term is a big int, creating a term from a big integer and to access the value of a big int from a term.

YAP_Bool YAP_IsBigNumTerm(YAP_Term _t_)
YAP_Term YAP_MkBigNumTerm(void \* _b_)
void \*YAP_BigNumOfTerm(YAP_Term _t_, void \* _b_)

YAP must support bignum for the configuration you are using (check the YAP configuration and setup). For now, YAP only supports the GNU GMP library, and `void \*` will be a cast for `mpz_t`. Notice that [YAP_BigNumOfTerm](@ref YAP_BigNumOfTerm) requires the number to be already initialized. As an example, we show how to print a bignum:

static int
p_print_bignum(void)
{
  mpz_t mz;
  if (!YAP_IsBigNumTerm(YAP_ARG1))
    return FALSE;

  mpz_init(mz);
  YAP_BigNumOfTerm(YAP_ARG1, mz);
  gmp_printf("Shows up as %Zd\n", mz);
  mpz_clear(mz);
  return TRUE;
}

Currently, no primitives are supplied to users for manipulating data base references.

A special typedef YAP_Atom is provided to describe Prolog \a atoms (symbolic constants). The two following primitives can be used to manipulate atom terms

YAP_Term YAP_MkAtomTerm(YAP_Atom at)
YAP_Atom YAP_AtomOfTerm(YAP_Term _t_)

The following primitives are available for associating atoms with their names

YAP_Atom YAP_LookupAtom(char \* _s_)
YAP_Atom YAP_FullLookupAtom(char \* _s_)
char \*YAP_AtomName(YAP_Atom _t_)

The function [YAP_LookupAtom](@ref YAP_LookupAtom) looks up an atom in the standard hash table. The function [YAP_FullLookupAtom](@ref YAP_FullLookupAtom) will also search if the atom had been "hidden": this is useful for system maintenance from C code. The functor [YAP_AtomName](@ref YAP_AtomName) returns a pointer to the string for the atom.

The following primitives handle constructing atoms from strings with wide characters, and vice-versa:

YAP_Atom YAP_LookupWideAtom(wchar_t \* _s_)
wchar_t \*YAP_WideAtomName(YAP_Atom _t_)

The following primitive tells whether an atom needs wide atoms in its representation:

int YAP_IsWideAtom(YAP_Atom _t_)

The following primitive can be used to obtain the size of an atom in a representation-independent way:

int YAP_AtomNameLength(YAP_Atom _t_)

The next routines give users some control over the atom garbage collector. They allow the user to guarantee that an atom is not to be garbage collected (this is important if the atom is hold externally to the Prolog engine, allow it to be collected, and call a hook on garbage collection:

int YAP_AtomGetHold(YAP_Atom _at_)
int YAP_AtomReleaseHold(YAP_Atom _at_)
int YAP_AGCRegisterHook(YAP_AGC_hook _f_)

A \a pair is a Prolog term which consists of a tuple of two Prolog terms designated as the \a head and the \a tail of the term. Pairs are most often used to build lists. The following primitives can be used to manipulate pairs:

YAP_Term YAP_MkPairTerm(YAP_Term _Head_, YAP_Term _Tail_)
YAP_Term YAP_MkNewPairTerm(void)
YAP_Term YAP_HeadOfTerm(YAP_Term _t_)
YAP_Term YAP_TailOfTerm(YAP_Term _t_)
YAP_Term YAP_MkListFromTerms(YAP_Term \* _pt_, YAP_Int \* _sz_)

One can construct a new pair from two terms, or one can just build a pair whose head and tail are new unbound variables. Finally, one can fetch the head or the tail.

The last function supports the common operation of constructing a list from an array of terms of size sz in a simple sweep.

Notice that the list constructors can call the garbage collector if there is not enough space in the global stack.

A \a compound term consists of a \a functor and a sequence of terms with length equal to the \a arity of the functor. A functor, described in C by the typedef Functor, consists of an atom and of an integer. The following primitives were designed to manipulate compound terms and functors

YAP_Term YAP_MkApplTerm(YAP_Functor _f_, unsigned long int _n_, YAP_Term[] _args_)
YAP_Term YAP_MkNewApplTerm(YAP_Functor _f_, int _n_)
YAP_Term YAP_ArgOfTerm(int argno,YAP_Term _ts_)
YAP_Term \*YAP_ArgsOfTerm(YAP_Term _ts_)
YAP_Functor YAP_FunctorOfTerm(YAP_Term _ts_)

The [YAP_MkApplTerm() function constructs a new term, with functor _f_ (of arity _n_), and using an array _args_ of _n_ terms with _n_ equal to the arity of the functor. YAP_MkNewApplTerm() builds up a compound term whose arguments are unbound variables. [YAP_ArgOfTerm](@ref YAP_ArgOfTerm) gives an argument to a compound term. `argno` should be greater or equal to 1 and less or equal to the arity of the functor. [YAP_ArgsOfTerm](@ref YAP_ArgsOfTerm) returns a pointer to an array of arguments.

