The S-Lang library provides an interpreter that when embedded into an application, makes the application extensible. Examples of programs that embed the interpreter include the jed editor and the slrn newsreader.
Embedding the interpreter is easy. The hard part is to decide what application specific built-in or intrinsic functions should be provided by the application. The S-Lang library provides some pre-defined intrinsic functions, such as string processing functions, and simple file input-output routines. However, the basic philosophy behind the interpreter is that it is not a standalone program and it derives much of its power from the application that embeds it.
Only one function needs to be called to embed the S-Lang interpreter
into an application: SLang_init_slang. This function
initializes the interpreter's data structures and adds some intrinsic
functions:
if (-1 == SLang_init_slang ())
exit (EXIT_FAILURE);
if ((-1 == SLang_init_slang ()) /* basic interpreter functions */
|| (-1 == SLang_init_slmath ()) /* sin, cos, etc... */
|| (-1 == SLang_init_stdio ()) /* stdio file I/O */
|| (-1 == SLang_init_posix_dir ()) /* mkdir, stat, etc. */
|| (-1 == SLang_init_posix_process ()) /* getpid, umask, etc. */
)
exit (EXIT_FAILURE);
if (-1 == SLang_init_all ())
exit (EXIT_FAILURE);
There are several ways of calling the interpreter. The most common
method used by both jed and slrn is to use the
SLang_load_file function to interprete a file. For example,
jed starts by loading a file called site.sl:
if (-1 == SLang_load_file ("site.sl"))
{
SLang_restart (1);
SLang_Error = 0;
}
SLang_load_file function returns zero upon if successful, or -1
upon failure. The SLang_restart function resets the
interpreter back to its default state; however, it does not reset
SLang_Error to zero. It is up to the application to
re-initialize the SLang_Error variable.
There are several other mechanisms for interacting with the
interpreter. For example, the SLang_load_string function
loads a string into the interpreter and interprets it:
if (-1 == SLang_load_string ("message (\"hello\");"))
{
SLang_restart (1);
SLang_Error = 0;
}
Typically, an interactive application will load a file via
SLang_load_file and then go into a loop that consists of
reading lines of input and sending them to the interpreter, e.g.,
while (EOF != fgets (buf, sizeof (buf), stdin))
{
if (-1 == SLang_load_string (buf))
SLang_restart (1);
SLang_Error = 0;
}
Both jed and slrn use another method of interacting with the interpreter. They read key sequences from the keyboard and map those key sequences to interpreter functions via the S-Lang keymap interface.
An intrinsic function is simply a function that is written in C and is made available to the interpreter as a built-in function. For this reason, the words `intrinsic' and `built-in' are often used interchangeably.
Applications are expected to add application specific functions to the interpreter. For example, jed adds nearly 300 editor-specific intrinsic functions. The application designer should think carefully about what intrinsic functions to add to the interpreter.
Intrinsic functions are required to follow a few rules to cooperate with the interpreter.
Intrinsic function must take only pointer arguments. This is because when the interpreter calls an intrinsic function, it passes value to the function by reference and not by value. For example, intrinsic with the declarations:
int intrinsic_0 (void);
int intrinsic_1 (char *s);
void intrinsic_2 (char *s, int *i);
void intrinsic_3 (int *i, double *d, double *e);
int invalid_1 (char *s, int len);
len parameter is not a pointer.
Intrinsic functions can only return void, int,
double, or char *. A function such as
int *invalid (void);
7.
Another restriction is that the intrinsic should regard all its parameters as pointers to constant objects and make no attempt to modify the value to which they point. For example,
void truncate (char *s)
{
s[0] = 0;
}
s.
There are two basic mechanisms for adding an intrinsic function to the
interpreter: SLadd_intrinsic_function and
SLadd_intrin_fun_table. Functions may be added to a specified
namespace via SLns_add_intrinsic_function and
SLns_add_intrin_fun_table functions.
As an specific example, consider a function that will cause the
program to exit via the exit C library function. It is not
possible to make this function an intrinsic because it does not meet
the specifications for an intrinsic function that were described
earlier. However, one can call exit from a function that is
suitable, e.g.,
void intrin_exit (int *code)
{
exit (*code);
}
SLadd_intrinsic_function routine:
if (-1 == SLadd_intrinsic_function ("exit", (FVOID_STAR) intrin_exit,
SLANG_VOID_TYPE, 1,
SLANG_INT_TYPE))
exit (EXIT_FAILURE);
intrin_exit is a function that returns nothing and takes a
single argument: a pointer to an integer (SLANG_INT_TYPE).
