SPUG: NDBM questions

[Colin Meyer]

On Thu, Mar 14, 2002 at 02:47:37PM -0800, Benjamin Franks wrote:
> 
> I have a small NDBM database that is normally written to by a C application.
> As the keys and values stored in an NDBM database are character strings,
> I'm casting ...  something roughly like.
> 
> 	struct custom_struct {
> 	  some complex struct stuff;
> 	};
> 
> 	datum key, val;
> 	struct custom_struct aaa;
> 
> 	key.dptr="bbb";
> 	key.dsize=strlen(key.dptr);
> 
> 	val.dptr=(char *) &aaa;
> 	val.dsize=sizeof(struct custom_struct);
> 
> 	/* db is pointer to DBM already opened */
> 	dbm_store(db, key, val, DBM_REPLACE);
> 
> Now when I read the data from the database at a later time, I use memcpy
> to move from the stored string back to my struct, as in:
> 
> 	val = dbm_fetch(db, key)
> 	memcpy((char *) &aaa, val.dptr, val.dsize);
> 	/* now work on struct aaa */
> 
> Ok, now for the question.  I'm writing a Perl application that reads from
> the same database.  However, if I tie a %hash to the ndbm file and then
> print out the keys and values, the values are garbled.  Is there a Perl
> equivalent to the C memcpy for byte string copies?

Perl doesn't grok C structs directly, so a memcpy in Perl doesn't
make sense.

You could attempt a scheme of digging into the struct with unpack(), but
that would be difficult and probably not very portable.

Another method would be to code a custom XS map so that Perl could
understand your particular custom_struct. The problem here is the XS
learning curve, which isn't entirely easy.

I think that Inline::C is appropriate for your problem. You can use your
same C code for accessing the NDBM file, and provide some "accessor"
functions to be made visible to the Perl side.

Take a look at the Inline::C-Cookbook manpage, and search for "Object
Oriented Inline" for some example code. I think that this is very much
what you are looking for.  

> 
> One more question.  Let's say I open a handle to this database (or a file
> in general) in a parent and fork off children.  The children inherit the
> open handle.  Now, if the children close the handle explicitly, does that
> mean it closed for the parent as well?  Or will the parent handle remain
> open?

A child may close a filehandle, and the same filehandle remains open in
the parent.  

Have fun,
-C.

> 
> Thanks,
> --Ben
> 
> 
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