Notice that the compound term constructors can call the garbage collector if there is not enough space in the global stack.

YAP allows one to manipulate the functors of compound term. The function [YAP_FunctorOfTerm](@ref YAP_FunctorOfTerm) allows one to obtain a variable of type YAP_Functor with the functor to a term. The following functions then allow one to construct functors, and to obtain their name and arity.

YAP_Functor YAP_MkFunctor(YAP_Atom _a_,unsigned long int _arity_)
YAP_Atom YAP_NameOfFunctor(YAP_Functor _f_)
YAP_Int YAP_ArityOfFunctor(YAP_Functor _f_)

Note that the functor is essentially a pair formed by an atom, and arity.

Constructing terms in the stack may lead to overflow. The routine

int YAP_RequiresExtraStack(size_t _min_)

verifies whether you have at least _min_ cells free in the stack, and it returns true if it has to ensure enough memory by calling the garbage collector and or stack shifter. The routine returns false if no memory is needed, and a negative number if it cannot provide enough memory.

You can set min to zero if you do not know how much room you need but you do know you do not need a big chunk at a single go. Usually, the routine would usually be called together with a long-jump to restart the code. Slots can also be used if there is small state.

Unification

YAP provides a single routine to attempt the unification of two Prolog terms. The routine may succeed or fail:

Int YAP_Unify(YAP_Term _a_, YAP_Term _b_)

The routine attempts to unify the terms _a_ and _b_ returning `TRUE` if the unification succeeds and `FALSE` otherwise. ### Strings {#Manipulating_Strings}

The YAP C-interface now includes an utility routine to copy a string represented as a list of a character codes to a previously allocated buffer

int YAP_StringToBuffer(YAP_Term _String_, char \* _buf_, unsigned int _bufsize_)

The routine copies the list of character codes _String_ to a previously allocated buffer _buf_. The string including a terminating null character must fit in _bufsize_ characters, otherwise the routine will simply fail. The _StringToBuffer_ routine fails and generates an exception if _String_ is not a valid string.

The C-interface also includes utility routines to do the reverse, that is, to copy a from a buffer to a list of character codes, to a difference list, or to a list of character atoms. The routines work either on strings of characters or strings of wide characters:

YAP_Term YAP_BufferToString(char \* _buf_)
YAP_Term YAP_NBufferToString(char \* _buf_, size_t _len_)
YAP_Term YAP_WideBufferToString(wchar_t \* _buf_)
YAP_Term YAP_NWideBufferToString(wchar_t \* _buf_, size_t _len_)
YAP_Term YAP_BufferToAtomList(char \* _buf_)
YAP_Term YAP_NBufferToAtomList(char \* _buf_, size_t _len_)
YAP_Term YAP_WideBufferToAtomList(wchar_t \* _buf_)
YAP_Term YAP_NWideBufferToAtomList(wchar_t \* _buf_, size_t _len_)

Users are advised to use the _N_ version of the routines. Otherwise, the user-provided string must include a terminating null character.

The C-interface function calls the parser on a sequence of characters stored at buf and returns the resulting term.

YAP_Term YAP_ReadBuffer(char \* _buf_,YAP_Term \* _error_)

The user-provided string must include a terminating null character. Syntax errors will cause returning `FALSE` and binding _error_ to a Prolog term.

These C-interface functions are useful when converting chunks of data to Prolog:

YAP_Term YAP_FloatsToList(double \* _buf_,size_t _sz_)
YAP_Term YAP_IntsToList(YAP_Int \* _buf_,size_t _sz_)

Notice that they are unsafe, and may call the garbage collector. They return 0 on error.

These C-interface functions are useful when converting Prolog lists to arrays:

YAP_Int YAP_IntsToList(YAP_Term t, YAP_Int \* _buf_,size_t _sz_)
YAP_Int YAP_FloatsToList(YAP_Term t, double \* _buf_,size_t _sz_)

They return the number of integers scanned, up to a maximum of sz, and -1 on error. ### Memory Allocation {#Memory_Allocation}

The next routine can be used to ask space from the Prolog data-base:

void \*YAP_AllocSpaceFromYAP(int _size_)

The routine returns a pointer to a buffer allocated from the code area, or `NULL` if sufficient space was not available.

