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 @settitle The FreeM Manual  @settitle The FreeM Manual
   
 @copying  @copying
 This manual is for FreeM, (version 0.64.0-rc0), which is a free and open-source implementation of the M programming language.  This manual is for FreeM, (version 0.64.0-rc1), which is a free and open-source implementation of the M programming language.
   
   
 Copyright @copyright{} 2014-2025 Coherent Logic Development LLC  Copyright @copyright{} 2014-2025 Coherent Logic Development LLC
Line 18  Permission is granted to copy, distribut Line 18  Permission is granted to copy, distribut
   
 @title The FreeM Manual  @title The FreeM Manual
 @subtitle @sc{The Official Manual of FreeM}  @subtitle @sc{The Official Manual of FreeM}
 @subtitle Version 0.64.0-rc0  @subtitle Version 0.64.0-rc1
 @c@vskip 10pt  @c@vskip 10pt
 @c@center @image{freem-logo-sm,,,,.png}  @c@center @image{freem-logo-sm,,,,.png}
 @author Serena Willis  @author Serena Willis
Line 73  This is the official manual for the Free Line 73  This is the official manual for the Free
 * Global Aliasing::                     Defining alternate names for globals.  * Global Aliasing::                     Defining alternate names for globals.
 * Global Mappings::                     Mapping global names to non-default namespaces.  * Global Mappings::                     Mapping global names to non-default namespaces.
   
 * Transaction Processing::              Transactions in FreeM.  
 * Asynchronous Event Handling::         Handling asynchronous events in FreeM.  * Asynchronous Event Handling::         Handling asynchronous events in FreeM.
 * Global Triggers::                     Responding to global accesses in M code.  * Global Triggers::                     Responding to global accesses in M code.
 * Synchronous Event Handling::          Synchronous events in FreeM.  * Synchronous Event Handling::          Synchronous events in FreeM.
Line 235  Steve Zeck (Code) Line 234  Steve Zeck (Code)
 $ @command{./freem} [@emph{OPTIONS}...] [[-r <entryref>] | [--routine=<entryref>]]  $ @command{./freem} [@emph{OPTIONS}...] [[-r <entryref>] | [--routine=<entryref>]]
 @end example  @end example
   
 When FreeM loads, it searches the @code{SYSTEM} namespace for the @code{^%SYSINIT} routine, and begins executing it.  When FreeM loads, it searches the @code{SYSTEM} namespace for the @code{%SYSINIT} routine, and begins executing it.
   
 When @code{-r} or @code{--routine} are passed on the command line, FreeM will load and run the specified routine after running @code{^%SYSINIT}. Beginning with FreeM 0.1.7, routines invoked in this manner are no longer required to perform their own namespace setup with @code{VIEW} commands.  When @code{-r} or @code{--routine} are passed on the command line, FreeM will load and run the specified routine after running @code{%SYSINIT}. Beginning with FreeM 0.1.7, routines invoked in this manner are no longer required to perform their own namespace setup with @code{VIEW} commands.
   
 @section %SYSINIT Routine  @section %SYSINIT Routine
   
 The @code{^%SYSINIT} routine runs every time a FreeM interpreter process starts. This routine defines some useful constants, enables handling of @code{TRIGGER} events, and handles the execution of code passed via the @code{-x\--execute} or routines passed via @code{-r|--routine}.  The @code{%SYSINIT} routine runs every time a FreeM interpreter process starts. This routine defines some useful constants, enables handling of @code{TRIGGER} events, and handles the execution of code passed via the @code{-x\--execute} or routines passed via @code{-r|--routine}.
   
 Do not modify the supplied @code{^%SYSINIT} routine to add site-specific startup items. Instead, create a @code{^LCLINIT} routine in the @code{USER} namespace of one or more environments. @code{^%SYSINIT} will automatically run @code{^LCLINIT} each time it starts.  Do not modify the supplied @code{%SYSINIT} routine to add site-specific startup items. Instead, create a @code{LCLINIT} routine in the @code{USER} namespace of one or more environments. @code{%SYSINIT} will automatically run @code{LCLINIT} each time it starts.
   
