Yes, the HCL C API Tookit is available for Download at Flexnet.
https://blog.thomashampel.com/blog/tomcat2000.nsf/dx/developers-new-c-api-toolkit-11.0.1-now-available.htm

It also includes the requested documentation , so I'm closing this idea as shipped.

PS: A new version 12 of the C API Toolkit will be released soon

I just noticed a download "HCL C API Toolkit for Notes/Domino 11.0.1" within the downloads of HCL software. It has been available since Feb 08, 2021.

So it seems this idea is already "implemented" by now.

does anybody have this DB downloaded?

C API Home - http://www.lotus.com/developers/devbase.nsf/homedata/homecapi

C++ API Home - http://www.lotus.com/developers/devbase.nsf/homedata/homecpp

The most convinient way would be to implement more functions (all?) in LotusScript as this is much easier to use and to integrate in existing code ...

Unfortunately the mentioned IBM download links are not working anymore. (The links themselves still work. But if you click your way through until you finally reach the download button, the files cannot be found on the server (with Download director) or an error message about missing permissions appears (with HTTP download). Even if you signed in before using a valid IBM id.).

So, currently there is no chance to get any documentation of any version of the C API?!

I would urge HCL to just make the latest version of the toolkit available for download (probably: C_API_TKT_NOTDOM_901WINEN.zip). Not more, not less. I think this is what most people voting for this idea want and it could be done by HCL without much effort.

http://www.ls2capi.com/web/ls2capi/ls2capihome.nsf/Content/SC_OSLOAD?OpenDocument&ExpandOutline=1.2

From HCL support I received this:

"Currently there is no C API Toolkit available for Domino v10.x and v11.x and there are no plans either."

This is really sad!

This are two ideas in one: 

1. one is about documentation
2. one is about making more C functions public (aka export them) and usable

The documentation is in the downloaded C api package (zip file). Done.

In the comments, a guest wrote, he could download the toolkit.

I found it on this page:

https://developer.ibm.com/collaboration/downloads-trials-code/ 

I clicked the "BM C and C++ Toolkits for Notes and Domino" link (https://www-01.ibm.com/marketing/iwm/iwm/web/preLogin.do?source=ESD-SUPPORT&lang=en_US&S_PKG=CIMW7EN)

After I logged in with my IBM ID I got to this page:

https://www-01.ibm.com/marketing/iwm/iwm/web/pickUrxNew.do?source=ESD-SUPPORT 

where I started the download. Worked.

About making more C functions available (for C/LotusScript/Java programmers): +100

The only "work" involved would be to declare them exported, copy the internal documentation to the public one (all internal functions are documented, aren't they?) and to support them.

This would bring more flexibility, more freedom, more use cases implementable with Notes and Domino.

I'd advise not to use C API from LS. It ties your code to one platform or is very complex to develop and maintain. Calling C API from Java with JNA is - to my experience - much simpler and much less prone to crashes.

@Thomas Hampel

I can open the overview, but there is no way to download the

IBM C API Toolkit for Notes/Domino 9.0.1 Multiplatform English eAssembly

After confirming the continue action I get the page with the message code 40.

The page is working fine, however you need to authenticate. Could it be that your IBMID is not an email address? e.g. if your username does not contain an @ sign, you might encounter an issue?

Broken download link still not fixed :(

@Thomas Hampel:

Sorry to bother you again, but the download link is broken:

Try this more general page instead, and make sure you are using an IBMID where the user name is an email address

https://www-01.ibm.com/marketing/iwm/iwm/web/pickUrxNew.do?source=ESD-SUPPORT

@Thomas Hempel:

Following the given link ends up with this messages ...

Die Verknüpfung, über die Sie hierher gelangt sind, ist ungültig. Laden Sie die vorherige Seite erneut, und versuchen Sie es noch einmal.

Nachrichtencode: 57e

You are looking for the C-API documentation?
See this https://www-01.ibm.com/marketing/iwm/iwm/web/reg/signup.do?source=ESD-SUPPORT&lang=en_US&S_PKG=CRP8NEN 

C-API from Lotus Script can be very complex depending on the plattform and the calls.

I would also prefer to have Lotus Script classes for this functionality that is missing..

Well if they could expose all C-API-Features to the LotusScript-Model, I am not the one to argue that ;)

Isn't it more efficient to create new LS functions instead of forcing us to the C-API?