The space allocated with [YAP_AllocSpaceFromYAP](@ref YAP_AllocSpaceFromYAP) can be released back to YAP by using:

void YAP_FreeSpaceFromYAP(void \* _buf_)

The routine releases a buffer allocated from the code area. The system may crash if `buf` is not a valid pointer to a buffer in the code area. ### Controlling YAP Streams from `C` {#Controlling_Streams}

The C-Interface also provides the C-application with a measure of control over the YAP Input/Output system. The first routine allows one to find a file number given a current stream:

int YAP_StreamToFileNo(YAP_Term _stream_)

This function gives the file descriptor for a currently available stream. Note that null streams and in memory streams do not have corresponding open streams, so the routine will return a negative. Moreover, YAP will not be aware of any direct operations on this stream, so information on, say, current stream position, may become stale.

A second routine that is sometimes useful is:

void YAP_CloseAllOpenStreams(void)

This routine closes the YAP Input/Output system except for the first three streams, that are always associated with the three standard Unix streams. It is most useful if you are doing `fork()`.

Last, one may sometimes need to flush all streams:

void YAP_CloseAllOpenStreams(void)

It is also useful before you do a `fork()`, or otherwise you may have trouble with unflushed output.

The next routine allows a currently open file to become a stream. The routine receives as arguments a file descriptor, the true file name as a string, an atom with the user name, and a set of flags:

void YAP_OpenStream(void \* _FD_, char \* _name_, YAP_Term _t_, int _flags_)

The available flags are `YAP_INPUT_STREAM`, `YAP_OUTPUT_STREAM`, `YAP_APPEND_STREAM`, `YAP_PIPE_STREAM`, `YAP_TTY_STREAM`, `YAP_POPEN_STREAM`, `YAP_BINARY_STREAM`, and `YAP_SEEKABLE_STREAM`. By default, the stream is supposed to be at position 0. The argument _name_ gives the name by which YAP should know the new stream. ### Utility Functions in `C` {#Utility_Functions}

The C-Interface provides the C-application with a a number of utility functions that are useful.

The first provides a way to insert a term into the data-base

void \*YAP_Record(YAP_Term _t_)

This function returns a pointer to a copy of the term in the database (or to NULL if the operation fails.

The next functions provides a way to recover the term from the data-base:

YAP_Term YAP_Recorded(void \* _handle_)

Notice that the semantics are the same as for recorded/3: this function creates a new copy of the term in the stack, with fresh variables. The function returns 0L if it cannot create a new term.

Last, the next function allows one to recover space:

int YAP_Erase(void \* _handle_)

Notice that any accesses using _handle_ after this operation may lead to a crash.

The following functions are often required to compare terms.

Succeed if two terms are actually the same term, as in ==/2:

int YAP_ExactlyEqual(YAP_Term t1, YAP_Term t2)

The next function succeeds if two terms are variant terms, and returns 0 otherwise, as =@=/2:

int YAP_Variant(YAP_Term t1, YAP_Term t2)

The next functions deal with numbering variables in terms:

int YAP_NumberVars(YAP_Term t, YAP_Int first_number)
YAP_Term YAP_UnNumberVars(YAP_Term t)
int YAP_IsNumberedVariable(YAP_Term t)

The next one returns the length of a well-formed list t, or -1 otherwise:

Int YAP_ListLength(YAP_Term t)

Last, this function succeeds if two terms are unifiable: =@=/2:

int YAP_Unifiable(YAP_Term t1, YAP_Term t2)

The second function computes a hash function for a term, as in term_hash/4.

YAP_Int YAP_TermHash(YAP_Term t, YAP_Int range, YAP_Int depth, int ignore_variables));

The first three arguments follow `term_has/4`. The last argument indicates what to do if we find a variable: if `0` fail, otherwise ignore the variable. ### From `C` back to Prolog {#Calling_YAP_From_C}

There are several ways to call Prolog code from C-code. By default, the YAP_RunGoal() should be used for this task. It assumes the engine has been initialized before:

YAP_Int YAP_RunGoal(YAP_Term Goal)

Execute query _Goal_ and return 1 if the query succeeds, and 0 otherwise. The predicate returns 0 if failure, otherwise it will return an _YAP_Term_.

Quite often, one wants to run a query once. In this case you should use Goal:

YAP_Int YAP_RunGoalOnce(YAP_Term Goal)

The `YAP_RunGoal()` function makes sure to recover stack space at the end of execution.