A user can call this function from within the interpreter
via
message ("Calling the exit function");
exit (0);
intrin_exit
function to execute, which in turn calls exit.
The most convenient mechanism for adding new intrinsic functions is
to create a table of SLang_Intrin_Fun_Type objects and add the
table via the SLadd_intrin_fun_table function. The table will
look like:
SLang_Intrin_Fun_Type My_Intrinsics [] =
{
/* table entries */
MAKE_INTRINSIC_N(...),
MAKE_INTRINSIC_N(...),
.
.
MAKE_INTRINSIC_N(...),
SLANG_END_TABLE
};
MAKE_INTRINSIC macros defined in slang.h. The main
macro is called MAKE_INTRINSIC_N and takes ?? arguments:
MAKE_INTRINSIC_N(name, funct-ptr, return-type, num-args,
arg-1-type, arg-2-type, ... arg-7-type)
name is the name of the intrinsic function that the
interpreter is to give to the function. func-ptr is a pointer
to the intrinsic function taking num-args and returning
ret-type. The final 7 arguments specifiy the argument
types. For example, the intrin_exit intrinsic described above
may be added to the table using
MAKE_INTRINSIC_N("exit", intrin_exit, SLANG_VOID_TYPE, 1,
SLANG_INT_TYPE, 0,0,0,0,0,0)
While MAKE_INTRINSIC_N is the main macro for constructing
table entries, slang.h defines other macros that may prove
useful. In particular, an entry for the intrin_exit function
may also be created using any of the following forms:
MAKE_INTRINSIC_1("exit", intrin_exit, SLANG_VOID_TYPE, SLANG_INT_TYPE)
MAKE_INTRINSIC_I("exit", intrin_exit, SLANG_VOID_TYPE)
slang.h for related macros. You are also encouraged to
look at, e.g., slang/src/slstd.c for a more extensive examples.
The table may be added via the SLadd_intrin_fun_table
function, e.g.,
if (-1 == SLadd_intrin_fun_table (My_Intrinsics, NULL))
{
/* an error occurred */
}
Earlier it was mentioned that intrinsics may be added to a specified
namespace. To this end, one must first get a pointer to the
namespace via the SLns_create_namespace function. The
following example illustrates how this function is used to add the
My_Intrinsics table to a namespace called my:
SLang_NameSpace_Type *ns = SLns_create_namespace ("my");
if (ns == NULL)
return -1;
return SLns_add_intrin_fun_table (ns, My_Intrinsics, "__MY__"));
The intrinsic functions described in the previous example were functions that took a fixed number of arguments. In this section we explore more complex intrinsics such as those that take a variable number of arguments.
Consider a function that takes two double precision numbers and returns the lesser:
double intrin_min (double *a, double *b)
{
if (*a < *b) return *a;
return *b;
}
MAKE_INTRINSIC_2("min", intrin_min, SLANG_DOUBLE_TYPE,
SLANG_DOUBLE_TYPE, SLANG_DOUBLE_TYPE)
double intrin_min_n (int *num_ptr)
{
double min_value, x;
unsigned int num = (unsigned int) *num_ptr;
if (-1 == SLang_pop_double (&min_value, NULL, NULL))
return 0.0;
num--;
while (num > 0)
{
num--;
if (-1 == SLang_pop_double (&x, NULL, NULL))
return 0.0;
if (x < min_value) min_value = x;
}
return min_value;
}
SLang_pop_double
function. A suitable table entry for it is
MAKE_INTRINSIC_I("min", intrin_min_n, SLANG_DOUBLE_TYPE)
variable xmin = min (x0, x1, x2, x3, x4, 5);
5 values.
The problem with this intrinsic function is that the user must explicitly specify how many numbers to compare. It would be more convenient to simply use
variable xmin = min (x0, x1, x2, x3, x4);
SLang_Num_Function_Args to obtain the necessary information:
double intrin_min (void)
{
double min_value, x;
unsigned int num = SLang_Num_Function_Args;
if (-1 == SLang_pop_double (&min_value, NULL, NULL))
return 0.0;
num--;
while (num > 0)
{
num--;
if (-1 == SLang_pop_double (&x, NULL, NULL))
return 0.0;
if (x < min_value) min_value = x;
}
return min_value;
}
MAKE_INTRINSIC_0("min", intrin_min, SLANG_DOUBLE_TYPE)
It is possible to access an application's global variables from
within the interpreter. The current implementation supports the
access of variables of type int, char *, and
double.
