298 lines
12 KiB
TeX
298 lines
12 KiB
TeX
%&latex -*- latex -*-
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\chapter{Changes from previous releases}
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\label{sec:changes}
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\newcommand{\itam}[1]{\item {#1} \\}
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\section{Changes from the previous release}
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This section details changes that have been made in scsh since
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the previous release.
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Scsh is now much more robust.
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All known bugs have been fixed.
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There have been many improvements and extensions made.
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These new features and changes are listed below, in no particular order;
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the relevant sections of the manual give the full details.
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Scsh now supports complete {\Posix}, including signal handlers.
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Early autoreaping of child processes is now handled by a \ex{SIGCHLD}
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signal handler, so children are reaped as early as possible with no
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user intervention required.
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A functional static heap linker is included in this release.
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It is ugly, limited in functionality, and extremely slow, but it works.
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It can be used to build scsh binaries that start up instantly.
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The regular expression system has been sped up.
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Regular-expression compilation is now provided,
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and the \ex{awk} macro has been rewritten to pre-compile
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regexps used in rules outside the loop.
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It is still, however, slower than it should be.
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Execing programs should be faster in this release, since we now use the
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\ex{CLOEXEC} status bit to get automatic closing of unrevealed
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port file descriptors.
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{scm}'s floating point support was inadvertently omitted from the last
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release. It has been reinstated.
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There is now a new command-line switch, \ex{-sfd \var{num}},
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which causes scsh to read its script from file descriptor \var{num}.
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\section{Changes from the penultimate release}
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This section details changes that have been made in scsh since
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the penultimate release.
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Scsh is now much more robust.
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All known bugs have been fixed.
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There have been many improvements and extensions made.
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We have also made made some incompatible changes.
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The sections below briefly describe these new features and changes;
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the relevant sections of the manual give the full details.
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{New features}
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This release incorporates several new features into scsh.
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\begin{itemize}
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\itam{Control of buffered I/O}
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Scsh now allows you to control the buffering policy used for doing I/O
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on a Scheme port.
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\itam{Here-strings}
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Scsh now has a new lexical feature, \verb|#<<|, that provides
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the ability to enter long, multi-line string constants in scsh programs.
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Such a string is called a ``here string,'' by analogy to the common
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shell ``here document'' \ex{<<} redirection.
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\itam{Delimited readers and read-line}
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Scsh now has a powerful set of delimited readers.
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These can be used to read input delimited by
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a newline character (\ex{read-line}),
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a blank line (\ex{read-paragraph}),
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or the occurrence of any character in an arbitrary set (\ex{read-delimited}).
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While these procedures can be applied to any Scheme input port,
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there is native-code support for performing delimited reads on
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Unix input sources, so doing block input with these procedures should be
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much faster than the equivalent character-at-a-time Scheme code.
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\itam{New system calls}
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With the sole exception of signal handlers, scsh now has all of {\Posix}.
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This release introduces
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\begin{itemize}
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\item \ex{select},
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\item full terminal device control,
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\item support for pseudo-terminal ``pty'' devices,
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\item file locking,
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\item process timing,
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\item \ex{set-file-times},
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\item \ex{seek} and \ex{tell}.
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\end{itemize}
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Note that having \ex{select}, pseudo-terminals, and tty device control means
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that it is now possible to implement interesting network protocols, such as
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telnet servers and clients, directly in Scheme.
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\itam{New command-line switches}
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There is a new set of command-line switches that make it possible
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to write shell scripts using the {\scm} module system.
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Scripts can use the new command-line switches to open dependent
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modules and load dependent source code.
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Scripts can also be written in the {\scm} module language,
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which allows you to use it both as a standalone shell script,
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and as a code module that can be loaded and used by other Scheme programs.
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\itam{Static heap linking}
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There is a new facility that allows you to compile a heap image
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to a \ex{.o} file that can be linked with the scsh virtual machine.
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This produces a standalone executable binary, makes startup time
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near-instantaneous, and greatly improves memory performance---the
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initial heap image is placed in the process' text pages,
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where it is shared by different scsh processes, and does not occupy
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space in the run-time heap.
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\oops{The static heap linker was not documented and installed in time
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for this release.}
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\end{itemize}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Incompatible improvements}
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Some features of scsh have been improved in ways that are
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not backwards-compatible with previous releases.
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These changes should not affect most code;
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however, please note the changes and modify your code accordingly.
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\begin{itemize}
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\itam{New process-object data-type returned by \ex{fork}}
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Previous releases were prone to fill up the kernel's process table
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if a program forked large numbers of processes and subsequently failed
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to use \ex{wait} to reclaim the entries in the kernel's process table.
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(This is a problem in standard C environments, as well.)
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Scsh 0.4 introduces a new mechanism for automatically managing subprocesses.
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Processes are no longer represented by an integer process id,
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which is impossible to garbage-collect, but by an
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abstract process data type that encapsulates the process id.
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All processes are represented using the new data structures;
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see the relevant section of the manual for further details.
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\itam{Better stdio/current-port synchronisation}
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The \ex{(begin \ldots)} process form now does a \ex{stdio->stdports}
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call before executing its body.
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This means that the Scheme code in the body ``sees'' any external
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redirections.
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For example, it means that if a \ex{begin} form in the middle of a pipeline
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performs I/O on the current input and output ports, it will be communicating
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with its upstream and downstream pipes.
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\Eg, this code works as intended without the need for explicit synchronisation:
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\begin{verbatim}
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(run (| (gunzip)
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;; Kill line 1 and insert doubled-sided
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;; code at head of Postscript.