Prolog terms are pointers: a problem users often find is that the term Goal may actually be moved around during the execution of YAP_RunGoal(), due to garbage collection or stack shifting. If this is possible, Goal will become invalid after executing YAP_RunGoal(). In this case, it is a good idea to save Goal slots, as shown next:

  long sl = YAP_InitSlot(scoreTerm);

  out = YAP_RunGoal(t);
  t = YAP_GetFromSlot(sl);
  YAP_RecoverSlots(1);
  if (out == 0) return FALSE;

@copydoc real

The following functions complement YAP_RunGoal:

`int` YAP_RestartGoal(`void`)
Look for the next solution to the current query by forcing YAP to backtrack to the latest goal. Notice that slots allocated since the last YAP_RunGoal() will become invalid.
`int` YAP_Reset(`yap_reset_t mode`)
Reset execution environment (similar to the abort/0 built-in). This is useful when you want to start a new query before asking all solutions to the previous query. 'modespecifies how deep the Reset will go and what to do next. It will be most often set toYAP_FULL_RESET`.
`int` YAP_ShutdownGoal(`int backtrack`)
Clean up the current goal. If backtrack is true, stack space will be recovered and bindings will be undone. In both cases, any slots allocated since the goal was created will become invalid.
`YAP_Bool` YAP_GoalHasException(`YAP_Term \*tp`)
Check if the last goal generated an exception, and if so copy it to the space pointed to by tp
`void` YAP_ClearExceptions(`void`)
Reset any exceptions left over by the system.

The YAP_RunGoal() interface is designed to be very robust, but may not be the most efficient when repeated calls to the same goal are made and when there is no interest in processing exception. The YAP_EnterGoal() interface should have lower-overhead:

`YAP_PredEntryPtr` YAP_FunctorToPred(`YAP_Functor` _f_) Return the predicate whose main functor is _f_.
`YAP_PredEntryPtr` YAP_AtomToPred(`YAP_Atom` _at_)
Return the arity 0 predicate whose name is at.
`YAP_PredEntryPtr` YAP_FunctorToPredInModule(`YAP_Functor` _f_, `YAP_Module` _m_),
Return the predicate in module m whose main functor is f.
`YAP_PredEntryPtr` YAP_AtomToPred(`YAP_Atom` _at_, `YAP_Module` _m_),
Return the arity 0 predicate in module m whose name is at.
`YAP_Bool` YAP_EnterGoal(`YAP_PredEntryPtr` _pe_),
YAP_Term \* array, YAP_dogoalinfo \* infop) Execute a query for predicate pe. The query is given as an array of terms Array. infop is the address of a goal handle that can be used to backtrack and to recover space. Succeeds if a solution was found.

Notice that you cannot create new slots if an YAP_ExnterGoal goal is open.
`YAP_Bool` YAP_RetryGoal(`YAP_dogoalinfo \*` _infop_) @anchor YAP_RetryGoal
Backtrack to a query created by [YAP_EnterGoal](@ref YAP_EnterGoal). The query is given by the handle infop. Returns whether a new solution could be be found.
`YAP_Bool` YAP_LeaveGoal(`YAP_Bool` _backtrack_, @anchor YAP_LeaveGoal
YAP_dogoalinfo \* infop) Exit a query query created by [YAP_EnterGoal](@ref YAP_EnterGoal). If backtrack is TRUE, variable bindings are undone and Heap space is recovered. Otherwise, only stack space is recovered, ie, LeaveGoal executes a cut.

Next, follows an example of how to use [YAP_EnterGoal](@ref YAP_EnterGoal):

void
runall(YAP_Term g)
{
    YAP_dogoalinfo goalInfo;
    YAP_Term *goalArgs = YAP_ArraysOfTerm(g);
    YAP_Functor *goalFunctor = YAP_FunctorOfTerm(g);
    YAP_PredEntryPtr goalPred = YAP_FunctorToPred(goalFunctor);

    result = YAP_EnterGoal( goalPred, goalArgs, &goalInfo );
    while (result)
       result = YAP_RetryGoal( &goalInfo );
    YAP_LeaveGoal(TRUE, &goalInfo);
}

YAP allows calling a new Prolog interpreter from C. One way is to first construct a goal G, and then it is sufficient to perform:

YAP_Bool YAP_CallProlog(YAP_Term _G_)

the result will be `FALSE`, if the goal failed, or `TRUE`, if the goal succeeded. In this case, the variables in _G_ will store the values they have been unified with. Execution only proceeds until finding the first solution to the goal, but you can call [findall/3](@ref findall) or friends if you need all the solutions.

