SWI-Prolog -- API-function PL_set_prolog

Documentation
- Reference manual
  - Foreign Language Interface
    - The Foreign Include File
      - Miscellaneous
        
        Dealing with Prolog flags from C
        
        PL_set_prolog_flag()
        
        PL_current_prolog_flag()
- Packages

Availability:C-language interface function

int PL_set_prolog_flag(const char *name, int type, ...)

Set/create a Prolog flag from C. name is the name of the affected flag. type is one of the values below, which also dictates the type of the final argument. The function returns TRUE on success and FALSE on failure. This function can be called before PL_initialise(), making the flag available to the Prolog startup code.

PL_BOOL: Create a boolean (true or false) flag. The argument must be an int.
PL_ATOM: Create a flag with an atom as value. The argument must be of type const char *.
PL_INTEGER: Create a flag with an integer as value. The argument must be of type intptr_t *.