glFenceSync

glFenceSync: man4/glFenceSync.xml

glFenceSync creates a new fence sync object, inserts a fence command into the GL command stream and associates it with that sync object, and returns a non-zero name corresponding to the sync object. When the specified condition of the sync object is satisfied by the fence command, the sync object is signaled by the GL, causing any glWaitSync, glClientWaitSync commands blocking in sync to. No other state is affected by glFenceSync or by the execution of the associated fence command. condition must be GL_SYNC_GPU_COMMANDS_COMPLETE. This condition is satisfied by completion of the fence command corresponding to the sync object and all preceding commands in the same command stream. The sync object will not be signaled until all effects from these commands on GL client and server state and the framebuffer are fully realized. Note that completion of the fence command occurs once the state of the corresponding sync object has been changed, but commands waiting on that sync object may not be unblocked until after the fence command completes.

glFenceSync is only supported if the GL version is 3.2 or greater, or if the ARB_sync extension is supported.