 @section Command-Line Options  @section Command-Line Options
 @cindex options, command-line  @cindex options, command-line
Line 278  Allows your M routines to be used as UNI Line 277  Allows your M routines to be used as UNI
 Selects the FreeM namespace to be entered on startup. Must be defined in @file{/etc/<environment>/freem.conf}.  Selects the FreeM namespace to be entered on startup. Must be defined in @file{/etc/<environment>/freem.conf}.
   
 @item @option{-r @emph{<entryref>}}, @option{--routine=@emph{<entryref>}}  @item @option{-r @emph{<entryref>}}, @option{--routine=@emph{<entryref>}}
 Causes @code{<entryref>} to be executed at load, instead of @code{^%SYSINIT}.  Causes @code{<entryref>} to be executed at load, instead of @code{%SYSINIT}.
   
 @item @option{--standard=@emph{<standard>}}  @item @option{--standard=@emph{<standard>}}
 Sets the default FreeM dialect to use for new routine buffers.  Sets the default FreeM dialect to use for new routine buffers.
Line 371  Attempting to start a FreeM interpreter Line 370  Attempting to start a FreeM interpreter
 The FreeM direct-mode environment is the mode entered when FreeM is invoked without the use of @option{-r @emph{<entryref>}} or @option{--routine=@emph{<entryref>}}:  The FreeM direct-mode environment is the mode entered when FreeM is invoked without the use of @option{-r @emph{<entryref>}} or @option{--routine=@emph{<entryref>}}:
   
 @example  @example
 Coherent Logic Development FreeM version 0.64.0-rc0 (x86_64-pc-linux-gnu)  Coherent Logic Development FreeM version 0.64.0-rc1 (x86_64-pc-linux-gnu)
 Copyright (C) 2014, 2020, 2021 Coherent Logic Development LLC  Copyright (C) 2014, 2020, 2021 Coherent Logic Development LLC
   
   
Line 435  Displays a list of @code{LOCK}s held in Line 434  Displays a list of @code{LOCK}s held in
 @item @command{rbuf}  @item @command{rbuf}
 Lists the status of all FreeM routine buffers.  Lists the status of all FreeM routine buffers.
   
 @anchor{dbstats}  
 @item @command{dbstats}  
 Shows statistics for the BerkeleyDB global handler.  
   
 @anchor{wh}  @anchor{wh}
 @item @command{wh}  @item @command{wh}
 Forces an immediate flush of this process's @code{readline} history buffer to disk.  Forces an immediate flush of this process's @code{readline} history buffer to disk.
Line 474  DEFAULT.USER> Line 469  DEFAULT.USER>
 @end example  @end example
   
 @item @command{!@emph{<external-command>}}  @item @command{!@emph{<external-command>}}
 Invokes a shell to run @emph{<external-command>} from within FreeM. This temporarily disables @command{SIGALRM} handling in FreeM, which may interrupt the use of event-driven M programming commands including @command{ESTART} and @command{ESTOP}.  Invokes a shell to run @emph{<external-command>} from within FreeM. This temporarily disables @command{SIGALRM} handling in FreeM, which may interrupt the use of event-driven M programming commands including @command{ASTART} and @command{ASTOP}.
   
 If the @command{>} character is supplied immediately preceding @emph{<external-command>}, FreeM will append the contents of an M local or global variable referenced in @code{^$JOB($JOB,"PIPE_GLVN")} to the standard input stream of @emph{<external-command>}.  If the @command{>} character is supplied immediately preceding @emph{<external-command>}, FreeM will append the contents of an M local or global variable referenced in @code{^$JOB($JOB,"PIPE_GLVN")} to the standard input stream of @emph{<external-command>}.
   
