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proc.md

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processes and IPC

local proc = require'proc'

A library for creating, controlling and communicating with child processes. Works on Windows, Linux and OSX.

Status

Tested on Windows and Linux.

Missing features:

  • named mutexes, semaphores and events.
  • setting CPU and RAM limits.
  • CPU and RAM monitoring.

API

proc.exec(opt | cmd,...) -> p spawn a child process proc.exec_luafile(opt | script,...) -> p spawn a process running a Lua script p:kill() kill process p:wait([expires]) -> status wait for a process to finish p:status() -> active|finished|killed|forgotten process status p:exit_code() -> code | nil,status get process exit code p:forget() close process handles proc.env(k) -> v get env. var proc.env(k, v) set env. var proc.env(k, false) delete env. var proc.env() -> env get all env. vars proc.esc(s, ['win'|'unix']) -> s escape string (but not quote) proc.quote_arg(s, ['win'|'unix']) -> s quote as cmdline arg

proc.exec(opt | cmd, [env], [cur_dir], [stdin], [stdout], [stderr], [autokill]) -> p

Spawn a child process and return a process object to query and control the process. Options can be given as separate args or in a table.

  • cmd can be either a string or an array containing the filepath of the executable to run and its command-line arguments.
  • env is a table of environment variables (if not given, the current environment is inherited).
  • cur_dir is the directory to start the process in.
  • stdin, stdout, stderr are pipe ends created with fs.pipe() to redirect the standard input, output and error streams of the process; you can also set any of these to true to have them opened (and closed) for you.
  • autokill kills the process when the calling process exits.

proc.exec_luafile(opt | script,...) -> p

Spawn a process running a Lua script, using the same LuaJIT executable as that of the running process. The process starts in the current directory unless otherwise specified. The arguments and options are the same as for exec(), except that cmd must be a Lua file instead of an executable file.

Programming Notes

Env vars

Only use uppercase env. var names because like file names, env. vars are case-sensitive on POSIX, but case-insensitive on Windows.

Only use proc.env() to read variables instead of os.getenv() because the latter won't see the changes made to variables.

Exit codes

Only use exit status codes in the 0..255 range because Windows exit codes are int32 but POSIX codes are limited to a byte.

In Windows, if you kill a process from Task Manager, exit_code() returns 1 instead of nil, 'killed', and status() returns 'finished' instead of 'killed'. You only get 'killed' when you kill the process yourself by calling kill().

Standard I/O redirection

The only way to safely redirect both stdin and stdout of child processes without potentially causing deadlocks is to use async pipes and perform the writes and the reads in separate [sock] threads.

Don't forget to close the stdin file when you're done with it to signal end-of-input to the child process.

Don't forget to check for a zero-length read which can happen any time and signals that the child process closed its end of the pipe.

Cleaning up

Always call forget() when you're done with the process, even after you killed it, but not before you're done with all its redirected pipes if any (because forget() also closes them).

Autkill caveats

In Linux, if you start your autokilled process from a thread other than the main thread, the process is killed when the thread finishes, IOW autokill is only portable if you start processes from the main thread.

In Windows, the autokill behavior is by default inherited by the child processes. In Linux it isn't. IOW autkill inheritance is not portable.