Re: about the magic(?) of palloc() and variable-length user-defined data type

"Seung Hyun Jeong" <jeongs@cs.man.ac.uk> writes:
> In the manual, I found this example:
> typedef struct {
>    int4 length;
>    char data[1];
> } text;

> 1) then, my first question is about the example for coding as follows:
> char buffer[40];
> ....
> text *destination = (text *) palloc(VARHDRSZ + 40);
> ....

> I cannot understand this mechanism... it looks like magic to me.

No, it's simply relying on the fact that C doesn't check array
subscripts.  Given the stated declaration for struct text, we can
access data[0], or we can (try to) access data[1], data[2], etc etc.
These latter array elements are off the end of the declared structure;
but if we've allocated sufficient memory, it'll work fine.

This is a very standard C programming trick to get around the language's
lack of explicit variable-sized arrays.  If you haven't seen it before,
you may need to spend more time with an introductory C textbook before
you start trying to make sense of the Postgres internals...

> I tried to see the source code, and found it just call MemoryContextAlloc(),
> but MemoryContextAlloc() just has an empty function body.

Not hardly.  Where are you looking?  MemoryContextAlloc is in
src/backend/utils/mmgr/mcxt.c, and the function pointer it invokes
generally points at AllocSetAlloc in src/backend/utils/mmgr/aset.c.

> But, I have no idea what I need to set to 'internallength' for
> variable-length data types.

You say "variable".

> If I want to make a stand-alone program which call internal functions,
> especially palloc(),
> to which library I need to link my program?

You don't.  There is no library built for the backend, only the server
executable.  It's pretty unclear to me what a standalone program would
want with these functions anyway ... they are of no value except to
something that plans to run inside a server backend process.
        regards, tom lane