Notice that during execution, garbage collection or stack shifting may have moved the terms

Module Manipulation in C

YAP allows one to create a new module from C-code. To create the new code it is sufficient to call:

YAP_Module YAP_CreateModule(YAP_Atom _ModuleName_)

Notice that the new module does not have any predicates associated and that it is not the current module. To find the current module, you can call:

YAP_Module YAP_CurrentModule()

Given a module, you may want to obtain the corresponding name. This is possible by using:

YAP_Term YAP_ModuleName(YAP_Module mod)

Notice that this function returns a term, and not an atom. You can [YAP_AtomOfTerm](@ref YAP_AtomOfTerm) to extract the corresponding Prolog atom. ### Miscellaneous C Functions {#Miscellaneous_ChYFunctions}

`void` YAP_Throw(`YAP_Term exception`)
`void` YAP_AsyncThrow(`YAP_Term exception`) @anchor YAP_Throw
Throw an exception with term exception, just like if you called throw/2. The function YAP_AsyncThrow is supposed to be used from interrupt handlers.
`int` YAP_SetYAPFlag(`yap_flag_t flag, int value`) @anchor YAP_SetYAPFlag
This function allows setting some YAP flags from C .Currently, only two boolean flags are accepted: YAPC_ENABLE_GC and YAPC_ENABLE_AGC. The first enables/disables the standard garbage collector, the second does the same for the atom garbage collector.`
`YAP_TERM` YAP_AllocExternalDataInStack(`size_t bytes`)
`void \*` YAP_ExternalDataInStackFromTerm(`YAP_Term t`)
`YAP_Bool` YAP_IsExternalDataInStackTerm(`YAP_Term t`) @anchor YAP_AllocExternalDataInStack
The next routines allow one to store external data in the Prolog execution stack. The first routine reserves space for sz bytes and returns an opaque handle. The second routines receives the handle and returns a pointer to the data. The last routine checks if a term is an opaque handle.

Data will be automatically reclaimed during backtracking. Also, this storage is opaque to the Prolog garbage compiler, so it should not be used to store Prolog terms. On the other hand, it may be useful to store arrays in a compact way, or pointers to external objects.
`int` YAP_HaltRegisterHook(`YAP_halt_hook f, void \*closure`) @anchor YAP_HaltRegisterHook
Register the function f to be called if YAP is halted. The function is called with two arguments: the exit code of the process (0 if this cannot be determined on your operating system) and the closure argument closure.
`int` YAP_Argv(`char \*\*\*argvp`) @anchor YAP_Argv
Return the number of arguments to YAP and instantiate argvp to point to the list of such arguments.

### Writing predicates in C {#Writing_C}

We will distinguish two kinds of predicates:

\a deterministic predicates which either fail or succeed but are not backtrackable, like the one in the introduction;
\a backtrackable predicates which can succeed more than once.

The first kind of predicates should be implemented as a C function with no arguments which should return zero if the predicate fails and a non-zero value otherwise. The predicate should be declared to YAP, in the initialization routine, with a call to

void YAP_UserCPredicate(char \* _name_, YAP_Bool \* _fn_(), unsigned long int _arity_); where _name_ is a string with the name of the predicate, _init_, _cont_, _cut_ are the C functions used to start, continue and when pruning the execution of the predicate, _arity_ is the predicate arity, and _sizeof_ is the size of the data to be preserved in the stack.
For the second kind of predicates we need three C functions. The first one is called when the predicate is first activated; the second one is called on backtracking to provide (possibly) other solutions; the last one is called on pruning. Note also that we normally also need to preserve some information to find out the next solution.

In fact the role of the two functions can be better understood from the following Prolog definition
```
       p :- start.
       p :- repeat,
                continue.
```
where start and continue correspond to the two C functions described above.

The interface works as follows:
- void YAP_UserBackCutCPredicate(char \* _name_, int \* _init_(), int \* _cont_(), int \* _cut_(), unsigned long int _arity_, unsigned int _sizeof_) @anchor YAP_UserBackCutCPredicate
  describes a new predicate where name is the name of the predicate, init, cont, and cut are the C functions that implement the predicate and arity is the predicate's arity.
- void YAP_UserBackCPredicate(char \* _name_, int \* _init_(), int \* _cont_(), unsigned long int _arity_, unsigned int _sizeof_) @anchor YAP_UserBackCPredicate
  describes a new predicate where name is the name of the predicate, init, and cont are the C functions that implement the predicate and arity is the predicate's arity.
- void YAP_PRESERVE_DATA( _ptr_, _type_); @anchor YAP_PRESERVE_DATA
- void YAP_PRESERVED_DATA( _ptr_, _type_); @anchor YAP_PRESERVED_DATA
- void YAP_PRESERVED_DATA_CUT( _ptr_, _type_); @anchor YAP_PRESERVED_DATA_CUT
- void YAP_cut_succeed( void ); @anchor YAP_cut_succeed
- void YAP_cut_fail( void ); @anchor YAP_cut_fail
As an example we will consider implementing in C a predicate n100(N) which, when called with an instantiated argument should succeed if that argument is a numeral less or equal to 100, and, when called with an uninstantiated argument, should provide, by backtracking, all the positive integers less or equal to 100.