Line 663  Please note that FreeM is not entirely s Line 658  Please note that FreeM is not entirely s
   
 Returns a comma-delimited list of error conditions currently present, and is writable. An empty @code{$ECODE} indicates no errors.  Returns a comma-delimited list of error conditions currently present, and is writable. An empty @code{$ECODE} indicates no errors.
   
   Writing a value in the format @code{,<error-code>,} into @code{$ECODE} will raise that error condition.
   
 @node $ESTACK  @node $ESTACK
 @section $ESTACK  @section $ESTACK
 @cindex $ESTACK  @cindex $ESTACK
Line 1242  The optional fourth argument is the fina Line 1239  The optional fourth argument is the fina
 Can be used on the left-hand side of an expression in order to @code{@ref{SET}} a value into a @code{d}-delimited piece of @code{s}, as in:  Can be used on the left-hand side of an expression in order to @code{@ref{SET}} a value into a @code{d}-delimited piece of @code{s}, as in:
   
 @example  @example
 ; ^jpw="this^is^a^piece"  ; ^snw="this^is^a^piece"
 SET $PIECE(^jpw,"^",2)="isn't" ; => "this^isn't^a^piece"  SET $PIECE(^snw,"^",2)="isn't" ; => "this^isn't^a^piece"
 @end example  @end example
   
 @node $QLENGTH()  @node $QLENGTH()
Line 1275  Returns the number of subscripts in @emp Line 1272  Returns the number of subscripts in @emp
 @code{$QSUBSCRIPT(@emph{expr V glvn},@emph{expr V n})}  @code{$QSUBSCRIPT(@emph{expr V glvn},@emph{expr V n})}
 @end example  @end example
   
 Returns the @emph{n}th subscript of @emph{glvn}.  In the RHS form, returns the @emph{n}th subscript of @emph{glvn}.
   
 @emph{Example}  @emph{Example}
   
Line 1283  Returns the @emph{n}th subscript of @emp Line 1280  Returns the @emph{n}th subscript of @emp
 @code{SET SUB=$QSUBSCRIPT("^GBL(1,2,3)",2) ; => 2}  @code{SET SUB=$QSUBSCRIPT("^GBL(1,2,3)",2) ; => 2}
 @end example  @end example
   
   @emph{Syntax}
   
   @example
   @code{SET $QSUBSCRIPT(@emph{expr V glvn},@emph{expr V n})=@emph{expr} ; => ^GBL(1,4,3)}
   @end example
   
   In the LHS form, sets the @emph{n}th subscript of @emph{glvn} to @emph{expr}.
   
 @node $QUERY()  @node $QUERY()
 @section $QUERY  @section $QUERY
 @cindex $QUERY  @cindex $QUERY
Line 1300  Returns the next subscripted reference i Line 1305  Returns the next subscripted reference i
   
 We will assume the following data structure exists:  We will assume the following data structure exists:
 @example  @example
 ^jpw(1)=1  ^snw(1)=1
 ^jpw(1,2)="foo"  ^snw(1,2)="foo"
 ^jpw(2)=3  ^snw(2)=3
 ^jpw(3)=""  ^snw(3)=""
 @end example  @end example
   
 The following code will retrieve the next subscripted name after @code{^jpw(1)}:  The following code will retrieve the next subscripted name after @code{^snw(1)}:
   
 @example  @example
 @code{SET NEXTNAM=$QUERY(^jpw(1)) ; => ^jpw(1,2)}  @code{SET NEXTNAM=$QUERY(^snw(1)) ; => ^snw(1,2)}
 @end example  @end example
   
 @node $RANDOM()  @node $RANDOM()
Line 2509  In the above @emph{inclusive} form, @cod Line 2514  In the above @emph{inclusive} form, @cod
 @cartouche  @cartouche
 @quotation  @quotation
 @emph{Note}  @emph{Note}
 The below @emph{argumentless} and @emph{exclusive} forms of @code{KSUBSCRIPTS} are not implemented in FreeM, as of version 0.64.0-rc0, but are planned for a future release.  The below @emph{argumentless} and @emph{exclusive} forms of @code{KSUBSCRIPTS} are not implemented in FreeM, as of version 0.64.0-rc1, but are planned for a future release.
 @end quotation  @end quotation
 @end cartouche  @end cartouche
   
