380 lines
13 KiB
TeX
380 lines
13 KiB
TeX
%&latex -*- latex -*-
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\chapter{Miscellaneous routines}
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\section{Integer bitwise ops}
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\label{sec:bitwise}
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\defun{arithmetic-shift} {i j} \integer
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\defunx {bitwise-and} {i j} \integer
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\defunx {bitwise-ior} {i j} \integer
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\defunx {bitwise-not} {i} \integer
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\defunx {bitwise-xor} {i j} \integer
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\begin{desc}
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These operations operate on integers representing semi-infinite
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bit strings, using a 2's-complement encoding.
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\ex{arithmetic-shift} shifts \var{i} by \var{j} bits.
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A left shift is $j > 0$; a right shift is $j < 0$.
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\end{desc}
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\section{List procedures}
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\dfn{nth}{list i}{object}{procedure \textbf{(obsolete)}}
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\begin{desc}
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Returns the $i^{\mathrm th}$ element of \var{list}.
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The first element (the car) is \ex{(nth \var{list} 0)},
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the second element is \ex{(nth \var{list} 1)}, and so on.
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This procedure is provided as it is useful for accessing elements
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from the lists returned by the field-readers (chapter~\ref{chapt:fr-awk}).
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The functionality of \ex{nth} is equivalent to that of \RnRS{}'s
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\ex{list-ref}. Therefore, \ex{nth} will go away in a future release.
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\end{desc}
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\section{Top level}
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\defun{repl}{}\undefined
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\begin{desc}
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This runs a {\scm} read-eval-print loop,
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reading forms from the current input port,
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and writing their values to the current output port.
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If you wish to try something dangerous,
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and want to be able to recover your shell state, you can
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fork off a subshell with the following form:
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\codex{(run (begin (repl)))}
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{\ldots}or, rephrased for the proceduralists:
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\codex{(wait (fork repl))}
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\end{desc}
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\section{Password encryption}
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\defun {crypt} {key salt} {encrypted value}
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Decrypts \var{key} by directly calling the \texttt{crypt} function
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using \var{salt} to perturb the hashing algorithm. \var{Salt} must be
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a two-character string consisting of digits, alphabetic characters,
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``.'' or ``\verb+\+''. The length of \var{key} may be at most eight.
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\section{Dot-Locking}
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Section \ref{sec:filelocking} already points out that {\Posix}'s file
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locks are almost useless in practice. To bypass this restriction other
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advisory locking mechanisms, based only on standard file operations,
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where invented. One of them is the so-called \emph{dot-locking} scheme
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where the lock of \textit{file-name} is represented by the file
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\textit{file-name}\texttt{.lock}. Care is taken that only one process
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may generate the lock for a given file.
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Here is scsh's interface to dot-locking:
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\defun {obtain-dot-lock} {file-name [interval retry-number stale-time]} {\boolean}
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\begin{desc}
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Tries to obtain the lock for \var{file-name}. If the file is already
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locked, the thread sleeps for \var{interval} seconds (default is 1)
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before it retries. If the lock cannot be obtained after
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\var{retry-number} attempts, the procedure returns \sharpf,
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otherwise \sharpt. The default value of \var{retry-number} is
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\sharpf which corresponds to an infinite number of retires.
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If \var{stale-time} is non-\sharpf, it specifies the minimum age a
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lock may have (in seconds) before it is considered \textit{stale}.
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\ex{Obtain-dot-lock} attempts to delete stale locks. If it was
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succcessful obtaining a lock after breaking it, \ex{obtain-dot-lock}
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returns \ex{broken}. If \var{stale-time} is \sharpf,
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\ex{obtain-dot-lock} never considers a lock stale. The default for
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\var{stale-time} is 300.
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Note that it is possible that \ex{obtain-dot-lock} breaks a lock
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but nevertheless fails to obtain it otherwise. If it is necessary
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to handle this case specially, use \ex{break-dot-lock} directly
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(see below) rather than specifying a non-\sharpf \var{stale-time}
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\end{desc}
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\defun {break-dot-lock} {file-name} {undefined}
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\begin{desc}
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Breaks the lock for \var{file-name} if one exists. Note that
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breaking a lock does \emph{not} imply a subsequent
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\ex{obtain-dot-lock} will succeed, as another party may have
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acquired the lock between \ex{break-dot-lock} and
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\ex{obtain-dot-lock}.
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\end{desc}
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\defun {release-dot-lock} {file-name} {\boolean}
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\begin{desc}
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Releases the lock for \var{file-name}. On success,
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\ex{release-dot-lock} returns \sharpt, otherwise \sharpf. Note that
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this procedure can also be used to break the lock for
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\var{file-name}.
