The plug-in API is defined in install-path/include/slapi-plugin.h. Observe that the header file includes ldap.h, the entry point for the standard and extended LDAP C APIs, and ldap_msg.h, the list of error message identifiers used by Directory Server.
In general, interfaces that Directory Server exposes are specified in install-path/include/. For details about specific features of the API, refer to Part II, Directory Server Plug-In API Reference.
To use the API, include slapi-plugin.h in the declaration section of your plug-in source:
As a rule, use appropriate macros in your Makefile or project file to tell the linker to look for header files in install-path/include/.