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fork(2) calls slow down as the parent process uses more memory due to the need to copy page tables. In many common uses of fork(), where it is followed by one of the exec family of functions to spawn child processes (Kernel#system, IO::popen, Process::spawn, etc.), it's possible to remove this overhead by using special process spawning interfaces (posix_spawn(), vfork(), etc.)

The posix-spawn library aims to implement a subset of the Ruby 1.9 Process::spawn interface in a way that takes advantage of fast process spawning interfaces when available and provides sane fallbacks on systems that do not.


  • Fast, constant-time spawn times across a variety of platforms.
  • A largish compatible subset of Ruby 1.9's Process::spawn interface and enhanced versions of Kernel#system, Kernel#`, etc. under Ruby >= 1.8.7 (currently MRI only).
  • High level POSIX::Spawn::Child class for quick (but correct!) non-streaming IPC scenarios.


The following benchmarks illustrate time needed to fork/exec a child process at increasing resident memory sizes on Linux 2.6 and MacOS X. Tests were run using the posix-spawn-benchmark program included with the package.


posix_spawn is faster than fork+exec, and executes in constant time when used with POSIX_SPAWN_USEVFORK.

fork+exec is extremely slow for large parent processes.


posix_spawn is faster than fork+exec, but neither is affected by the size of the parent process.


This library includes two distinct interfaces: POSIX::Spawn::spawn, a lower level process spawning interface based on the new Ruby 1.9 Process::spawn method, and POSIX::Spawn::Child, a higher level class geared toward easy spawning of processes with simple string based standard input/output/error stream handling. The former is much more versatile, the latter requires much less code for certain common scenarios.


The POSIX::Spawn module (with help from the accompanying C extension) implements a subset of the Ruby 1.9 Process::spawn interface, largely through the use of the IEEE Std 1003.1 posix_spawn(2) systems interfaces. These are widely supported by various UNIX operating systems.

In its simplest form, the POSIX::Spawn::spawn method can be used to execute a child process similar to Kernel#system:

require 'posix/spawn'
pid  = POSIX::Spawn::spawn('echo', 'hello world')
stat = Process::waitpid(pid)

The first line executes echo with a single argument and immediately returns the new process's pid. The second line waits for the process to complete and returns a Process::Status object. Note that spawn does not wait for the process to finish execution like system and does not reap the child's exit status -- you must call Process::waitpid (or equivalent) or the process will become a zombie.

The spawn method is capable of performing a large number of additional operations, from setting up the new process's environment, to changing the child's working directory, to redirecting arbitrary file descriptors.

See the Ruby 1.9 Process::spawn documentation for details and the STATUS section below for a full account of the various Process::spawn features supported by POSIX::Spawn::spawn.

system, popen4, and `

In addition to the spawn method, Ruby 1.9 compatible implementations of Kernel#system and Kernel#` are provided in the POSIX::Spawn module. The popen4 method can be used to spawn a process with redirected stdin, stdout, and stderr objects.

POSIX::Spawn as a Mixin

The POSIX::Spawn module can also be mixed in to classes and modules to include spawn and all utility methods in that namespace:

require 'posix/spawn'

class YourGreatClass
  include POSIX::Spawn

  def speak(message)
    pid = spawn('echo', message)

  def calculate(expression)
    pid, in, out, err = popen4('bc')
    [in, out, err].each { |io| io.close if !io.closed? }


The POSIX::Spawn::Child class includes logic for executing child processes and reading/writing from their standard input, output, and error streams. It's designed to take all input in a single string and provides all output as single strings and is therefore not well-suited to streaming large quantities of data in and out of commands. That said, it has some benefits:

  • Simple - requires little code for simple stream input and capture.
  • Internally non-blocking (using select(2)) - handles all pipe hang cases due to exceeding PIPE_BUF limits on one or more streams.
  • Potentially portable - abstracts lower-level process and stream management APIs so the class can be made to work on platforms like Java and Windows where UNIX process spawning and stream APIs are not supported.