Line 2544  In the above @emph{inclusive} form, @cod Line 2549  In the above @emph{inclusive} form, @cod
 @cartouche  @cartouche
 @quotation  @quotation
 @emph{Note}  @emph{Note}
 The below @emph{argumentless} and @emph{exclusive} forms of @code{KVALUE} are not implemented in FreeM, as of version 0.64.0-rc0, but are planned for a future release.  The below @emph{argumentless} and @emph{exclusive} forms of @code{KVALUE} are not implemented in FreeM, as of version 0.64.0-rc1, but are planned for a future release.
 @end quotation  @end quotation
 @end cartouche  @end cartouche
   
Line 3300  WATCH[@emph{:postcondition}] [+|-|?]@emp Line 3305  WATCH[@emph{:postcondition}] [+|-|?]@emp
 @end example  @end example
   
   
 The following example demonstrates turning watchpoint processing on and adding a watchpoint for global variable @code{^jpw(1)}. It then changes the value of @code{^jpw(1)}.  The following example demonstrates turning watchpoint processing on and adding a watchpoint for global variable @code{^snw(1)}. It then changes the value of @code{^snw(1)}.
   
 @example  @example
 DEFAULT.USER> WATCH  DEFAULT.USER> WATCH
     
 Watchpoints enabled.  Watchpoints enabled.
     
 DEFAULT.USER> WATCH +^JPW(1)  DEFAULT.USER> WATCH +^SNW(1)
     
 Added '^JPW("1")' to the watchlist.  Added '^SNW("1")' to the watchlist.
     
 DEFAULT.USER> SET ^JPW(1)="new value"  DEFAULT.USER> SET ^SNW(1)="new value"
     
 >> WATCHPOINT:  ^JPW("1") => 'new value' (changed 1 times)  >> WATCHPOINT:  ^SNW("1") => 'new value' (changed 1 times)
   
 @end example  @end example
   
 The following example will remove that watchpoint:  The following example will remove that watchpoint:
   
 @example  @example
 DEFAULT.USER> WATCH -^JPW(1)  DEFAULT.USER> WATCH -^SNW(1)
     
 Removed '^JPW("1")' from the watchlist.  Removed '^SNW("1")' from the watchlist.
     
 DEFAULT.USER> WATCH ?^JPW(1)  DEFAULT.USER> WATCH ?^SNW(1)
     
 '^JPW("1")' is not being watched.  '^SNW("1")' is not being watched.
 @end example  @end example
   
 @node WITH  @node WITH
Line 4166  Forces a number to positive, whether pos Line 4171  Forces a number to positive, whether pos
 @section Unary -  @section Unary -
 @cindex operators, unary -  @cindex operators, unary -
   
   Forces a number to negative, whether positive or negative. Also forces numeric coercion of strings.
   
 @node +  @node +
 @section + (Add)  @section + (Add)
 @cindex operators, +  @cindex operators, +
   
   @emph{Syntax}
   
   @example
   S X=1+2 ; => 3
   @end example
   
   Adds numbers together.
   
 @node +=  @node +=
 @section += (Add/Assign)  @section += (Add/Assign)
 @cindex operators, +=  @cindex operators, +=
   
   @emph{Syntax}
   
   @example
   S X=5
   S X+=3 ; => 8
   @end example
   
   Increments the variable on the LHS by the value on the RHS.
   
 @node ++  @node ++
 @section ++ (Postfix Increment)  @section ++ (Postfix Increment)
 @cindex operators, ++  @cindex operators, ++
   
   Increments a variable by 1.
   
 @node -  @node -
 @section - (Subtract)  @section - (Subtract)
 @cindex operators, -  @cindex operators, -
   
   Subtracts one number from another.
   
