The Standard ML Basis Library

The `Unix` structure

Synopsis

signature UNIX (* OPTIONAL *) structure Unix :> UNIX (* OPTIONAL *)

The Unix structure provides several high-level functions for creating and communicating with separate processes, in analogy with the popen interface provided in the Unix operating system. This module provides a more flexible interface than that provided by the OS.Process.system function. Using this module, a program can invoke a separate process and obtain input and output streams connected to the standard output and input streams, respectively, of the other process.

Interface

type ('a,'b) proc type signal datatype exit_status = W_EXITED | W_EXITSTATUS of Word8.word | W_SIGNALED of signal | W_STOPPED of signal val fromStatus : OS.Process.status -> exit_status val executeInEnv : string * string list * string list -> ('a, 'b) proc val execute : string * string list -> ('a, 'b) proc val textInstreamOf : (TextIO.instream, 'a) proc -> TextIO.instream val binInstreamOf : (BinIO.instream, 'a) proc -> BinIO.instream val textOutstreamOf : ('a, TextIO.outstream) proc -> TextIO.outstream val binOutstreamOf : ('a, BinIO.outstream) proc -> BinIO.outstream val streamsOf : (TextIO.instream, TextIO.outstream) proc -> TextIO.instream * TextIO.outstream val reap : ('a, 'b) proc -> OS.Process.status val kill : ('a, 'b) proc * signal -> unit val exit : Word8.word -> 'a

Description

type ('a,'b) proc

A type representing a handle for an operating system process.

type signal

A Unix-like signal which can be sent to another process. Note that signal values must be obtained from some other structure. For example, an implementation providing the Posix module would probably equate the signal and Posix.Signal.signal types.

datatype exit_status = W_EXITED | W_EXITSTATUS of Word8.word | W_SIGNALED of signal | W_STOPPED of signal

These values represent the ways in which a Unix process might stop. They correspond to, respectively, successful termination, termination with the given exit value, termination upon receipt of the given signal, and stopping upon receipt of the given signal. The value carried by W_EXITSTATUS will be non-zero.

If an implementation provides both the Posix and Unix structures, then Posix.Process.exit_status and exit_status must be the same type.

fromStatus sts

returns a concrete view of the given status.

executeInEnv (cmd, args, env)

asks the operating system to execute the program named by the string cmd with the argument list args and the environment env. The program is run as a child process of the calling program; the return value of this function is an abstract proc value naming the child process. Strings in the env list typically have the form "name=value" (see OS.Process.getEnv).

The executeInEnv function raises the OS.SysErr exception if it fails. Reasons for failure include insufficient memory, too many processes, and the case where cmd does not name an executable file. If the child process fails to execute the command (i.e., the execve call fails), then it should exit with a status code of 126.

execute (cmd, args)

asks the operating system to execute the program named by the string cmd with the argument list args. The program is run as a child process of the calling program and it inherits the calling process's environment; the return value of this function is an abstract proc value naming the child process. The failure semantics of this function are the same as for executeInEnv.

For implementations providing the Posix modules, this function is equivalent to

          fun execute (cmd, args) =
	        executeInEnv (cmd, args, Posix.ProcEnv.environ ())

textInstreamOf pr

binInstreamOf pr

These return a text or binary instream connected to the standard output stream of the process pr.

Note that multiple calls to these functions on the same proc value will result in multiple streams that all share the same underlying open file descriptor, which can lead to unpredictable effects because of the state inherent in file descriptors.

textOutstreamOf pr

binOutstreamOf pr

These return a text or binary outstream connected to the standard input stream of the process pr.

streamsOf pr

returns a pair of input and output text streams associated with pr. This function is equivalent to (textInstream pr, textOutstream pr) and is provided for backward compatibility.

reap pr

closes the input and output streams associated with pr, and then suspends the current process until the system process corresponding to pr terminates. It returns the exit status given by pr when it terminated. If reap is applied again to pr, it should immediately return the previous exit status.

Implementation note:

Typically, one cannot rely on the underlying operating system to provide the exit status of a terminated process after it has done so once. Thus, the exit status probably needs to be cached. Also note that reap should not return until the process being monitored has terminated. In particular, implementations should be careful not to return if the process has only been suspended.

kill (pr,s)

sends the signal s to the process pr.

exit st

executes all actions registered with OS.Process.atExit, flushes and closes all I/O streams opened using the Library, then terminates the SML process with termination status st.

Discussion

Note that the interpretation of the string cmd in the execute and executeInEnv functions depends very much on the underlying operating system. Typically, the cmd argument will be a full pathname.

The semantics of Unix necessitates that processes that have terminated need to be reaped. If this is not done, information concerning the dead process continues to reside in system tables. Thus, a program using execute or executeInEnv should invoke reap on any subprocess it creates.

Implementation note:

Although the flavor of this module is heavily influenced by Unix, and the module is simple to implement given the Posix subsystem, the functions are specified at a sufficiently high-level that implementations, including non-Unix ones, could provide this module without having to supply all of the Posix modules.

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Generated April 12, 2004
Last Modified May 9, 2003
Comments to John Reppy.

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