SDL_GetDirect3D9AdapterIndex - Man Page

#include <SDL3/SDL_system.h>

int SDL_GetDirect3D9AdapterIndex(SDL_DisplayID displayID);

The returned adapter index can be passed to IDirect3D9::CreateDevice and controls on which monitor a full screen application will appear.

displayID

the instance of the display to query.

Returns the D3D9 adapter index on success or -1 on failure; call SDL_GetError() for more information.

This function is available since SDL 3.2.0.