question about fopen fd limit

盛慧华 hhsheng at
Thu Dec 22 04:01:47 UTC 2016

hi all,

   we are from Chinese Game Develop Corp, Netease.
   and One of our product using FreeBsd as its OS platform.
   This Game has Millions of players online , and Each Server may holds 25000+ tcp connection at the same time.Thanks to BSD and kqueue :)

   for example, it's one of our server , netstat cmd to list connections overall...
   netstat -an | grep 13396 (it's our listening port) | wc -l 

    recently we do some performance optimize and promote this connect limit to 28000+ or 30000+.
  But we find Freebsd has a limit that this huge online number will take 28000+ fd, and bsd FILE * struct's 
  fd only support to SHORT . such as ..

struct __sFILE {
 short _file; /* (*) fileno, if Unix descriptor, else -1 */

  so if our server want to fopen some file when we still hold this online number, the fd amount may easily exceed
32767, and fopen definitely return a err code. then the server will appear some fataly ERROR. 

  we do a simple test and confirm this situation.

  then in fopen's code , we notice that we can use open to return a fd instread of fopen to avoid this overflow, 
   as below

68 /* 
1 * File descriptors are a full int, but _file is only a short. 
2 * If we get a valid file descriptor that is greater than 
3 * SHRT_MAX, then the fd will get sign-extended into an 
4 * invalid file descriptor. Handle this case by failing the 
5 * open. 
6 */

  BUT ... so many c lib FILE series function needs a FILE * pointer as input argument, we can't convert all of them to fd, 
or it will be a rather suffering things to us. 

  and even in BSD 10 , it seems this short limit still there , but other OS as debian , FILE strucnt's fileno is a int .
  we found an unoffical patch easily change this fileno to unsigned , but we are a very stready project, we can't afford the risk
to use an unoffical patch.

  so, do you have any plan to change this fopen fd limit to UNSIGNED SHORT or int in the future ? ushort is enough for us . 
  if you do , we are really glad and excited~~~~~~~if you don't ,it donen't matter,  plz give us a reply so that we may need to 
  find some other plan to resolved this suffering thing.
  LoL, thank you !!!!!

yours sincerely
                                       winson sheng

winson sheng

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