Program works in foreground, but not in background using nohup

Question

I am currently writing a server backend for my iOS game. The server is written in C++ and compiled & run on a remove Ubuntu Server. I start the server through SSH using

sudo nohup ./mygameserver &

The server communications is written in TCP. The main function with the read-loop is written using standard C socket.h/netdb.h/in.h, and uses select() to accept many users on a nonblocking listening socket.

When I run the server in the foreground through SSH, everything seems to be working fine. It receives all packets I send in the right order, and with correct header info. When I use nohup and disconnect from SSH however... Everything seems to crash. A typical log from a user connect when the server software runs without SSH/in nohup mode looks like this:

CONNECTED TO SQL
Server starting...
Second thread initialized
Entering order loop
Got a new message of type: 0
Got a new message of type: 0
Got a new message of type: 0
Got a new message of type: 0
<this line continues ad infinitum>

I really have no idea why. I've made sure every print goes to nohup.out instead of std::cout, and the server sends an update to MySQL every 10 seconds to avoid timeouts.

Any ideas or input on what's wrong here? Let me know if you want some code samples, I just don't know which ones are interesting to this problem in particular. Thanks in advance.

Answer 1

I found out what was wrong.

In my server program I have a readSockets() function which is called after a call to select() in the main server loop. The readSockets() function responded to newline character pushed to stdin (for reasons I don't know) by nohup on startup, and as stdin is, in fact, also a FILE* connected to a file descriptor, my readSockets() function responded to stdin as a connecting client.

This obviously made the server crash, as stdin was never flushed (and was therefore read every time select() had returned). This again blocked the thread for other users.