what is (*vector) ?

[RocTheEngy]

Greetings c.l.c...

I am trying to understand some structure definitions in working code
(e.g. compiles, runs & produces expected results) that I have. I
think I've trimmed the code to what is relevant to my question...

Which is: What is the (*vector) in the "struct function" declaration.
What is that line doing?

My best guess is that it is a function definition that returns a struct
variable * and takes two paramters; an int (argc) and a pointer to a
struct variable (argv). I'm not quite sure what the (*vector) is ...
other than the pointer to the function itself?

Is that the name of the struct [function] member, because I see code
like fnc_start.vector = null; later in the progam... but i never see it
assigned to a "real" function address (for fnc_start and fnc_end,
anyhow) - So I'm skeptical of my understanding...

Any insight/explaination would be greatly appreciated. Thanks.



#define MAXVARNAMESIZE 32

struct variable
{
char name[ MAXVARNAMESIZE + 1 ];
int type;
char *array;
size_t array_units;
struct variable *next;
};

struct function
{
char name[ MAXVARNAMESIZE + 1 ];
int arguments;
int type;
struct variable * (*vector) ( int argc, struct variable *argv ); /*

function pointer ? */
struct function *next;
};

extern struct function fnc_start, fnc_end;

 

[Kenneth Brody]

RocTheEngy wrote:
[...]

Which is: What is the (*vector) in the "struct function" declaration.
What is that line doing?

My best guess is that it is a function definition that returns a struct
variable * and takes two paramters; an int (argc) and a pointer to a
struct variable (argv). I'm not quite sure what the (*vector) is ...
other than the pointer to the function itself? [...]
struct variable * (*vector) ( int argc, struct variable *argv ); /*
function pointer ? */

[...]

Close. "vector" isn't a function, but rather a pointer to a function,
with the return type and arguments as you described.

--
+-------------------------+--------------------+-----------------------------+
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody/at\spamcop.net | www.fptech.com | #include <std_disclaimer.h> |
+-------------------------+--------------------+-----------------------------+
Don't e-mail me at: <mailto:[email protected]>

 

[Eric Sosman]

RocTheEngy wrote On 10/05/05 16:23,:

Greetings c.l.c...

I am trying to understand some structure definitions in working code
(e.g. compiles, runs & produces expected results) that I have. I
think I've trimmed the code to what is relevant to my question...

Which is: What is the (*vector) in the "struct function" declaration.
What is that line doing?

My best guess is that it is a function definition that returns a struct
variable * and takes two paramters; an int (argc) and a pointer to a
struct variable (argv). I'm not quite sure what the (*vector) is ...
other than the pointer to the function itself?

That's right: the element named `vector' is a pointer.
It doesn't point to data the way an `int*' or `char*' does;
instead it can point to a function. If you point it at a
function of the proper type (that is, at a function that
takes an `int' and a `struct variable*' as arguments and
returns a `struct variable*' result), you can use it to
call that function without actually knowing its name:

ptr2 = fnc_start.vector(5, ptr1);

The program might point `vector' to different functions at
different times (just as a `char*' can point to different
characters at different times), so the line above might
call a different actual function each time it's executed.

Is that the name of the struct [function] member, because I see code
like fnc_start.vector = null; later in the progam... but i never see it
assigned to a "real" function address (for fnc_start and fnc_end,
anyhow) - So I'm skeptical of my understanding...

Observe that `null' is not `NULL'. Perhaps there's a
function named null() somewhere -- given the suggestive name,
I'd imagine it would be a rather short function, easily
overlooked.

Any insight/explaination would be greatly appreciated. Thanks.



#define MAXVARNAMESIZE 32

struct variable
{
char name[ MAXVARNAMESIZE + 1 ];
int type;
char *array;
size_t array_units;
struct variable *next;
};

struct function
{
char name[ MAXVARNAMESIZE + 1 ];
int arguments;
int type;
struct variable * (*vector) ( int argc, struct variable *argv ); /*

function pointer ? */
struct function *next;
};

extern struct function fnc_start, fnc_end;

It looks like the program intends to set up a linked list
of these structs, probably with fnc_start at the head and
fnc_end at the tail. Then (I'd guess) it might insert other
`struct function' instances at various spots in between them,
probably with their `vector' elements pointing at assorted
functions that might be scattered all over the place.

This sort of thing -- not necessarily with this exact
data structure -- is often found in "frameworks," where a
piece of the program controls the execution of other pieces
it might not even know about when it's compiled. If there's
a loop in the "framework" that walks along a list of these
structs and calls all the functions they point to, then a
previously unknown sub-system can "install" itself by adding
an appropriate struct to the list. The framework doesn't need
to know anything about the new sub-system; it just calls
everything that has been "registered" in the list.

 

[RocTheEngy]

hope this posts correctly (trying to use the 'correct' google
instructions found in other threads...)

RocTheEngy wrote On 10/05/05 16:23,:

Greetings c.l.c...

[snip]

My best guess is that it is a function definition that returns a struct
variable * and takes two paramters; an int (argc) and a pointer to a
struct variable (argv). I'm not quite sure what the (*vector) is ...
other than the pointer to the function itself?

