# CGGetOnlineDisplayList(_:_:_:) Provides a list of displays that are online (active, mirrored, or sleeping). ``` func CGGetOnlineDisplayList(_ maxDisplays: UInt32, _ onlineDisplays: UnsafeMutablePointer?, _ displayCount: UnsafeMutablePointer?) -> CGError ``` ## Parameters `maxDisplays` The size of the `onlineDisplays` array. This value determines the maximum number of display IDs that can be returned. `onlineDisplays` A pointer to storage provided by the caller for an array of display IDs. On return, the array contains a list of the online displays. If you pass `NULL`, on return the display count contains the total number of online displays. `displayCount` A pointer to a display count variable provided by the caller. On return, the display count contains the actual number of displays returned in the `onlineDisplays` array. This value is at most `maxDisplays`. ## Return Value A result code. See `Core Graphics Data Types and Constants`. ## Discussion If the framebuffer hardware is connected, a display is considered connected or online. When hardware mirroring is used, a display can be online but not active or drawable. Programs that manipulate display settings (such as gamma tables) need access to all displays, including hardware mirrors, which are not drawable. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)