POSIX::Spawn::Child takes the standard spawn arguments when instantiated, and runs the process to completion after writing all input and reading all output:

>> require 'posix/spawn'
>> child ='git', '--help')

Retrieve process output written to stdout / stderr, or inspect the process's exit status:

>> child.out
=> "usage: git [--version] [--exec-path[=GIT_EXEC_PATH]]\n ..."
>> child.err
=> ""
>> child.status
=> #<Process::Status: pid=80718,exited(0)>

Use the :input option to write data on the new process's stdin immediately after spawning:

>> child ='bc', :input => '40 + 2')
>> child.out

Additional options can be used to specify the maximum output size (:max) and time of execution (:timeout) before the child process is aborted. See the POSIX::Spawn::Child docs for more info.

Reading Partial Results spawns the process immediately when instantiated. As a result, if it is interrupted by an exception (either from reaching the maximum output size, the time limit, or another factor), it is not possible to access the out or err results because the constructor did not complete.

If you want to get the out and err data was available when the process was interrupted, use the alternate form to create the child without immediately spawning the process. Call exec! to run the command at a place where you can catch any exceptions:

>> child ='git', 'log', :max => 100)
>> begin
?>   child.exec!
?> rescue POSIX::Spawn::MaximumOutputExceeded
?>   # limit was reached
?> end
>> child.out
"commit fa54abe139fd045bf6dc1cc259c0f4c06a9285bb\n..."

Please note that when the MaximumOutputExceeded exception is raised, the actual combined out and err data may be a bit longer than the :max value due to internal buffering.


The POSIX::Spawn::spawn method is designed to be as compatible with Ruby 1.9's Process::spawn as possible. Right now, it is a compatible subset.

These Process::spawn arguments are currently supported to any of Spawn::spawn, Spawn::system, Spawn::popen4, and

env: hash
  name => val : set the environment variable
  name => nil : unset the environment variable
  commandline                 : command line string which is passed to a shell
  cmdname, arg1, ...          : command name and one or more arguments (no shell)
  [cmdname, argv0], arg1, ... : command name, argv[0] and zero or more arguments (no shell)
options: hash
  clearing environment variables:
    :unsetenv_others => true   : clear environment variables except specified by env
    :unsetenv_others => false  : don't clear (default)
  current directory:
    :chdir => str : Not thread-safe when using posix_spawn (see below)
  process group:
    :pgroup => true or 0 : make a new process group
    :pgroup => pgid      : join to specified process group
    :pgroup => nil       : don't change the process group (default)
      FD              : single file descriptor in child process
      [FD, FD, ...]   : multiple file descriptor in child process
      FD                        : redirect to the file descriptor in parent process
      :close                    : close the file descriptor in child process
      string                    : redirect to file with open(string, "r" or "w")
      [string]                  : redirect to file with open(string, File::RDONLY)
      [string, open_mode]       : redirect to file with open(string, open_mode, 0644)
      [string, open_mode, perm] : redirect to file with open(string, open_mode, perm)
    FD is one of follows
      :in     : the file descriptor 0 which is the standard input
      :out    : the file descriptor 1 which is the standard output
      :err    : the file descriptor 2 which is the standard error
      integer : the file descriptor of specified the integer
      io      : the file descriptor specified as io.fileno

These options are currently NOT supported:

options: hash
  resource limit: resourcename is core, cpu, data, etc.  See Process.setrlimit.
    :rlimit_resourcename => limit
    :rlimit_resourcename => [cur_limit, max_limit]
    :umask => int
      [:child, FD]              : redirect to the redirected file descriptor
  file descriptor inheritance: close non-redirected non-standard fds (3, 4, 5, ...) or not
    :close_others => false : inherit fds (default for system and exec)
    :close_others => true  : don't inherit (default for spawn and IO.popen)

The :chdir option provided by Posix::Spawn::Child, Posix::Spawn#spawn, Posix::Spawn#system and Posix::Spawn#popen4 is not thread-safe because processes spawned with the posix_spawn(2) system call inherit the working directory of the calling process. The posix-spawn gem works around this limitation in the system call by changing the working directory of the calling process immediately before and after spawning the child process.


Copyright (c) by Ryan Tomayko and Aman Gupta.

See the COPYING file for more information on license and redistribution.