That's right: the element named `vector' is a pointer.
It doesn't point to data the way an `int*' or `char*' does;
instead it can point to a function. If you point it at a
function of the proper type (that is, at a function that
takes an `int' and a `struct variable*' as arguments and
returns a `struct variable*' result), you can use it to
call that function without actually knowing its name:

ptr2 = fnc_start.vector(5, ptr1);

So "vector" also becomes the name of the member to reference... okay
that's part of my confusion. Clearer.

The program might point `vector' to different functions at
different times (just as a `char*' can point to different
characters at different times), so the line above might
call a different actual function each time it's executed.

Is that the name of the struct [function] member, because I see code
like fnc_start.vector = null; later in the progam... but i never see it
assigned to a "real" function address (for fnc_start and fnc_end,
anyhow) - So I'm skeptical of my understanding...

Observe that `null' is not `NULL'. Perhaps there's a
function named null() somewhere -- given the suggestive name,
I'd imagine it would be a rather short function, easily
overlooked.

You're right. I goofed when I searched for the varnames in the code
and incorrectly wrote "null" when I posted. Your assumption is
correct, because now that I paid more attention I see:

fnc_end.vector = fnc_null; /* in a few places */

/* fnc_null code: */
struct variable *fnc_null( int argc, struct variable *argv )
{
static struct variable nvar;
static int init = FALSE;

/* But when is this NOT FALSE? */
if ( init == FALSE )
{
init = TRUE;
var_make( &nvar, INTEGER ); }

return &nvar; /* return the address of a local var? */
/* i have to read up on 'static' i guess... */
}
/* end fnc_null code */

....

/* elsewhere in the code */
f->vector = NULL; /* an example of what I goofed up and you corrected
*/

....


[snip]

It looks like the program intends to set up a linked list
of these structs, probably with fnc_start at the head and
fnc_end at the tail. Then (I'd guess) it might insert other
`struct function' instances at various spots in between them,
probably with their `vector' elements pointing at assorted
functions that might be scattered all over the place.

This sort of thing -- not necessarily with this exact
data structure -- is often found in "frameworks," where a
piece of the program controls the execution of other pieces
it might not even know about when it's compiled. If there's
a loop in the "framework" that walks along a list of these
structs and calls all the functions they point to, then a
previously unknown sub-system can "install" itself by adding
an appropriate struct to the list. The framework doesn't need
to know anything about the new sub-system; it just calls
everything that has been "registered" in the list.

The code IS an from a language interpreter... and with that
explanation, I bet you're correct again. Probably, as it reads in the
"code" it: tokenizes it, formats it into its internal structs, and then
sets up this data structure to "run" everything once the entire program
is read in. "Run" would essentailly mean walk the linked list...

Wow... I've been reading this code for days (wondering what all the
extra 'overhead' was for) and you picked up on all that from a couple
struct definitions... Now I can take a different approach to
understanding what is going on. lmao... NICE!

Mucho Thanks!

 

[Eric Sosman]

RocTheEngy wrote On 10/05/05 17:31,:

[...]
Wow... I've been reading this code for days (wondering what all the
extra 'overhead' was for) and you picked up on all that from a couple
struct definitions... Now I can take a different approach to
understanding what is going on. lmao... NICE!

I'm expecting to receive the Nobel Prize for Programming
any day now.

Seriously, it's not a matter of genius but of experience.
I've been reading and writing C for almost thirty years, have
seen lots and lots of gadgetry invented by people smarter
than I am, and have learned to recognize some recurring
patterns. Read as much code as you can, learn several different
programming languages, and become proficient in at least two
or three; a broader perspective never hurts.

By the way, not all recurring patterns are good ones.
But even the Frequently Committed Errors are worth becoming
familiar with: not only will you improve your ability to spot
them when debugging somebody else's code, but you'll be more
likely to avoid them in your own.

Here's the acid test: Go back and read some code that you
yourself wrote two or three years earlier. If you can't see
how to improve it, you've ceased to learn and it's time to
trim your hair into points. ;-)

 

[RocTheEngy]

hope this posts correctly (trying to use the 'correct' google
instructions found in other threads...)

[big snip]

The code IS an from a language interpreter... and with that
explanation, I bet you're correct again. Probably, as it reads in the
"code" it: tokenizes it, formats it into its internal structs, and then
sets up this data structure to "run" everything once the entire program
is read in. "Run" would essentailly mean walk the linked list...

It would not be difficult to store the data structure in a file for
later retrieval and "execution". It would be a nice feature to add to
this interpreter... (I realize "something" would have to be done with
the function pointers when they are retrieved... but I think I can
handle that).

Would it be possible to write a "stub" C program and attach the
datafile created above? I'm not sure how to even research this part...
My imagination is that another utility would append the datafile
created above to the end of the "stub" program which contains all the
relevant function code, and have to 'patch' the stub somehow so it knew
the offset of where the data begins. The stub program is esentially
the 'framework' mentioned: it reads itself (from memory or disk?)
skipping over N

 bytes until it arrived at the data structure...
Then, read in the data structure, populating a linked list, and walk
the linked list again...  IIRC, flavors of BASIC, REXX and others did
something like that.

Is there a way/convention to accomplish the above in C without the
post-compile 'patching' of code in the "stub"?  I know I've way over
simplified process (possibly totally off-base), so can someone direct
me to information where I can research this, or do I need to have
intimate knowledge of compilers, linkers, etc... ?