I want to add plugin suport to an network app i am developing,

the easiest way of doing this is passing the internal objects as parameters to funcions inside libraries (.so or .dll)

something like

procedure plugin(const parameter : tobject): tobject; external;

is it allowed ?

or if i cant use objects, can i pass ansistrings ? (i can serialize deserialize the object internal states)

i know that passing ansistrings or tobjects would make the plugin system compatible with pascal only, but i dont care about that.

thanks in advance

is it allowed ?

Yes, but watch out that is/as operator might not work as expected.

can i pass ansistrings ?

Yes, but only if you know what you're doing. NEVER modify the passed strings (it would confuse the reference counting). Use const parameter all the time.

i know that passing ansistrings or tobjects would make the plugin system compatible with pascal only

Perhaps even worse, to FPC only.

leledumbo wrote

> can i pass ansistrings ? Yes, but only if you know what you're doing. NEVER modify the passed strings (it would confuse the reference counting). Use const parameter all the time.

Or, alternatively, use the "cmem" unit in both the application and the plugin (or the "sharemem" unit under Windows).

Jonas

It will almost certainly lead to memory leaks, because a simple a:=b;
inside the library with b passed on from the main program, will lead to problems as soon as the library is unloaded.

This is also true for strings that are properties of objects.

Michael.

It shouldn't matter with cmem/sharemem, because
a) unloading a library from within one of its own routines is not possible, so there cannot be any local variables in the shared libraries holding a reference at such a point b) global variables are finalised when the library is unloaded

Jonas

This is correct. I didn't know about sharemem, though. I should document it :-)

Michael.

this seems to be something that i'm learning about... it appears to be a bit(???) different than what i am used to with TP/BP 6/7...

in this case, does a const parameter point to the original or a copy of the original?

Read the docs. If the size of the argument is greater than native pointer size, it would be passed by reference. Otherwise, it's passed by value. A new constref modifier would always pass it by reference.

leledumbo wrote

Read the docs. If the size of the argument is greater than native pointer size, it would be passed by reference.

If the docs say that somewhere, they are wrong. How a "const" parameter is passed is an implementation detail (which hence could change at any time), except for "stdcall" routines on Windows platforms (for COM compatibility).

The only thing 'const' means is that the programmer tells to the compiler that this parameter will not change inside the called routine (if it nevertheless does, the generated code may be invalid). Based on that information, the compiler can perform additional checks (catching direct assignments to the parameter inside the routine) and optimizations (passing it by value/reference depending on the size and target platform, not performing reference count adjustments, ...).

Jonas

I have adapted the docs.

Michael.