[fpc-pascal] fp libraries do not like cmem ?
ewald at yellowcouch.org
Fri Feb 14 20:51:53 CET 2014
On 14 Feb 2014, at 20:28, Fred van Stappen wrote:
> >What I have been wondering for some time now (perhaps you wrote it in a
> >mail and I missed it): do you use the same memory manager in the library
> >and in your test program?
> Oops, indeed i forget that point...
> The test now are with cmem only in program, not in library.
> I will test with cmem into library too...
That that is issue number 1: if your program wants to free something that your library allocated, both operations need to be done by the same memory manager.
> >The example here might be extremely over-simplified, but replace
> > `a: Integer` with `mystring: String` and we're roughly at your example.
> Hum, absolutely, it is the same example...
> So, how must i do to use PChar ?
The way I use to pass strings from C/C++ libraries is to allocate a buffer, copy the contents of the string into the buffer and return the pointer to that buffer. The same method can be used here. Also don't forget to provide a second function to free these buffers (in this way your library can have a different memory manager than your application).
So you might try something along the lines of:
Function ConvertToPChar(str: String): PChar;
Result:= Getmem(Length(str) + 1); // Allocating the buffer somewhere on the heap
If Length(str) > 0 Then
Move(str, Result, Length(str)); // The content
Result[l]Length(str)]:= #0; // The null termination
..., where I decided to return `standard` null-terminated strings, since your library is meant to be `universal`.
Next, the procedure to free buffers using the memory manager of the library:
Procedure FreeBuffer(Buffer: Pointer);
That's it for the library part. In your program part, don't forget to free these buffers once they are no longer needed.
Also note that this is a good approach with respect to `universiality`, but if you need to pass a lot of strings to and fro you would be better off with a more pascalish version. The thing that comes to mind here is providing a function in your library that replaces the current memory manager with the one passed to it as an argument. This would allow you to simply return a string.
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