MLtonProcess - MLton Standard ML Compiler (SML Compiler)

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    signature MLTON_PROCESS =
       sig
          type pid

          val spawn: {args: string list, path: string} -> pid
          val spawne: {args: string list, env: string list, path: string} -> pid
          val spawnp: {args: string list, file: string} -> pid

          type ('stdin, 'stdout, 'stderr) t

          type input
          type output

          type none
          type chain
          type any

          exception MisuseOfForget
          exception DoublyRedirected

          structure Child:
            sig
              type ('use, 'dir) t

              val binIn: (BinIO.instream, input) t -> BinIO.instream
              val binOut: (BinIO.outstream, output) t -> BinIO.outstream
              val fd: (Posix.FileSys.file_desc, 'dir) t -> Posix.FileSys.file_desc
              val remember: (any, 'dir) t -> ('use, 'dir) t
              val textIn: (TextIO.instream, input) t -> TextIO.instream
              val textOut: (TextIO.outstream, output) t -> TextIO.outstream
            end

          structure Param:
            sig
              type ('use, 'dir) t

              val child: (chain, 'dir) Child.t -> (none, 'dir) t
              val fd: Posix.FileSys.file_desc -> (none, 'dir) t
              val file: string -> (none, 'dir) t
              val forget: ('use, 'dir) t -> (any, 'dir) t
              val null: (none, 'dir) t
              val pipe: ('use, 'dir) t
              val self: (none, 'dir) t
            end

          val create:
             {args: string list,
              env: string list option,
              path: string,
              stderr: ('stderr, output) Param.t,
              stdin: ('stdin, input) Param.t,
              stdout: ('stdout, output) Param.t}
             -> ('stdin, 'stdout, 'stderr) t
          val getStderr: ('stdin, 'stdout, 'stderr) t -> ('stderr, input) Child.t
          val getStdin:  ('stdin, 'stdout, 'stderr) t -> ('stdin, output) Child.t
          val getStdout: ('stdin, 'stdout, 'stderr) t -> ('stdout, input) Child.t
          val kill: ('stdin, 'stdout, 'stderr) t * Posix.Signal.signal -> unit
          val reap: ('stdin, 'stdout, 'stderr) t -> Posix.Process.exit_status
       end

## Spawn

The `spawn` functions provide an alternative to the `fork`/`exec` idiom that is typically used to create a new process. On most platforms, the `spawn` functions are simple wrappers around `fork`/`exec`. However, under Windows, the `spawn` functions are primitive. All `spawn` functions return the process id of the spawned process. They differ in how the executable is found and the environment that it uses.

- `spawn {args, path}`
  starts a new process running the executable specified by `path` with the arguments `args`. Like `Posix.Process.exec`.

- `spawne {args, env, path}`
  starts a new process running the executable specified by `path` with the arguments `args` and environment `env`. Like `Posix.Process.exece`.

- `spawnp {args, file}`
  search the `PATH` environment variable for an executable named `file`, and start a new process running that executable with the arguments `args`. Like `Posix.Process.execp`.

## Create

`MLton.Process.create` provides functionality similar to `Unix.executeInEnv`, but provides more control control over the input, output, and error streams. In addition, `create` works on all platforms, including Cygwin and MinGW (Windows) where `Posix.fork` is unavailable. For greatest portability programs should still use the standard `Unix.execute`, `Unix.executeInEnv`, and `OS.Process.system`.

The following types and sub-structures are used by the `create` function. They provide static type checking of correct stream usage.

### Child

- `('use, 'dir) Child.t`
  This represents a handle to one of a child's standard streams. The `'dir` is viewed with respect to the parent. Thus a `('a, input) Child.t` handle means that the parent may input the output from the child.

- `Child.{bin,text}{In,Out} h`
  These functions take a handle and bind it to a stream of the named type. The type system will detect attempts to reverse the direction of a stream or to use the same stream in multiple, incompatible ways.

- `Child.fd h`
  This function behaves like the other `Child.*` functions; it opens a stream. However, it does not enforce that you read or write from the handle. If you use the descriptor in an inappropriate direction, the behavior is undefined. Furthermore, this function may potentially be unavailable on future MLton host platforms.

- `Child.remember h`
  This function takes a stream of use `any` and resets the use of the stream so that the stream may be used by `Child.*`. An `any` stream may have had use `none` or `'use` prior to calling `Param.forget`. If the stream was `none` and is used, `MisuseOfForget` is raised.

### Param

- `('use, 'dir) Param.t`
  This is a handle to an input/output source and will be passed to the created child process. The `'dir` is relative to the child process. Input means that the child process will read from this stream.

- `Param.child h`
  Connect the stream of the new child process to the stream of a previously created child process. A single child stream should be connected to only one child process or else `DoublyRedirected` will be raised.

- `Param.fd fd`
  This creates a stream from the provided file descriptor which will be closed when `create` is called. This function may not be available on future MLton host platforms.