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(begin (read-line) ; Eat first line.
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(display "%!PS-Adobe-2.0\\n")
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(display "statusdict /setduplexmode known ")
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(display "{statusdict begin true ")
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(display "setduplexmode end} if\n")
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(exec-epf (cat)))
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(lpr))
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(< paper.ps))\end{verbatim}
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Arranging for the \ex{begin} process form to synchronise
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the current I/O ports with stdio means that all process forms now
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see their epf's redirections.
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\itam{\ex{file-match} more robust}
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The \ex{file-match} procedure now catches any error condition
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signalled by a match procedure,
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and treats it as if the procedure had simply returned {\sharpf},
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\ie, match failure.
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This means \ex{file-match} no longer gets blown out of the water by
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trying to apply a function like \ex{file-directory?} to a dangling symlink,
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and other related OS errors.
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\itam{Standard input now unbuffered}
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Scsh's startup code now makes the initial current input port
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(corresponding to file descriptor 0) unbuffered.
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This keeps the shell from ``stealing'' input meant for subprocesses.
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However, it does slow down character-at-a-time input processing.
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If you are writing a program that is tolerant of buffered input,
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and wish the efficiency gains, you can reset the buffering policy
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yourself.
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\itam{``writeable'' now spelled ``writable''}
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We inconsistently spelled \ex{file-writable?} and \ex{file-not-writable?}
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in the manual and the implementation.
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We have now standardised on the common spelling ``writable'' in both.
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The older bindings still exist in release 0.4, but will go away in future
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releases.
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\itam{\protect\ex{char-set-member?} replaced}
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We have de-released the \ex{char-set-member?} procedure.
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The scsh 0.3 version of this procedure took arguments
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in the following order:
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\codex{(char-set-member? \var{char} \var{char-set})}
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This argument order is in accordance with standard mathematical useage
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(\ie, $x \in S$), and also consistent with the R4RS
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\ex{member}, \ex{memq} and \ex{memv} procedures.
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It is, however, exactly opposite from the argument order
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used by the \ex{char-set-member?} in MIT Scheme's character-set library.
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If we left things as they were, we risked problems with code
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ported over from MIT Scheme.
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On the other hand, changing to conformance with MIT Scheme meant
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inconsistency with common mathematical notation and other long-standing
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Scheme procedures.
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Either way was bound to introduce confusion.
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We've taken the approach of simply removing the \ex{char-set-member?}
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procedure altogether, and replacing it with a new procedure:
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\codex{(char-set-contains? \var{cset} \var{char})}
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Note that the argument order is consistent with the name.
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\itam{\ex{file-attributes} now \ex{file-info}}
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In keeping with the general convention in scsh of naming procedures
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that retrieve information about system resources \ex{\ldots-info}
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(\eg, \ex{tty-info}, \ex{user-info}, \ex{group-info}),
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the \ex{file-attributes} procedure is now named \ex{file-info}.
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We continue to export a \ex{file-attributes} binding for the current
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release, but it will go away in future releases.
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\itam{Renaming of I/O synchronisation procedures}
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The \ex{(stdio->stdports \var{thunk})} procedure has been
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renamed \ex{with-stdio-ports*};
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there is now a corresponding \ex{with-stdio-ports} special form.
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The \ex{stdio->stdports} procedure is now a nullary procedure
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that side-effects the current set of current I/O port bindings.
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\itam{New meta-arg line-two syntax}
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Scsh now uses a simplified grammar for describing command-line
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arguments read by the ``meta-arg'' switch from line two of a shell script.
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If you were using this feature in previous releases, the three incompatible
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changes of which to be aware are:
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(1) tab is no longer allowed as an argument delimiter,
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(2) a run of space characters is not equivalent to a single space,
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(3) empty arguments are written a different way.
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\end{itemize}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Backwards-compatible improvements}
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Some existing features in scsh have been improved in ways that will
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not effect existing code.
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\begin{itemize}
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\itam{Improved error reporting}
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Exception handlers that print out error messages and warnings now
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print their messages on the error output port,
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instead of the current output port.
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Previous releases used the current output port,
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a problem inherited from Scheme 48.
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Previous scsh releases flushed the Scheme 48 debugging tables when
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creating the standard scsh heap image.
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This trimmed the size of the heap image, but made error messages much
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less comprehensible.
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We now retain the debugging tables.
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This bloats the heap image up by about 600kb. And worth it, too.
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(We also have some new techniques for eliminating the run-time memory
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penalty imposed by these large heap images.
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Scsh's new static-heap technology allows for this data to be linked
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into the text pages of the vm's binary, where it will not be touched
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by the GC or otherwise affect the memory system until it is referenced.)
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Finally, scsh now generates more informative error messages for syscall
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errors.
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For example, a file-open error previously told you what the error was
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(\eg, ``Permission denied,'' or ``No such file or directory''),
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but not which file you had tried to open.
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We've improved this.
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\itam{Closing a port twice allowed}
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Scsh used to generate an error if you attempted to close a port
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that had already been closed.
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This is now allowed.
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The close procedure returns a boolean to indicate whether the port had
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already been closed or not.
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\itam{Better time precision}
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The \ex{time+ticks} procedure now returns sub-second precision on OS's
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that support it.
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\itam{Nicer print-methods for basic data-types}
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Scsh's standard record types now print more informatively.
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For example, a process object includes the process id in its
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printed representation: the process object for process id 2653
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prints as \verb|#{proc 2653}|.
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\end{itemize}
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