Procedures
A primitive procedure is a Scheme-callable procedure that is implemented in C. Primitive procedures are created in MzScheme with the function scheme_make_prim_w_arity, which takes a C function pointer, the name of the primitive, and information about the number of Scheme arguments that it takes; it returns a Scheme procedure value.
The C function implementing the procedure must take two arguments: an
integer that specifies the number of arguments passed to the
procedure, and an array of Scheme_Object * arguments. The
number of arguments passed to the function will be checked using the
arity information. (The arity information provided to
scheme_make_prim_w_arity is also used for the Scheme
arity procedure.) The procedure implementation is not allowed
to mutate the input array of arguments, although it may mutate the
arguments themselves when appropriate (e.g., a fill in a vector
argument).
The function scheme_make_closed_prim_w_arity is similar to scheme_make_prim_w_arity, but it takes an additional void * argument; this argument is passed back to the C function when the closure is invoked. In this way, closure-like data from the C world can be associated with the primitive procedure.
To work well with MzScheme threads, a C function that performs substantial or unbounded work should occasionally call SCHEME_USE_FUEL; see section 8.2 for details.
5.1 Library Functions
¤ Scheme_Object *scheme_make_prim_w_arity(Scheme_Prim *prim,
char *name,
int mina, int maxa)
Creates a primitive procedure value, given the C function pointer
prim. The form of prim is defined by:
typedef Scheme_Object *(*Scheme_Prim)(int argc, Scheme_Object **argv);
The value mina should be the minimum number of arguments that
must be supplied to the procedure. The value maxa should be the
maximum number of arguments that can be supplied to the procedure, or
-1 if the procedure can take arbitrarily many arguments. The
mina and maxa values are used for automatically checking
the argument count before the primitive is invoked, and also for the
Scheme arity procedure. The name argument is used to
report application arity errors at run-time.
¤ Scheme_Object *scheme_make_folding_prim(Scheme_Prim *prim,
char *name,
int mina, int maxa, short folding)
Like scheme_make_prim_w_arity, but if folding is
non-zero, the compiler assumes that an application of the procedure
to constant values can be folded to a constant. For example, +,
zero?, and string-length are folding primitives, but
display and cons are not.
¤ Scheme_Object *scheme_make_prim(Scheme_Prim *prim)
Same as scheme_make_prim_w_arity, but the arity (0, -1) and the name ``UNKNOWN'' is assumed. This function is provided for backward compatibility only.
¤ Scheme_Object *scheme_make_closed_prim_w_arity(Scheme_Closed_Prim *prim, void *data,
char *name, int mina, int maxa)
Creates a primitive procedure value; when the C function prim is
invoked, data is passed as the first parameter. The form of
prim is defined by:
typedef Scheme_Object *(*Scheme_Closed_Prim)(void *data, int argc, Scheme_Object **argv);
¤ Scheme_Object *scheme_make_closed_prim(Scheme_Closed_Prim *prim, void *data)
Creates a closed primitive procedure value. This function is provided for backward compatibility only.