SDL_GetPrimaryDisplay - Man Page

#include <SDL3/SDL_video.h>

SDL_DisplayID SDL_GetPrimaryDisplay(void);

Returns the instance ID of the primary display on success or 0 on failure; call SDL_GetError() for more information.

This function should only be called on the main thread.

This function is available since SDL 3.2.0.

SDL_GetDisplays(3)