 @node -=  @node -=
 @section -= (Subtract/Assign)  @section -= (Subtract/Assign)
 @cindex operators, -=  @cindex operators, -=
   
   @emph{Syntax}
   
   @example
   S X=5
   S X-=3 ; => 2
   @end example
   
   Decrements the variable on the LHS by the value on the RHS.
   
 @node --  @node --
 @section -- (Postfix Decrement)  @section -- (Postfix Decrement)
 @cindex operators, --  @cindex operators, --
   
   Decrements the variable by one.
   
 @node *  @node *
 @section * (Multiply)  @section * (Multiply)
 @cindex operators, *  @cindex operators, *
   
   Multiplies one number by another.
   
 @node *=  @node *=
 @section *= (Multiply/Assign)  @section *= (Multiply/Assign)
 @cindex operators, *=  @cindex operators, *=
   
   
   
 @node /  @node /
 @section / (Divide)  @section / (Divide)
 @cindex operators, /  @cindex operators, /
Line 4361  An @code{INTEGER} is an interpretation o Line 4404  An @code{INTEGER} is an interpretation o
 @cindex data types, REAL  @cindex data types, REAL
 @cindex types, REAL  @cindex types, REAL
   
   A @code{REAL} is a numeric interpretation of data including a fractional part.
   
 @node STRING  @node STRING
 @section STRING  @section STRING
 @cindex data types, STRING  @cindex data types, STRING
 @cindex types, STRING  @cindex types, STRING
   
   A @code{STRING} is any data in FreeM.
   
 @node Custom Types (Classes)  @node Custom Types (Classes)
 @section Custom Types (Classes)  @section Custom Types (Classes)
 @cindex data types, custom  @cindex data types, custom
Line 4380  See @ref{Classes}. Line 4427  See @ref{Classes}.
 @cindex variables, global  @cindex variables, global
 @cindex data  @cindex data
   
   @menu
   * Globals Overview::            Basics of FreeM persistent storage.
   * Creating Globals::            How to create globals.
   * Removing Globals::            How to remove globals.
   * Global Storage::              How globals are stored.
   @end menu
   
   @node Globals Overview
   @section Globals Overview
   
   FreeM supports typical M globals, which are often described as persistent, hierachical sparse arrays. Globals make it relatively simple to include persistent data in an application without requiring the developer to use an external database management system, and offer syntax and semantics so similar to M local variables and structured system variables that moving from one to the other is seamless.
   
   Each global comprises three elements:
   
   @itemize @bullet
   @item
   An alphabetic name beginning with a caret (@code{^}) or a caret and a percent sign (@code{^%})
   @item
   Optionally, one or more comma-delimited subscripts, enclosed in parentheses
   @item
   A value of up to 255 characters in length
   @end itemize
   
   The percent sign will force the named global into the @code{SYSTEM} namespace of the current FreeM environment.
   
   @node Creating Globals
   @section Creating Globals
   @cindex globals, creating
   
   To create a global, you can use the @code{SET} command:
   
   @example
   SET ^MYGLOBAL("foo","bar")="this is the data value"
   @end example
   
   @node Removing Globals
   @section Removing Globals
   @cindex globals, removing
   
   To remove an entire global, you can use the @code{KILL} command with the unsubscripted name of the global:
   
   @example
   KILL ^MYGLOBAL
   @end example
   
   If you only want to remove part of a global, i.e., beginning at a certain subscript level, use the @code{KILL} command with a subscripted name:
   
   @example
   KILL ^MYGLOBAL("foo")
   @end example
   
   This will remove only the @code{"foo"} subscript and all of its children.
   
   If you only want to remove the data value at a specific subscript level, leaving the subscript itself intact, use @code{KVALUE}:
   
   @example
   KVALUE ^MYGLOBAL("foo")
   @end example
   
   @node Global Storage
   @section Global Storage
   @cindex globals, storage
   
   FreeM globals are stored in @code{$PREFIX/var/freem/@emph{<environment-name>}/@emph{<namespace-name>}/globals} in a binary format.
   
