如何在UNIX / tmp目录中创建文件，以便所有用户都可以读取和写入文件？

I am trying to create a file in the /tmp directory (working on a Linux UBUNTU 7.10), that has read/write/execute access for any user. So I am using the "open(fileName,O_CREAT|O_RDWR,0777)" function to create the file (from a C program) in user1 account and I would like user2 to be able to write to the specific file. However, when i check the /tmp directory (with ls -l) I see that I do not have the write access permission for user2 (considering the fact that user1 created it, I have write access for user1, but user2, who is considered to be "others" does not have any access). I have tried to use mode 0766 in the open function (and such combinations of 7 and 6 for modes), so that I may get write access for user2, but I still don't have the required access.I am trying to create a file in the /tmp direct