4 ms·

A call to unlink removes the name from the filesystem, but keeps the file around as long as it's still opened by a process; and mmap keeps the file opened, even

• mehrdadn 9y ago
  > A call to unlink removes the name from the filesystem, but keeps the file around as long as it's still opened by a process; and mmap keeps the file opened, even if you close the original file descriptor. Are you familiar with Windows or just assuming all OSes works like *nix systems? That's definitely not at all close to how Windows file systems behave.
  • pjmlp 9y ago
    Not even all UNIXes. It is implementation defined on the UNIX OS variant and mounted file system. The beauty of POSIX is how it appears to be portable, while leaving quite a few things being implementation defined.
    • mehrdadn 9y ago
      Today I learned!
    • gpderetta 9y ago
      Not sure what UNIX variant are you thinking, but from POSIX: > The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file. I'm sure there have been unix variants that got it wrong and might not even have been in posix from the beginning (it is there since at least SUSv2); still I think this is traditional unix behavior.
  • zuzun 9y ago
    Your comment contains no details whatsoever, then you described perfectly sane mmap behavior, so what do I know?
    • throwaway2048 9y ago
      Windows dosent have the concept of an inode, so there is nothing for a file handle (what windows calls a file descriptor) to bind to except a file name, this means that a filename cannot be removed while it is being accessed somehow. This is the cause of a lot of woes on windows, such as random access denied errors when attempting to delete stuff, and what forces a reboot when you update nearly anything on windows. One of my favourite things related to this is that the Windows XP explorer had a bug where clicking on a file would generate a dynamic preview of an audio/video file in the sidebar, which would lock the file, rendering it impossible to delete because you had to click on it first....
• benmmurphy 9y ago
  this reminds me how updating libraries in linux can be kind of dangerous depending on how you perform it. for example if you unlink the library then create a new library with the same name then the system behaves probably how you want it to. existing processes use the original library until they are restarted. however, if you open the existing library with O_TRUNC and overwrite the library then existing processes will use the new code which is super dangerous. by default cp will do the O_TRUNC behaviour, and tar will use the unlink behaviour. its interesting to think that stuff might be accidentally working for some people because of this choice in behaviour for tar. i haven't actually tested this with a real library. this is what i've assumed happens from playing around with mmap(PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE).
  • zrm 9y ago
    Both of those are the wrong way to do it. :) O_TRUNC for the reasons you stated and unlink because it causes a race between the unlink and the copy when a new process that tries to use the library will either fail because it doesn't exist or get a copy of the library that isn't completely written yet. The correct way to do it is to write the new file to the same filesystem at a different path and then, once it's completely written, move it to the intended path. Move (i.e. rename(2)) is atomic on sane filesystems so anything that opens the file will get one complete copy or the other.