There are two basic methods of making an intrinsic variable
available to the interpreter. The most straight forward method is
to use the function SLadd_intrinsic_variable:
int SLadd_intrinsic_variable (char *name, VOID_STAR addr,
unsigned char data_type,
int read_only);
I is an integer variable, e.g.,
int I;
I_Variable via a
statement such as
if (-1 == SLadd_intrinsic_variable ("I_Variable", &I,
SLANG_INT_TYPE, 0))
exit (EXIT_FAILURE);
S is declared as
char *S;
if (-1 == SLadd_intrinsic_variable ("S_Variable", &S,
SLANG_STRING_TYPE, 1))
exit (EXIT_FAILURE);
S available as a read-only variable with the name
S_Variable. Note that if a pointer variable is made available
to the interpreter, its value is managed by the interpreter and
not the application. For this reason, it is recommended that such
variables be declared as read-only.
It is important to note that if S were declared as an array of
characters, e.g.,
char S[256];
char *S_Ptr = S;
S_Ptr available as a read-only variable.
One should not make the mistake of trying to use the same address for different variables as the following example illustrates:
int do_not_try_this (void)
{
static char *names[3] = {"larry", "curly", "moe"};
unsigned int i;
for (i = 0; i < 3; i++)
{
int value;
if (-1 == SLadd_intrinsic_variable (names[i], (VOID_STAR) &value,
SLANG_INT_TYPE, 1))
return -1;
}
return 0;
}
The most convenient method for adding many intrinsic variables to
the interpreter is to create an array of SLang_Intrin_Var_Type
objects and then add the array via SLadd_intrin_var_table.
For example, the array
static SLang_Intrin_Var_Type Intrin_Vars [] =
{
MAKE_VARIABLE("I_Variable", &I, SLANG_INT_TYPE, 0),
MAKE_VARIABLE("S_Variable", &S_Ptr, SLANG_STRING_TYPE, 1),
SLANG_END_TABLE
};
if (-1 == SLadd_intrin_var_table (Intrin_Vars, NULL))
exit (EXIT_FAILURE);
MAKE_VARIABLE macro correspond to the parameters of the
SLadd_intrinsic_variable function.
Finally, variables may be added to a specific namespace via the SLns_add_intrin_var_table and SLns_add_intrinsic_variable functions.
An aggregate data object is an object that can contain more than one data value. The S-Lang interpreter supports several such objects: arrays, structure, and associative arrays. In the following sections, information about interacting with these objects is given.
An intrinsic function may interact with an array in several different ways. For example, an intrinsic may create an array and return it. The basic functions for manipulating arrays include:
SLang_create_array
SLang_pop_array_of_type
SLang_push_array
SLang_free_array
SLang_get_array_element
SLang_set_array_element
The first example shows how to create an return an array of strings to the interpreter. In particular, the names of the four seasons of the year will be returned:
void months_of_the_year (void)
{
static char *seasons[4] =
{
"Spring", "Summer", "Autumn", "Winter"
};
SLang_Array_Type *at;
int i, four;
four = 4;
at = SLang_create_array (SLANG_STRING_TYPE, 0, NULL, &four, 1);
if (at == NULL)
return;
/* Now set the elements of the array */
for (i = 0; i < 4; i++)
{
if (-1 == SLang_set_array_element (at, &i, &seasons[i]))
{
SLang_free_array (at);
return;
}
}
(void) SLang_push_array (at, 0);
SLang_free_array (at);
}
SLang_create_array function was used to create a 1 dimensional
array of 4 strings. Since this function could fail, its return value
was checked. Then the SLang_set_array_element function was
used to set the elements of the newly created array. Note that the
address containing the value of the array element was passed and not
the value of the array element itself. That is,
SLang_set_array_element (at, &i, seasons[i])
1. When it was pushed, the reference
count was bumped up to 2. Then since it was nolonger needed by
the function, SLang_free_array was called to decrement the
reference count back to 1. For convenience, the second
argument to SLang_push_array determines whether or not it is to
also free the array. So, instead of the two function calls:
(void) SLang_push_array (at, 0);
SLang_free_array (at);
(void) SLang_push_array (at, 1);
The second example returns a diagonal array of a specified size to the stack. A diagonal array is a 2-d array with all elements zero except for those along the diagonal, which have a value of one:
void make_diagonal_array (int n)
{
SLang_Array_Type *at;
int dims[2];
int i, one;
dims[0] = dims[1] = n;
at = SLang_create_array (SLANG_INT_TYPE, 0, NULL, dims, 2);
if (at == NULL)
return;
one = 1;
for (i = 0; i < n; i++)
{
dims[0] = dims[1] = i;
if (-1 == SLang_set_array_element (at, dims, &one))
{
SLang_free_array (at);
return;
}
}
(void) SLang_push_array (at, 1);
}
Now consider an example that acts upon an existing array. In particular, consider one that computes the trace of a 2-d matrix, i.e., the sum of the diagonal elements:
double compute_trace (void)
{
SLang_Array_Type *at;
double trace;
int dims[2];
if (-1 == SLang_pop_array_of_type (&at, SLANG_DOUBLE_TYPE))
return 0.0;
/* We want a 2-d square matrix. If the matrix is 1-d and has only one
element, then return that element. */
trace = 0.0;
if (((at->num_dims == 1) && (at->dims[0] == 1))
|| ((at->num_dims == 2) && (at->dims[0] == at->dims[1])))
{
double dtrace;
int n = at->dims[0];
for (i = 0; i < n; i++)
{
dims[0] = dims[1] = i;
(void) SLang_get_array_element (at, &dims, &dtrace);
trace += dtrace;
}
}
else SLang_verror (SL_TYPE_MISMATCH, "Expecting a square matrix");
SLang_free_array (at);
return trace;
}
SLang_pop_array_of_type was used to pop an
array of doubles from the stack. This function will make implicit
typecasts in order to return an array of the requested type.