   Global files have a header of the following format:
   
   @verbatim
   typedef struct global_header {
       
       char magic[5]; /* FRMGL */
       int format_version;
       char host_triplet[40];
       char host_id[256];
       
       unsigned long block_size;
       unsigned long last_transaction_id;
   
       long created;
       long last_backup;
       
   } global_header;
   @end verbatim
   
 @node Concurrency Control  @node Concurrency Control
 @chapter Concurrency Control  @chapter Concurrency Control
 @cindex concurrency control  @cindex concurrency control
 @cindex locking  @cindex locking
 @cindex transaction processing  @cindex transaction processing
   
   @menu
   * Concurrency Control Overview::                Basics of concurrency control.
   * Advisory Locks::                              Coordinating access voluntarily.
   * Transaction Processing::                      Ensuring logical consistency.
   @end menu
   
   @node Concurrency Control Overview
   @section Concurrency Control Overview
   
   Multitasking, multi-user FreeM applications must concern themselves with concurrent access to globals in order to maintain logical consistency and prevent concurrent writes from interleaving.
   
   In FreeM, there are two mechanisms provided for managing concurrent global access: advisory locks, and transaction processing.
   
   @node Advisory Locks
   @section Advisory Locks
   
   @node Transaction Processing
   @section Transaction Processing
   @cindex transaction processing
   
   FreeM implements a significant subset of the transaction processing features from @emph{ANSI X11.1-1995}. This allows a series of global operations to be conducted all at once, either in batch mode (where concurrency is not disturbed), or in serial mode (where writes are guaranteed to be atomic, consistent, isolated, and durable).
   
   @subsection Theory of Operation
   
   FreeM uses a pessimistic concurrency control mechanism for @code{SERIAL} transactions, meaning that any @code{TSTART} command that includes the @code{SERIAL} transaction parameter will cause the process to acquire the transaction processing mutex, which prevents any process but the one holding the mutex from performing any data access (read or write) until either @code{TCOMMIT} or @code{TROLLBACK} is called, either committing or rolling back the transaction, respectively.
   
   Any transaction in between its @code{TSTART} and @code{TCOMMIT}/@code{TROLLBACK} is said to be @emph{in-flight}. During the in-flight stage, pending global operations are held only in memory and after-image journals.
   
   FreeM maintains a list of all globals affected during a transaction in-flight. When a @code{TCOMMIT} is reached, FreeM will generate a @emph{checkpoint} of each global data file to be changed by the transaction. These checkpoints allow all FreeM globals to be restored to their pre-transaction state if a @code{TCOMMIT} should fail part of the way through its operation. 
   
   Checkpoints can have one of two modes:
   
   @table @asis
   
   @item @code{CP_REMOVE}
   Used for globals that did not exist prior to the beginning of this transaction. Simply marks the entire global data file for deletion in case of @code{TCOMMIT} failure.
   
   @item @code{CP_RESTORE}
   Used for globals that @emph{did} exist prior to the beginning of this transaction. In this case, the entire global data file is copied to a new file with a @code{.chk} extension. In cases of @code{TCOMMIT} failure, @code{CP_RESTORE} checkpoint files will be restored over the partially-modified live data file.
   
   @end table
   
   The below example shows a few global operations and checkpoints for a transaction in-flight using the @code{trantab} direct-mode command:
   
   @verbatim
   TL1:DEFAULT.USER> trantab
    $TLEVEL 1*
     Operations for Transaction ID: 6ea14aad-b8f1-47f9-9f52-4f513f892bc0 [RESTARTABLE SERIAL]
   
      OP. NO.   ACTION         KEY/DATA
      -------   ------         --------
      1         SET            ^FOO=3
      2         KILL           ^FOO
      3         SET            ^snw=10
      4         SET            ^BRANDNEW=6
   
     Global checkpoints:
   
