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 ofKernel#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
.
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.
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)
Process::waitpid(pid)
end
def calculate(expression)
pid, in, out, err = popen4('bc')
in.write(expression)
in.close
out.read
ensure
[in, out, err].each { |io| io.close if !io.closed? }
Process::waitpid(pid)
end
end
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 exceedingPIPE_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 = POSIX::Spawn::Child.new('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 = POSIX::Spawn::Child.new('bc', :input => '40 + 2')
>> child.out
"42\n"
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.
POSIX::Spawn::Child.new
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 POSIX::Spawn::Child.build
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 = POSIX::Spawn::Child.build('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 Spawn::Child.new
:
env: hash
name => val : set the environment variable
name => nil : unset the environment variable
command...:
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)
redirection:
key:
FD : single file descriptor in child process
[FD, FD, ...] : multiple file descriptor in child process
value:
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:
:umask => int
redirection:
value:
[: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.