For the purposes of this section, we shall differentiate structures according to whether or not they correspond to an application defined C structure. Those that do are called intrinsic structures, and those do not are called S-Lang interpreter structures.
The following simple example shows how to create and return a structure to the stack with a string an integer field:
int push_struct_example (char *string_value, int int_value)
{
char *field_names[2];
unsigned char field_types[2];
VOID_STAR field_values[2];
field_names[0] = "string_field";
field_types[0] = SLANG_STRING_TYPE;
field_values[0] = &string_value;
field_names[1] = "int_field";
field_types[1] = SLANG_INT_TYPE;
field_values[1] = &int_value;
if (-1 == SLstruct_create_struct (2, field_names,
field_types, field_values))
return -1;
return 0;
}
SLstruct_create_struct was used to push a
structure with the specified field names and values onto the
interpreter's stack.
Here we show how to make intrinsic structures available to the interpreter. The simplest interface is to structure pointers and not to the actual structures themselves. The latter would require the interpreter to be involved with the creation and destruction of the structures. Dealing with the pointers themselves is far simpler.
As an example, consider an object such as
typedef struct _Window_Type
{
char *title;
int row;
int col;
int width;
int height;
} Window_Type;
width,
height), and location (row, col).
We can make variables of type Window_Type available to the
interpreter via a table as follows:
static SLang_IStruct_Field_Type Window_Type_Field_Table [] =
{
MAKE_ISTRUCT_FIELD(Window_Type, title, "title", SLANG_STRING_TYPE, 1),
MAKE_ISTRUCT_FIELD(Window_Type, row, "row", SLANG_INT_TYPE, 0),
MAKE_ISTRUCT_FIELD(Window_Type, col, "col", SLANG_INT_TYPE, 0),
MAKE_ISTRUCT_FIELD(Window_Type, width, "width", SLANG_INT_TYPE, 0),
MAKE_ISTRUCT_FIELD(Window_Type, height, "height", SLANG_INT_TYPE, 0),
SLANG_END_TABLE
};
Window_Type structure.
Here, the title has been declared as a read-only field. Using
MAKE_ISTRUCT_FIELD(Window_Type, title, "title", SLANG_STRING_TYPE, 0),
Now suppose that My_Window is a pointer to a Window_Type
object, i.e.,
Window_Type *My_Window;
SLadd_istruct_table function:
if (-1 == SLadd_istruct_table (Window_Type_Field_Table,
(VOID_STAR) &My_Window,
"My_Window"))
exit (1);
My_Win whose value
corresponds to to the My_Win structure. This would permit one to
access the fields of My_Window via S-Lang statements such as
define set_width_and_height (w,h)
{
My_Win.width = w;
My_Win.height = h;
}
It is extremely important to understand that the interface described in
this section does not allow the interpreter to create new instances of
Window_Type objects. The interface merely defines an association or
correspondence between an intrinsic structure pointer and a S-Lang
variable. For example, if the value of My_Window is NULL, then
My_Win would also be NULL.
One should be careful in allowing read/write access to character string
fields. If read/write access is allowed, then the application should
always use the SLang_create_slstring and SLang_free_slstring
functions to set the character string field of the structure. Finally,
note that access to character array fields is not permitted via this
interface. That is, a structure such as
typedef struct
{
char name[32];
}
Name_Type;
char name[32] is not a
SLANG_STRING_TYPE object.