---
created_at: '2016-05-30T21:52:01.000Z'
title: Things Unix can do atomically (2010)
url: https://rcrowley.org/2010/01/06/things-unix-can-do-atomically.html
author: turrini
points: 207
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comment_text: 
num_comments: 62
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created_at_i: 1464645121
_tags:
- story
- author_turrini
- story_11803431
objectID: '11803431'

---
This is a catalog of things UNIX-like/POSIX-compliant operating systems
can do atomically, making them useful as building blocks for thread-safe
and multi-process-safe programs without mutexes or read/write locks. 
The list is by no means exhaustive and I expect it to be updated
frequently for the foreseeable future.

The philosophy here is to let the kernel do as much work as possible. 
At my most pessimistic, I trust the kernel developers more than a trust
myself.  More practically, it’s stupid to spend CPU time locking around
an operation that’s already atomic.  Added 2010-01-07.

## Operating on a pathname

The operations below are best left to local filesystems.  More than a
few people have written in crying foul if any of these techniques are
used on an NFS mount.  True.  When there are multiple kernels involved,
the kernel can’t very well take care of all the locking for us.  Added
2010-01-06.

  - `mv -T <oldsymlink> <newsymlink>` atomically changes the target of
    `<newsymlink>` to the directory pointed to by `<oldsymlink>` and is
    indispensable when deploying new code.  Updated 2010-01-06: both
    operands are symlinks.  (So this isn’t a system call, it’s still
    useful.)  A reader pointed out that `ln -Tfs <directory> <symlink>`
    accomplishes the same thing without the second symlink.  Added
    2010-01-06.  Deleted 2010-01-06: `strace(1)` shows that `ln -Tfs
    <directory> <symlink>` actually calls `symlink(2)`, `unlink(2)`, and
    `symlink(2)` once more, disqualifying it from this page.  `mv -T
    <oldsymlink> <newsymlink>` ends up calling `rename(2)` which can
    atomically replace `<newsymlink>`.  Caveat 2013-01-07: this does not
    apply to Mac OS X, whose
    [`mv(1)`](https://developer.apple.com/library/mac/#documentation/Darwin/Reference/Manpages/man1/mv.1.html)
    doesn’t call `rename(2)`.  [`mv(1)`](http://linux.die.net/man/1/mv).
  - `link(oldpath, newpath)` creates a new hard link called `newpath`
    pointing to the same inode as `oldpath` and increases the link count
    by one.  This will fail with the error code `EEXIST` if `newpath`
    already exists, making this a useful mechanism for locking a file
    amongst threads or processes that can all agree upon the name
    `newpath`.  I prefer this technique for whole-file locking because
    the lock is visible to `ls(1)`. 
    [`link(2)`](http://linux.die.net/man/2/link).
  - `symlink(oldpath, newpath)` operates very much like `link(2)` but
    creates a symbolic link at a new inode rather than a hard link to
    the same inode.  Symbolic links can point to directories, which hard
    links cannot, making them a perfect analogy to `link(2)` when
    locking entire directories.  This will fail with the error code
    `EEXIST` if `newpath` already exists, making this a perfect analogy
    to `link(2)` that works for directories, too.  Be careful of
    symbolic links whose target inode has been removed ("dangling"
    symbolic links) — `open(2)` will fail with the error code `ENOENT`. 
    It should be mentioned that inodes are a finite resource (this
    particular machine has 1,245,184 inodes). 
    [`symlink(2)`](http://linux.die.net/man/2/symlink).  Added
    2010-01-07
  - `rename(oldpath, newpath)` can change a pathname atomically,
    provided `oldpath` and `newpath` are on the same filesystem.  This
    will fail with the error code `ENOENT` if `oldpath` does not exist,
    enabling interprocess locking much like `link(oldpath, newpath)`
    above.  I find this technique more natural when the files in
    question will be `unlink`ed later. 
    [`rename(2)`](http://linux.die.net/man/2/rename).
  - `open(pathname, O_CREAT | O_EXCL, 0644)` creates and opens a new
    file.  (Don’t forget to set the mode in the third argument\!) 
    `O_EXCL` instructs this to fail with the error code `EEXIST` if
    `pathname` exists.  This is a useful way to decide which process
    should handle a task: whoever successfully creates the file. 
    [`open(2)`](http://linux.die.net/man/2/open).
  - `mkdir(dirname, 0755)` creates a new directory but fails with the
    error code `EEXIST` if `dirname` exists.  This provides for
    directories the same mechanism `link(2)` `open(2)` with `O_EXCL`
    provides for files. 
    [`mkdir(2)`](http://linux.die.net/man/2/mkdir).  Added 2010-01-06;
    edited 2013-01-07.

## Operating on a file descriptor

  - `fcntl(fd, F_GETLK, &lock)`, `fcntl(fd, F_SETLK, &lock)`, and
    `fcntl(fd, F_SETLKW, &lock)` allow cooperating processes to lock
    regions of a file to serialize their access.  `lock` is of type
    `struct flock` and describes the type of lock and the region being
    locked.  `F_SETLKW` is particularly useful as it blocks the calling
    process until the lock is acquired.  There is a “mandatory locking”
    mode but Linux’s implementation is unreliable as it’s subject to a
    race condition.  [`fcntl(2)`](http://linux.die.net/man/2/fcntl).
  - `fcntl(fd, F_GETLEASE)` and `fcntl(fd, F_SETLEASE, lease)` ask the
    kernel to notify the calling process with `SIGIO` when another
    process `open`s or `truncate`s the file referred to by `fd`.  When
    that signals arrives, the lease needs to be removed by `fcntl(fd,
    F_SETLEASE, F_UNLCK)`.  `fcntl(fd, F_NOTIFY, arg)` is similar but
    doesn’t block other processes, so it isn’t useful for
    synchronization.  [`fcntl(2)`](http://linux.die.net/man/2/fcntl).
  - `mmap(0, length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)` returns
    a pointer from which a file’s contents can be read and written by
    normal memory operations.  By making frequent use of `msync(addr,
    length, MS_INVALIDATE)`, data written in this manner can be shared
    between processes that both map the same file. 
    [`mmap(2)`](http://linux.die.net/man/2/mmap),
    [`msync(2)`](http://linux.die.net/man/2/msync).

## Operating on virtual memory

  - `__sync_fetch_and_add`, `__sync_add_and_fetch`,
    `__sync_val_compare_and_swap`, and friends provide a full barrier so
    “no memory operand will be moved across the operation, either
    forward or backward.” These operations are the basis for most (all?)
    lock-free algorithms.  [GCC Atomic
    Builtins](http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html).

Something I should add to my repertoire?  Race condition?  Let me know
at <r@rcrowley.org> or [@rcrowley](http://twitter.com/rcrowley) and I’ll
fix it.