To do that we first declare a structure, which can only consist of Prolog terms, containing the information to be preserved on backtracking and a pointer variable to a structure of that type.
```
#include "YAPInterface.h"

static int start_n100(void);
static int continue_n100(void);

typedef struct {
    YAP_Term next_solution;
   } n100_data_type;

n100_data_type *n100_data;
```
We now write the C function to handle the first call:
```
static int start_n100(void)
{
      YAP_Term t = YAP_ARG1;
      YAP_PRESERVE_DATA(n100_data,n100_data_type);
      if(YAP_IsVarTerm(t)) {
          n100_data->next_solution = YAP_MkIntTerm(0);
          return continue_n100();
       }
      if(!YAP_IsIntTerm(t) || YAP_IntOfTerm(t)<0 || YAP_IntOfTerm(t)>100) {
          YAP_cut_fail();
      } else {
          YAP_cut_succeed();
      }
}
```
The routine starts by getting the dereference value of the argument. The call to [YAP_PRESERVE_DATA](@ref YAP_PRESERVE_DATA) is used to initialize the memory which will hold the information to be preserved across backtracking. The first argument is the variable we shall use, and the second its type. Note that we can only use [YAP_PRESERVE_DATA](@ref YAP_PRESERVE_DATA) once, so often we will want the variable to be a structure. This data is visible to the garbage collector, so it should consist of Prolog terms, as in the example. It is also correct to store pointers to objects external to YAP stacks, as the garbage collector will ignore such references.

If the argument of the predicate is a variable, the routine initializes the structure to be preserved across backtracking with the information required to provide the next solution, and exits by calling continue_n100 to provide that solution.

If the argument was not a variable, the routine then checks if it was an integer, and if so, if its value is positive and less than 100. In that case it exits, denoting success, with [YAP_cut_succeed](@ref YAP_cut_succeed), or otherwise exits with [YAP_cut_fail](@ref YAP_cut_fail) denoting failure.

The reason for using for using the functions [YAP_cut_succeed](@ref YAP_cut_succeed) and [YAP_cut_fail](@ref YAP_cut_fail) instead of just returning a non-zero value in the first case, and zero in the second case, is that otherwise, if backtracking occurred later, the routine continue_n100 would be called to provide additional solutions.

The code required for the second function is
```
static int continue_n100(void)
{
      int n;
      YAP_Term t;
      YAP_Term sol = YAP_ARG1;
      YAP_PRESERVED_DATA(n100_data,n100_data_type);
      n = YAP_IntOfTerm(n100_data->next_solution);
      if( n == 100) {
           t = YAP_MkIntTerm(n);
           YAP_Unify(sol,t);
           YAP_cut_succeed();
        }
       else {
           YAP_Unify(sol,n100_data->next_solution);
           n100_data->next_solution = YAP_MkIntTerm(n+1);
           return(TRUE);
        }
}
```
Note that again the macro [YAP_PRESERVED_DATA](@ref YAP_PRESERVED_DATA) is used at the beginning of the function to access the data preserved from the previous solution. Then it checks if the last solution was found and in that case exits with [YAP_cut_succeed](@ref YAP_cut_succeed) in order to cut any further backtracking. If this is not the last solution then we save the value for the next solution in the data structure and exit normally with 1 denoting success. Note also that in any of the two cases we use the function YAP_unify to bind the argument of the call to the value saved in n100_state-\>next_solution.

Note also that the only correct way to signal failure in a backtrackable predicate is to use the [YAP_cut_fail](@ref YAP_cut_fail) macro.

Backtrackable predicates should be declared to YAP, in a way similar to what happened with deterministic ones, but using instead a call to
In this example, we would have something like
```
void
init_n100(void)
{
  YAP_UserBackCutCPredicate("n100", start_n100, continue_n100, cut_n100, 1, 1);
}
```
The argument before last is the predicate's arity. Notice again the last argument to the call. function argument gives the extra space we want to use for PRESERVED_DATA. Space is given in cells, where a cell is the same size as a pointer. The garbage collector has access to this space, hence users should use it either to store terms or to store pointers to objects outside the stacks.

The code for cut_n100 could be:
```
static int cut_n100(void)
{
  YAP_PRESERVED_DATA_CUT(n100_data,n100_data_type*);

  fprintf("n100 cut with counter %ld\n",
YAP_IntOfTerm(n100_data->next_solution));
  return TRUE;
}
```
Notice that we have to use [YAP_PRESERVED_DATA_CUT](@ref YAP_PRESERVED_DATA_CUT): this is because the Prolog engine is at a different state during cut.