- `Param.forget h`
  This hides the type of the actual parameter as `any`. This is useful if you are implementing an application which conditionally attaches the child process to files or pipes. However, you must ensure that your use after `Child.remember` matches the original type.

- `Param.file s`
  Open the given file and connect it to the child process. Note that the file will be opened only when `create` is called. So any exceptions will be raised there and not by this function. If used for `input`, the file is opened read-only. If used for `output`, the file is opened read-write.

- `Param.null`
  In some situations, the child process should have its output discarded. The `null` param when passed as `stdout` or `stderr` does this. When used for `stdin`, the child process will either receive `EOF` or a failure condition if it attempts to read from `stdin`.

- `Param.pipe`
  This will connect the input/output of the child process to a pipe which the parent process holds. This may later form the input to one of the `Child.*` functions and/or the `Param.child` function.

- `Param.self`
  This will connect the input/output of the child process to the corresponding stream of the parent process.

### Process

- `type ('stdin, 'stdout, 'stderr) t`
  represents a handle to a child process. The type arguments capture how the named stream of the child process may be used.

- `type any`
  bypasses the type system in situations where an application does not want the it to enforce correct usage. See `Child.remember` and `Param.forget`.

- `type chain`
  means that the child process's stream was connected via a pipe to the parent process. The parent process may pass this pipe in turn to another child, thus chaining them together.

- `type input, output`
  record the direction that a stream flows. They are used as a part of `Param.t` and `Child.t` and is detailed there.

- `type none`
  means that the child process's stream my not be used by the parent process. This happens when the child process is connected directly to some source.

The types `BinIO.instream`, `BinIO.outstream`, `TextIO.instream`, `TextIO.outstream`, and `Posix.FileSys.file_desc` are also valid types with which to instantiate child streams.

- `exception MisuseOfForget`
  may be raised if `Child.remember` and `Param.forget` are used to bypass the normal type checking. This exception will only be raised in cases where the `forget` mechanism allows a misuse that would be impossible with the type-safe versions.

- `exception DoublyRedirected`
  raised if a stream connected to a child process is redirected to two separate child processes. It is safe, though bad style, to use the a `Child.t` with the same `Child.*` function repeatedly.

- `create {args, path, env, stderr, stdin, stdout}`
  starts a child process with the given command-line `args` (excluding the program name). `path` should be an absolute path to the executable run in the new child process; relative paths work, but are less robust. Optionally, the environment may be overridden with `env` where each string element has the form `"key=value"`. The `std*` options must be provided by the `Param.*` functions documented above.

  Processes which are `create`d must be either `reap`ed or `kill`ed.

- `getStd{in,out,err} proc`
  gets a handle to the specified stream. These should be used by the `Child.*` functions. Failure to use a stream connected via pipe to a child process may result in runtime dead-lock and elicits a compiler warning.

- `kill (proc, sig)`
  terminates the child process immediately. The signal may or may not mean anything depending on the host platform. A good value is `Posix.Signal.term`.

- `reap proc`
  waits for the child process to terminate and return its exit status.

## Important usage notes

When building an application with many pipes between child processes, it is important to ensure that there are no cycles in the undirected pipe graph. If this property is not maintained, deadlocks are a very serious potential bug which may only appear under difficult to reproduce conditions.

The danger lies in that most operating systems implement pipes with a fixed buffer size. If process A has two output pipes which process B reads, it can happen that process A blocks writing to pipe 2 because it is full while process B blocks reading from pipe 1 because it is empty. This same situation can happen with any undirected cycle formed between processes (vertexes) and pipes (undirected edges) in the graph.

It is possible to make this safe using low-level I/O primitives for polling. However, these primitives are not very portable and difficult to use properly. A far better approach is to make sure you never create a cycle in the first place.

For these reasons, the `Unix.executeInEnv` is a very dangerous function. Be careful when using it to ensure that the child process only operates on either `stdin` or `stdout`, but not both.

## Example use of MLton.Process.create

The following example program launches the `ipconfig` utility, pipes its output through `grep`, and then reads the result back into the program.

    open MLton.Process
    val p =
            create {args = [ "/all" ],
                    env = NONE,
                    path = "C:\\WINDOWS\\system32\\ipconfig.exe",
                    stderr = Param.self,
                    stdin = Param.null,
                    stdout = Param.pipe}
    val q =
            create {args = [ "IP-Ad" ],
                    env = NONE,
                    path = "C:\\msys\\bin\\grep.exe",
                    stderr = Param.self,
                    stdin = Param.child (getStdout p),
                    stdout = Param.pipe}
    fun suck h =
            case TextIO.inputLine h of
                    NONE => ()
                    | SOME s => (print ("'" ^ s ^ "'\n"); suck h)

    val () = suck (Child.textIn (getStdout q))

Last edited on 2009-06-15 22:22:05 by MatthewFluet.