Ternary conditional operator is not shortcut of if-else


Reference can be initialized by ?:, but cannot by if-else.
The same is true for const initialization and initialization list on constructors.
Consider this code snippet:

	int a = 0, b = 1, bCondition = false;
	int & c = bCondition ? a : b;
	c = 2;
	_ASSERT(b == 2);
	(bCondition ? a : b) = 3;
	_ASSERT(b == 3);

The root distinction is that ?: is expression but if-else is statement.

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UDP behind NAT


There are challenges when communicating through NAT.
1. client is behind NAT. It doesn’t know it’s own public IP, which is needed by, e.g. RTSP
The public IP can be learned from STUN server.
2. server is behind NAT. No private-to-public port map in NAT initially.
server talks to STUN to establish the map, and STUN tells client server’s public IP-port.
With STUN, for both restricted and unrestricted NAT, two hosts can talk to each other directly.
TCP is not discussed here, which is more complicated.

A few words on NAT type:
Cone: one private IP-port maps to one public IP-port, when talking to multiple destinations
Symmetric: one private IP-port maps to different public IP-port pairs for different destinations

Start code in PES


There are two types of start code in H.264 PES preceding each NALU: the three byte and four byte version; I call them 001 and 0001 for short.
It has been confusing me for years and I am still not clear of when to use one over the other.
Some say 0001 is used before the first NALU in a coded video sequence. However, based on my test, most NALUs are preceded with 0001, the four byte version, regardless of the NALU’s type and location.
I examined three .h264 files, Party.h264 from my IP camera, Gym2017bshort.h264 from my Nexus phone camera, BengSi.h264 from a flash file.
For all three, vast majority of NALUs are preceded with 0001. Very few are preceded with 001, for example, SEI and first I-slice start with 001.
Sample files:
http://riowing.net/media/Party.h264
http://riowing.net/media/Gym2017bshort.h264
http://riowing.net/media/BengSi.h264

StartCode

Auto start with systemd


Three standards to start a program upon boot:
1. System V aka sysV classic init: up to Ubuntu 9.04
2. Upstart: for Ubuntu 9.10 to 14.10
3. systemd Ubuntu 15.04 and higher
This is Ubuntu 18.04 LTS running on AWS ec2 m5, systemd, using sshd service as example.
Current runlevel is 5, as seen by $runlevel, and it’s mapped to graphical.target as seen by $ls -l /lib/systemd/system/runlevel5.target and can be confirmed by $systemctl get-default
Not sure why it’s defaulted to graphic as this system doesn’t even have graphics installed.
Both symbol links, /etc/systemd/system/sshd.service and /etc/systemd/system/multi-user.target.wants/ssh.service points to /lib/systemd/system/ssh.service
To enable a service, create such a link, which can be achieved by:
sudo systemctl enable sshd.service
How rc.local works:
/lib/systemd/system/rc.local.service -> /lib/systemd/system/rc-local.service
“ExecStart=/etc/rc.local start” is in rc-local.service
There is no symbol link of rc.local in /etc/systemd/ because it’s pulled by /lib/systemd/system-generators/systemd-rc-local-generator automatically, which is binary.