Disallow non-portable and signal values as exit statuses.

phases/ephemeral/docs.md

@@ -2389,9 +2389,12 @@ The number of events stored.
 ---
 
 #### <a href="#exit" name="exit"></a> `exit(rval: exitcode)`
-Terminate the process normally. An exit code of 0 indicates successful
-termination of the program. The meanings of other values is dependent on
-the environment.
+Terminate the process normally. An exit code of `$exitcode::success`
+reports successful completion of the program. An exit code of
+`$exitcode::failure` or any other value less than 126 reports a
+failure, and the value is provided to the environment. If a value
+of 126 or greater is given, this function behaves as if it were
+implemented by an `unreachable` instruction.
 
 ##### Params
 - <a href="#exit.rval" name="exit.rval"></a> `rval`: [`exitcode`](#exitcode)

phases/ephemeral/witx/typenames.witx

@@ -638,8 +638,18 @@
   )
 )
 
-;;; Exit code generated by a process when exiting.
-(typename $exitcode u32)
+;;; Exit code generated by a program when exiting.
+(typename $exitcode u8)
+
+;;; Indicate the program exited successfully.
+;;;
+;;; Note: This is similar to `EXIT_SUCCESS` in POSIX.
+(@witx const $exitcode $success 0)
+
+;;; Indicate the program exited unsuccessfully.
+;;;
+;;; Note: This is similar to `EXIT_FAILURE` in POSIX.
+(@witx const $exitcode $failure 1)
 
 ;;; Flags provided to `sock_recv`.
 (typename $riflags

phases/ephemeral/witx/wasi_ephemeral_proc.witx

@@ -8,9 +8,12 @@
 (use "typenames.witx")
 
 (module $wasi_ephemeral_proc
-  ;;; Terminate the process normally. An exit code of 0 indicates successful
-  ;;; termination of the program. The meanings of other values is dependent on
-  ;;; the environment.
+  ;;; Terminate the process normally. An exit code of `$exitcode::success`
+  ;;; reports successful completion of the program. An exit code of
+  ;;; `$exitcode::failure` or any other value less than 126 reports a
+  ;;; failure, and the value is provided to the environment. If a value
+  ;;; of 126 or greater is given, this function behaves as if it were
+  ;;; implemented by an `unreachable` instruction.
   (@interface func (export "exit")
     ;;; The exit code returned by the process.
     (param $rval $exitcode)