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# Caveats for implementing `Signal.trap` callbacks

As with implementing signal handlers in C or most other languages, all code passed to `Signal.trap` must be reentrant. If you are not familiar with reentrancy, you need to read up on it at Wikipedia or elsewhere before reading the rest of this document.

Most importantly, “thread-safety” does not guarantee reentrancy; and methods such as Mutex#lock and Mutex#synchronize which are commonly used for thread-safety even prevent reentrancy.

## An implementation detail of the Ruby VM

The Ruby VM defers `Signal.trap` callbacks from running until it is safe for its internal data structures, but it does not know when it is safe for data structures in YOUR code. Ruby implements deferred signal handling by registering short C functions with only async-signal-safe functions as signal handlers. These short C functions only do enough to tell the VM to run callbacks registered via `Signal.trap` later in the main Ruby `Thread`.

## Unsafe methods to call in `Signal.trap` blocks

When in doubt, consider anything not listed as safe below as being unsafe.

- Mutex#lock, Mutex#synchronize and any code using them are explicitly unsafe. This includes `Monitor` in the standard library which uses Mutex to provide reentrancy.

- `Dir.chdir` with block

- any `IO` write operations when `IO#sync` is false; including `IO#write`, `IO#write_nonblock`, `IO#puts`. Pipes and sockets default to ‘IO#sync = true’, so it is safe to write to them unless `IO#sync` was disabled.

- `File#flock`, as the underlying flock(2) call is not specified by POSIX

## Commonly safe operations inside `Signal.trap` blocks

- Assignment and retrieval of local, instance, and class variables

- Most object allocations and initializations of common types including `Array`, `Hash`, `String`, `Struct`, `Time`.

- Common `Array`, `Hash`, `String`, `Struct` operations which do not execute a block are generally safe; but beware if iteration is occurring elsewhere.

- `Hash#[]`, `Hash#[]=` (unless `Hash.new` was given an unsafe block)

- `Thread::Queue#push` and `Thread::SizedQueue#push` (since Ruby 2.1)

- Creating a new `Thread` via `Thread.new`/Thread.start can used to get around the unusability of Mutexes inside a signal handler

- `Signal.trap` is safe to use inside blocks passed to `Signal.trap`

- arithmetic on `Integer` and `Float` (‘+’, ‘-’, ‘%’, ‘*’, ‘/’)

  Additionally, signal handlers do not run between two successive local variable accesses, so shortcuts such as ‘+=’ and ‘-=’ will not trigger a data race when used on `Integer` and `Float` classes in signal handlers.

## System call wrapper methods which are safe inside `Signal.trap`

Since Ruby has wrappers around many async-signal-safe C functions the corresponding wrappers for many `IO`, `File`, `Dir`, and `Socket` methods are safe.

(Incomplete list)

- `Dir.chdir` (without block arg)

- `Dir.mkdir`

- `Dir.open`

- `File#truncate`

- `File.link`

- `File.open`

- `File.readlink`

- `File.rename`

- `File.stat`

- `File.symlink`

- `File.truncate`

- `File.unlink`

- `File.utime`

- `IO#close`

- `IO#dup`

- `IO#fsync`

- `IO#read`

- `IO#read_nonblock`

- `IO#stat`

- `IO#sysread`

- `IO#syswrite`

- `IO.select`

- `IO.pipe`

- `Process.clock_gettime`

- `Process.exit!`

- `Process.fork`

- `Process.kill`

- `Process.pid`

- `Process.ppid`

- `Process.waitpid`

…