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\end{desc}
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\defun{with-dot-lock*} {file-name thunk} {value(s) of thunk}
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\dfnx{with-dot-lock} {file-name body \ldots} {value(s) of body}{syntax}
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\begin{desc}
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This procedure obtains the requested lock, and then calls
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\ex{(\var{thunk})}. When \var{thunk} returns, the lock is released.
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A non-local exit (\eg, throwing to a saved continuation or raising
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an exception) also causes the lock to be released.
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After a normal return from \var{thunk}, its return values are
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returned by \ex{with-dot-lock*}. The \ex{with-dot-lock} special
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form is equivalent syntactic sugar.
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\end{desc}
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\section{Syslog facility}
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\label{syslog-facility}
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(Note: the functionality presented in this section is still somewhat
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experimental and thus subject to interface changes.)
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The procedures in this section provide access to the 4.2BSD syslog
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facility present in most POSIX systems. The functionality is in a
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structure called \ex{syslog}. There's an additional structure
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\ex{syslog-channels} documented below. The scsh interface to
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the syslog facility differs significantly from that of the Unix
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library functionality in order to support multiple simultaneous
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connections to the syslog facility.
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Log messages carry a variety of parameters beside the text of the
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message itself, namely a set of options controlling the output format
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and destination, the facility identifying the class of programs the
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message is coming from, an identifier specifying the conrete program,
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and the level identifying the importance of the message. Moreover, a
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log mask can prevent messages at certain levels to be actually sent to
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the syslog daemon.
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\subsection*{Log options}
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A log option specifies details of the I/O behavior of the syslog
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facility. A syslog option is an element of a finite type (see
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the \scm~manual) constructed by the
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\ex{syslog-option} macro. The syslog facility works with sets of
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options which are represented as enum sets (see
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the \scm~manual).
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\dfn{syslog-option}{option-name}{option}{syntax}
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\defun{syslog-option?}{x}{boolean}
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\defun{make-syslog-options}{list}{options}
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\dfn{syslog-options}{option-name \ldots}{options}{syntax}
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\defun{syslog-options?}{x}{boolean}
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\begin{desc}
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\ex{Syslog-option} constructs a log option from the name of an
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option. (The possible names are listed below.) \ex{Syslog-option?}
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is a predicate for log options. Options are comparable using
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\ex{eq?}. \ex{Make-syslog-options} constructs a set of options
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from a list of options. \ex{Syslog-options} is a macro which
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expands into an expression returning a set of options from names.
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\ex{Syslog-options?} is a predicate for sets of options.
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\end{desc}
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%
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Here is a list of possible names of syslog options:
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\begin{description}
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\item[\ex{console}]
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If syslog cannot pass the message to syslogd it will attempt to
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write the message to the console.
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\item[\ex{delay}]
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Delay opening the connection to syslogd immediately until the first
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message is logged.
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\item[\ex{no-delay}]
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Open the connection to syslogd immediately. Normally
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the open is delayed until the first message is logged.
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Useful for programs that need to manage the order in which
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file descriptors are allocated.
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\noindent\textbf{NOTA BENE:}
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The \ex{delay} and \ex{no-delay} options are included for
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completeness, but do not have the expected effect in the present
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Scheme interface: Because the Scheme interface has to multiplex
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multiple simultaneous connections to the syslog facility over a
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single one, open and close operations on that facility happen at
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unpredictable times.
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\item[\ex{log-pid}]
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Log the process id with each message: useful for identifying
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instantiations of daemons.
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\end{description}
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\subsection*{Log facilities}
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A log facility identifies the originator of a log message from a
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finite set known to the system. Each originator is identified by a
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name:
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\dfn{syslog-facility}{facility-name}{facility}{syntax}
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\defun{syslog-facility?}{x}{boolean}
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\begin{desc}
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\ex{Syslog-facility} is macro that expands into an expression
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returning a facility for a given name. \ex{Syslog-facility?} is a
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predicate for facilities. Facilities are comparable via \ex{eq?}.
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\end{desc}
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%
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Here is a list of possible names of syslog facilities:
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\begin{description}
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\item[\ex{authorization}]
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The authorization system: login, su, getty, etc.
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\item[\ex{cron}]
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The cron daemon.
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\item[\ex{daemon}]
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System daemons, such as routed, that are not provided for explicitly
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by other facilities.
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\item[\ex{kernel}]
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Messages generated by the kernel.
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\item[\ex{lpr}]
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The line printer spooling system: lpr, lpc, lpd, etc.
