srcQueueFamilyIndex

srcQueueFamilyIndex is a field found in several Vulkan structure types that describe memory or layout transitions, most notably VkImageMemoryBarrier and VkBufferMemoryBarrier. It indicates the source queue family from which ownership of a resource is being transferred. When a queue family owns a resource, its operations are performed by that queue. If the resource must be used by another queue family, a ownership transfer is required; the srcQueueFamilyIndex value specifies which family is handing off the resource.

In practice, srcQueueFamilyIndex is used in conjunction with dstQueueFamilyIndex. For a barrier that signals a transition between queue families, both fields must be set to the respective family indices. If the resource is not undergoing an ownership transfer, both fields are set to VK_QUEUE_FAMILY_IGNORED, meaning the barrier applies only to access masks and layout changes, not queue ownership.

The value should be derived from the Vulkan instance's vkGetPhysicalDeviceQueueFamilyProperties call, which returns the number of available queue families. Using an invalid index will produce a validation error or a GPU fault. When both indices are equal, the barrier is effectively a stand‑alone sync without ownership change.

Typical usage is seen in code that copies data from a compute queue to a graphics queue: the compute queue issues a Write-only barrier with dstQueueFamilyIndex set to the graphics queue, and the graphics queue issues a Read-only barrier with srcQueueFamilyIndex set to the compute queue. The Vulkan validation layers will verify that the indices are correct and that any required access masks are set.