pcap_next_ex still have timeout even when pcap_set_immediate_mode

Describe the bug

In my test program, I opened a TAP adapter using pcap_create(), and pcap_set_timeout() to set timeout which should have no effect due to the following immediate mode, and pcap_set_immediate_mode(), then pcap_activate().

But when I use pcap_next_ex() to poll a received packet, there is still a timeout waitting in immediate mode. I expect it should return immediately, even if no packets are available.

When I set timeout to -1 which is not allowed in the document, the pcap_next_ex() become immediate. But the side effect is pcap_loop() thread takes too much CPU.

I didn't mixed pcap_next_ex() and pcap_loop() in one program, just init two ways with same configuration.

Diagnostic information

• Windows 11 Version 23H2, OS Build 22631.4037)
• Npcap 1.79
• TAP-Windows 9.9.2

Reproduce the report on Windows 11 with Npcap 1.79 using pcap_create(), pcap_set_timeout(), pcap_set_immediate_mode(), pcap_activate(), and pcap_next_ex(). Trace how immediate mode and timeout settings are handled for TAP adapters, comparing pcap_next_ex() with pcap_loop(). Done means the documented configuration returns promptly when no packet is available without causing excessive CPU use in the loop case.