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\item[\ex{mail}]
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The mail system.
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\item[\ex{news}]
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The network news system.
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\item[\ex{user}]
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Messages generated by random user processes.
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\item[\ex{uucp}]
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The uucp system.
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\item[\ex{local0} \ex{local1} \ex{local2} \ex{local3} \ex{local4} \ex{local5} \ex{local6} \ex{local7}]
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Reserved for local use.
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\end{description}
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\subsection*{Log levels}
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A log level identifies the importance of a message from a fixed set
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of possible levels.
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\dfn{syslog-level}{level-name}{level}{syntax}
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\defun{syslog-level?}{x}{boolean}
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%
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\begin{desc}
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\ex{Syslog-level} is macro that expands into an expression returning
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a facility for a given name. \ex{Syslog-level?} is a predicate for
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facilities. Levels are comparable via \ex{eq?}.
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\end{desc}
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%
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Here is a list of possible names of syslog levels:
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\begin{description}
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\item[\ex{emergency}]
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A panic condition. This is normally broadcast to all users.
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\item[\ex{alert}]
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A condition that should be corrected immediately, such as a
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corrupted system database.
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\item[\ex{critical}]
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Critical conditions, e.g., hard device errors.
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\item[\ex{error}]
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Errors.
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\item[\ex{warning}]
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Warning messages.
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\item[\ex{notice}]
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Conditions that are not error conditions, but should possibly be
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handled specially.
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\item[\ex{info}]
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Informational messages.
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\item[\ex{debug}]
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Messages that contain information normally of use only when
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debugging a program.
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\end{description}
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\subsection*{Log masks}
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A log masks can mask out log messages at a set of levels. A log
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mask is an enum set of log levels.
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\defun{make-syslog-mask}{list}{mask}
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\dfn{syslog-mask}{level-name \ldots}{mask}{syntax}
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\defvar{syslog-mask-all}{mask}
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\defun{syslog-mask-upto}{level}{mask}
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\defun{syslog-mask?}{x}{boolean}
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\begin{desc}
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\ex{Make-syslog-mask} constructs a mask from a list of levels.
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\ex{Syslog-mask} is a macro which constructs a mask from names of
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levels. \ex{Syslog-mask-all} is a predefined log mask containing
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all levels. \ex{Syslog-mask-upto} returns a mask consisting of all
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levels up to and including a certain level, starting with
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\ex{emergency}.
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\end{desc}
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\subsection*{Logging}
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Scheme~48 dynamically maintains implicit connections to the syslog
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facility specifying a current identifier, current options, a current
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facility and a current log mask. This implicit connection is held in
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a thread fluid (see
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Section~\ref{sec:ps_interac}). Hence, every thread
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maintains it own implicit connection to syslog. Note that the
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connection is not implicitly preserved across a \ex{spawn}, but it
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is preserved across a \ex{fork-thread}:
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\defun{with-syslog-destination}{string options facility mask thunk}{value}
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\defun{set-syslog-destination!}{string options facility mask}{undefined}
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%
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\begin{desc}
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\ex{With-syslog-destination} dynamically binds parameters of the
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implicit connection to the syslog facility and runs \var{thunk}
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within those parameter bindings, returning what \var{thunk}
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returns. Each of the parameters may be \ex{\#f} in which case the
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previous values will be used. \ex{Set-syslog-destination!} sets the
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parameters of the implicit connection of the current thread.
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\end{desc}
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\defun{syslog}{level message}{undefined}
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\defun{syslog}{level message [string options syslog-facility]}{undefined}
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%
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\begin{desc}
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\ex{Syslog} actually logs a message. Each of the parameters of the
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implicit connection (except for the log mask) can be explicitly
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specified as well for the current call to \ex{syslog}, overriding
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the parameters of the channel. The parameters revert to their
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original values after the call.
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\end{desc}
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\subsection*{Syslog channels}
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%
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The \ex{syslog-channels} structure allows direct manipulation of
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syslog channels, the objects that represent connections to the syslog
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facility. Note that it is
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not necessary to explicitly open a syslog channel to do logging.
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\defun{open-syslog-channel}{string options facility mask}{channel}
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\defun{close-syslog-channel}{channel}{undefined}
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\defun{syslog}{level message channel}{undefined}
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%
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\begin{desc}
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\ex{Open-syslog-channel} and \ex{close-syslog-channel} create and
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destroy a connection to the syslog facility, respectively. The
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specified form of calling \ex{syslog} logs to the specified channel.
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\end{desc}
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%%% Local Variables:
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%%% mode: latex
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%%% TeX-master: "man"
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%%% End:
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