I was working on a C++ daemon process that communicated over a TCP socket. At the time, we were using the Poco library's facilities to do the standard daemon s

I was working on a C++ daemon process that communicated over a TCP socket. At the time, we were using the Poco library's facilities to do the standard daemon startup stuff (get rid of the controlling pty, point standard fds to /dev/null, etc). Anyway, one of our field installations wasn't working, so I took a look. It turned out that the communications over the TCP socket weren't working -- where the client process expected a few header bytes containing the message length, it was getting wacky values. I tried a bunch of stuff, and in the end, I displayed the header as ASCII, and it showed up as "SQL: INS". This blew me away; this looked like some debugging output that normally goes to standard output when the process wasn't running in daemon mode. As it turns out, the Poco library didn't read Steven's UNIX book all that closely, and they closed all of the file descriptors when turning a process into a daemon, instead of reopening them to point to /dev/null. So, standard output was closed, and its file descriptor was reused for the TCP socket. Of course, things like "cout" always assume that standard output is at a particular descriptor, so all the standard output from the program was getting written to the TCP socket. Boy, that was confusing.