ble term. Other strings are more volatile. Several functions provide a BUF_* flag that can be set to either BUF_STACK (default) or BUF_MALLOC. Strings returned by a function accepting BUF_MALLOCmust be freed using PL_free(). Strings returned using BUF_STACK are pushed on a stack that is cleared when a foreign predicate returns control back to Prolog. More fine grained control may be needed if functions that return strings are called outside the context of a foreign predicate or a foreign predicate creates many strings during its execution. Temporary strings are scoped using these macros:

void PL_STRINGS_MARK()

void PL_STRINGS_RELEASE()

These macros must be paired and create a C block ({...}). Any string created using BUF_STACK after PL_STRINGS_MARK() is released by the corresponding PL_STRINGS_RELEASE(). These macros should be used like below

  ...
  PL_STRINGS_MARK();
  <operations involving strings>
  PL_STRINGS_RELEASE();
  ...

The Prolog flag string_stack_tripwire may be used to set a tripwire to help finding places where scoping strings may help reducing resources.

12.4.14 Foreign Code and Modules

Modules are identified via a unique handle. The following functions are available to query and manipulate modules.

module_t PL_context()

Return the module identifier of the context module of the currently active foreign predicate. If there is no currently active predicate it returns a handle to the user module.

int PL_strip_module(term_t +raw, module_t *m, term_t -plain)

Utility function. If raw is a term, possibly holding the module construct <module>:<rest>, this function will make plain a reference to <rest> and fill module * with <module>. For further nested module constructs the innermost module is returned via module *. If raw is not a module construct, raw will simply be put in plain. The value pointed to by m must be initialized before calling PL_strip_module(), either to the default module or to NULL. A NULL value is replaced by the current context module if raw carries no module. The following example shows how to obtain the plain term and module if the default module is the user module:

{ module m = PL_new_module(PL_new_atom("user"));
  term_t plain = PL_new_term_ref();

  PL_strip_module(term, &m, plain);
  ...
}

atom_t PL_module_name(module_t module)

Return the name of module as an atom.

module_t PL_new_module(atom_t name)

Find an existing module or create a new module with the name name. Currently aborts the process with a fatal error on failure. Future versions may raise a resource exception and return (module_t)0.

12.4.15 Prolog exceptions in foreign code

This section discusses PL_exception() and PL_raise_exception(), the interface functions to detect and generate Prolog exceptions from C code. PL_raise_exception() from the C interface registers the exception term and returns FALSE. If a foreign predicate returns FALSE, while an exception term is registered, a Prolog exception will be raised by the virtual machine. This implies for a foreign function that implements a predicate and wishes to raise an exception, the function shall call PL_raise_exception(), perform any necessary cleanup and return the return code of PL_raise_exception() or explicitly FALSE. Calling PL_raise_exception() outside the context of a function implementing a foreign predicate results in undefined behaviour.

Note that many of the C API functions may call PL_raise_exception() and return FALSE. The user shall test for this, cleanup and make the foreign function return FALSE.

PL_exception() may be used to inspect the currently registered exception. It is normally called after a call to PL_next_solution() returns FALSE, and returns a term reference to an exception term if an exception is pending, and (term_t)0 otherwise. It may also be called after, e.g.,