If no work is required at cut, we can use:
```
void
init_n100(void)
{
  YAP_UserBackCutCPredicate("n100", start_n100, continue_n100, NULL, 1, 1);
}
```
in this case no code is executed at cut time.

Changes to the C-Interface in YAP4

YAP4 includes several changes over the previous load_foreign_files/3 interface. These changes were required to support the new binary code formats, such as ELF used in Solaris2 and Linux.
```
+ All Names of YAP objects now start with  _YAP__. This is
  designed to avoid clashes with other code. Use `YAPInterface.h` to
  take advantage of the new interface. `c_interface.h` is still
  available if you cannot port the code to the new interface.

+ Access to elements in the new interface always goes through
<em>functions</em>. This includes access to the argument registers,
`YAP_ARG1` to `YAP_ARG16`. This change breaks code such as
`unify(\&ARG1,\&t)`, which is nowadays:
```
```
{
   YAP_Unify(ARG1, t);
}
```
```
+ `cut_fail()` and `cut_succeed()` are now functions.

+ The use of `Deref` is deprecated. All functions that return
```
Prolog terms, including the ones that access arguments, already dereference their arguments.
```
+ Space allocated with PRESERVE_DATA is ignored by garbage
```
collection and stack shifting. As a result, any pointers to a Prolog stack object, including some terms, may be corrupted after garbage collection or stack shifting. Prolog terms should instead be stored as arguments to the backtrackable procedure.

@defgroup YAPAsLibrary Using YAP as a Library @ingroup c-interface

YAP can be used as a library to be called from other programs. To do so, you must first create the YAP library:
```
make library
make install_library
```
This will install a file libyap.a in LIBDIR and the Prolog headers in INCLUDEDIR. The library contains all the functionality available in YAP, except the foreign function loader and for YAP's startup routines.

To actually use this library you must follow a five step process:
The next program shows how to use this system. We assume the saved program contains two facts for the procedure b:
```
#include "YAP/YAPInterface.h"
#include <stdio.h>

int
main(int argc, char *argv[]) {
  if (YAP_FastInit("saved_state") == YAP_BOOT_ERROR)
    exit(1);
  if (YAP_RunGoal(YAP_MkAtomTerm(YAP_LookupAtom("do")))) {
    printf("Success\n");
    while (YAP_RestartGoal())
      printf("Success\n");
  }
  printf("NO\n");
}
```
The program first initializes YAP, calls the query for the first time and succeeds, and then backtracks twice. The first time backtracking succeeds, the second it fails and exits.

To compile this program it should be sufficient to do:
```
cc -o exem -I../YAP4.3.0 test.c -lYAP -lreadline -lm
```
You may need to adjust the libraries and library paths depending on the Operating System and your installation of YAP.

Note that YAP4.3.0 provides the first version of the interface. The interface may change and improve in the future.

The following C-functions are available from YAP:
```
+ YAP_CompileClause(`YAP_Term`  _Clause_)
```
Compile the Prolog term Clause and assert it as the last clause for the corresponding procedure.
```
+ YAP_MkExo(`YAP_PredEntryPtr` _pred_, `size_t` _sz_, `void *` _uid_)
```
Predicate pred is an exo-predicate that needs sz bytes of contiguous storage. If uid is non-null associate user-defined code with pred.
```
+ YAP_AssertTuples(`YAP_PredEntryPtr` pred, `const YAP_Term *`  _Facts_,
```
size_t nb) Add the array of nb Prolog term Facts to the table Predicate.
```
+ `int` YAP_ContinueGoal(`void`)
```
Continue execution from the point where it stopped.
```
+ `void` YAP_Error(`int`  _ID_,`YAP_Term`  _Cause_,`char \*`
```
error_description) Generate an YAP System Error with description given by the string error_description. ID is the error ID, if known, or 0. Cause is the term that caused the crash.
```
+ `void` YAP_Exit(`int`  _exit_code_)
```
Exit YAP immediately. The argument exit_code gives the error code and is supposed to be 0 after successful execution in Unix and Unix-like systems.
```
+ `YAP_Term` YAP_GetValue(`Atom`  _at_)
```
Return the term value associated with the atom at. If no such term exists the function will return the empty list.
```
+ YAP_FastInit(`char \*`  _SavedState_)
```
Initialize a copy of YAP from SavedState. The copy is monolithic and currently must be loaded at the same address where it was saved. YAP_FastInit is a simpler version of YAP_Init.
```
+ YAP_Init( _InitInfo_)
```
Initialize YAP. The arguments are in a C structure of type YAP_init_args.

