As I read the article "What Is Multi-Threading?" in the February issue of LJ my mind went back a couple of months ago to the time I decided it would be fun to write a multi-threaded FTP daemon to replace the wu-ftpd we were using on a very heavily hit FTP server. As the author explains in his article, threads make a lot of sense for server applications. Just the memory savings on 250 copies of the FTP daemon makes it all worth investigating. BUT, just as you were about to go out and make all of you favorite server applications multi-threaded, I thought a couple of notes from my project might come in handy.
First, if you plan on allowing a high number of concurrent connections to your server, a single multi-threaded process will not do. Most OS's, Linux included limit the number of file descriptors a process is allowed to have open at any one time. You can usually use getrlimit() and setrlimit() to give your process the maximum number of file descriptors allowed, rather than the default (usually 64), but, even still most operating system (NOFILE) hard limits are set to 1024. In the case of an FTP server you must keep in mind that you will need at least three file descriptors for every client connection. (1 for commands, 1 for file transfers, and 1 to open the file or directory listing to transfer.) This quickly adds up. Supporting 500 concurrent connections would require an absolute minimum of 1500 descriptors, and that is not even counting the ones you need just to get up and running (like the socket used to listen for incoming connections.) The best way I have found to solve this problem is to fork() a predetermined number of child processes that all accept file descriptors that are passed from the parent and then create a thread to handle the incoming descriptor/connection. On Linux you would use the proc filesystem to pass the descriptor. On other OS's such as Solaris (that support Streams) you would use ioctl() with the I_SENDFD and I_RECVFD functions.
This has another advantage as well. In addition to accepting file descriptors from the parent process which is listening for connections on port n, you can now receive connections from any process that chooses to pass clients on to your multi-threaded server through a named pipe. A good example might be a small appliction that is started by inetd and then decides (by say IP address) whether to pass your connection to the multi-threaded server or to the standard ftpd. (This was useful in my case, since our ftpd was for anonymous FTP only. The daemon did not support any functions unneccesary for typical anonymous FTP such as chmod or delete. On the otherhand, we wanted employees of the company to be able to do just that while still logging in as anonymous. So, if you came from an IP address that we knew was ours, the inetd application exec()'d ftpd after clearing the close-on-exec flag. If you came from the outside world you went directly to the multi-threaded FTP daemon which also limited your access beyond what the file system already provided.)
Just when you finally think you have out smarted the file descriptor problem, here comes another one: fopen(). The standard i/o fuctions like fopen(), fprintf(), fgets(), etc., are extremely useful when working with a command driven application like FTP. Unfortunately the fileno element of the FILE struct is usually defined as an unsigned char. Simply put, once you have more than 255 open file descriptors in a single process you can no longer reliably use fopen(), fprintf(), etc. The solution here: don't use these functions. Instead use open(), read(), write(), etc. A possible second solution is to make sure you have enough child processes accepting file descriptors to keep each process from exceeding the 255 limit.
If you choose to write such a multi-threaded server, you will also have to deal with the possibility of concurrent threads in multiple processes accessing a delicate resource. (i.e. even something as simple as a global count of the number of concurrent connections.) In this case you will still want to use a Mutex to protect data, but, the mutex will need to mmap()'d by all child processes, so that a lock in thread A in process 1 will also block thread C in process 2. In the case of a resource such as a "current user count" you will want that variable to be included in the mmap()'ing anyway.
Aside from all of this, threads really are fun. Threaded applications are a great deal more painful to debug, and given the OS and stdio limits I have mentioned there may even be more programming overhead, but, the trade off in system performance and resource utilization for major client/server applications is worth it. Besides, this is the stuff that makes programming fun!
I hope this is helpful.
Andrew L. Sandoval