The fields of InitInfo are char \* SavedState, int HeapSize, int StackSize, int TrailSize, int NumberofWorkers, int SchedulerLoop, int DelayedReleaseLoad, int argc, char \*\* argv, int ErrorNo, and char \* ErrorCause. The function returns an integer, which indicates the current status. If the result is YAP_BOOT_ERROR booting failed.

If SavedState is not NULL, try to open and restore the file SavedState. Initially YAP will search in the current directory. If the saved state does not exist in the current directory YAP will use either the default library directory or the directory given by the environment variable YAPLIBDIR. Note that currently the saved state must be loaded at the same address where it was saved.

If HeapSize is different from 0 use HeapSize as the minimum size of the Heap (or code space). If StackSize is different from 0 use HeapSize as the minimum size for the Stacks. If TrailSize is different from 0 use TrailSize as the minimum size for the Trails.

The NumberofWorkers, NumberofWorkers, and DelayedReleaseLoad are only of interest to the or-parallel system.

The argument count argc and string of arguments argv arguments are to be passed to user programs as the arguments used to call YAP.

If booting failed you may consult ErrorNo and ErrorCause for the cause of the error, or call YAP_Error(ErrorNo,0L,ErrorCause) to do default processing.
```
+ `void` YAP_PutValue(`Atom`  _at_, `YAP_Term`  _value_)
```
Associate the term value with the atom at. The term value must be a constant. This functionality is used by YAP as a simple way for controlling and communicating with the Prolog run-time.
```
+ `YAP_Term` YAP_Read(`IOSTREAM \*Stream`)
```
Parse a Term from the stream Stream.
```
+ `YAP_Term` YAP_Write(`YAP_Term`  _t_)
```
Copy a Term t and all associated constraints. May call the garbage collector and returns 0L on error (such as no space being available).
```
+ `void` YAP_Write(`YAP_Term`  _t_, `IOSTREAM`  _stream_, `int`  _flags_)
```
Write a Term t using the stream stream to output characters. The term is written according to a mask of the following flags in the flag argument: YAP_WRITE_QUOTED, YAP_WRITE_HANDLE_VARS, YAP_WRITE_USE_PORTRAY, and YAP_WRITE_IGNORE_OPS.
```
+ `int` YAP_WriteBuffer(`YAP_Term`  _t_, `char \*`  _buff_, `size_t`
```
size, int flags) Write a YAP_Term t to buffer buff with size size. The term is written according to a mask of the following flags in the flag argument: YAP_WRITE_QUOTED, YAP_WRITE_HANDLE_VARS, YAP_WRITE_USE_PORTRAY, and YAP_WRITE_IGNORE_OPS. The function will fail if it does not have enough space in the buffer.
```
+ `char \*` YAP_WriteDynamicBuffer(`YAP_Term`  _t_, `char \*`  _buff_,
```
size_t size, size_t *lengthp, size_t *encodingp, int flags) Write a YAP_Term t to buffer buff with size size. The code will allocate an extra buffer if buff is NULL or if buffer does not have enough room. The variable lengthp is assigned the size of the resulting buffer, and encodingp will receive the type of encoding (currently only PL_ENC_ISO_LATIN_1 and PL_ENC_WCHAR are supported)
```
+ `void` YAP_InitConsult(`int`  _mode_, `char \*`  _filename_)
```
Enter consult mode on file filename. This mode maintains a few data-structures internally, for instance to know whether a predicate before or not. It is still possible to execute goals in consult mode.

If mode is TRUE the file will be reconsulted, otherwise just consulted. In practice, this function is most useful for bootstrapping Prolog, as otherwise one may call the Prolog predicate compile/1 or consult/1 to do compilation.

Note that it is up to the user to open the file filename. The YAP_InitConsult function only uses the file name for internal bookkeeping.
```
+ `void` YAP_EndConsult(`void`)
```
Finish consult mode.

Some observations:
```
+ The system will core dump if you try to load the saved state in a
```
different address from where it was made. This may be a problem if your program uses mmap. This problem will be addressed in future versions of YAP.
```
+ Currently, the YAP library will pollute the name
```
space for your program.
```
+ The initial library includes the complete YAP system. In
```
the future we plan to split this library into several smaller libraries (e.g. if you do not want to perform Input/Output).
```
+ You can generate your own saved states. Look at  the
```
boot.yap and init.yap files.

48 KiB Raw Blame History

The Foreign Code Interface

Terms

Unification

Module Manipulation in C

Changes to the C-Interface in YAP4

48 KiB

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