      GLOBAL                        MODE                FILES
      ------                        ----                -----
      ^BRANDNEW                     CP_REMOVE           IN:   /usr/local/var/freem/USER/globals/^BRANDNEW
      ^snw                          CP_RESTORE          IN:   /usr/local/var/freem/USER/globals/^snw
                                                        OUT:  /usr/local/var/freem/USER/globals/^snw.23390.1.chk
      ^FOO                          CP_RESTORE          IN:   /usr/local/var/freem/USER/globals/^FOO
                                                        OUT:  /usr/local/var/freem/USER/globals/^FOO.23390.1.chk
   @end verbatim
   
   In the above example, @code{IN} files are the live data file that will be overwritten or removed, and @code{OUT} files are the checkpoints themselves. Note that @code{OUT} files are only used for @code{CP_RESTORE} checkpoints.
   
   @subsection Using Transaction Processing
   
   To use transactions in FreeM, you need to be familiar with three commands:
   
   @itemize @bullet
   @item
   @code{TSTART}
   @item
   @code{TCOMMIT}
   @item
   @code{TROLLBACK}
   @end itemize
   
   With transaction processing, global variable operations occurring between @code{TSTART} and @code{TCOMMIT} commands will be contained within the transaction.
   
   The atomicity, consistency, isolation, and durability facets of FreeM transaction hinge on the transaction mode.
   
   @subsubsection BATCH Transactions
   @code{BATCH} transactions offer higher performance, and allow other applications aside from the one doing the transaction to continue normal operations until the transaction is committed with @code{TCOMMIT}. In batch mode, other processes are only locked out of normal operation during the commit phase of the transaction.
   
   The effect of this is that the operations within the batch transaction will not be interleaved with global writes from other applications, but the entire lifetime of the transaction is not guaranteed to be serialized with respect to the transaction processing activities of other running applications in the environment.
   
   @subsubsection SERIAL Transactions
   @code{SERIAL} transactions offer full ACID compliance at the expense of multiprocessing performance. In serial mode, a @code{TSTART} blocks all activity from all other FreeM processes in the environment, and this blocking effect is not released until the transaction is committed with @code{TCOMMIT} or rolled back with @code{TROLLBACK} (or due to abnormal conditions in the environment that preclude the successful completion of the transaction).
   
 @node Local Variables  @node Local Variables
 @chapter Local Variables  @chapter Local Variables
 @cindex variables, local  @cindex variables, local
Line 4438  See @ref{Classes}. Line 4663  See @ref{Classes}.
 @cindex programming, object-oriented  @cindex programming, object-oriented
   
 @menu  @menu
 * Classes::             The basis of object-oriented programming.  * Classes::                         The basis of object-oriented programming.
   * Inheritance::                     Basing one class upon another.
   * Methods::                         Attaching code to a class.
   * Public and Private Variables::    Managing class member access.
   * Instantiating Objects::           Creating instances of classes.
   * Determining Object Class::        Getting object information at runtime.
 @end menu  @end menu
   
 @node Classes  @node Classes
 @section Classes  @section Classes
   
 @menu  @menu
 * Inheritance::         Basing one class upon another.  * Class Overview::                  Class basics.
 * Methods::             Attaching code to a class.  * Constructors::                    Managing object creation.
 * Public Variables::    Variables visible outside of a class.  * Destructors::                     Cleaning up.
 * Private Variables::   Variables only visible within a class.  * Runtime Polymorphism::            Selecting methods at runtime.
 @end menu  @end menu
   
   @node Class Overview
   @subsection Class Overview
   
   A @emph{class} is the primary organizing concept of FreeM support for object-oriented programming, and in FreeM, is simply an M routine with a few special properties:
   
   @example
   MYCLASS(THIS,INIT):OBJECT ; Constructor for MYCLASS, inherits OBJECT
     ; two private variables
     S THIS("NUMERATOR"):PRIVATE=$P(INIT,"/",1)
     S THIS("DENOMINATOR"):PRIVATE=$P(INIT,"/",2)
     Q
     ;
   DESTROY(THIS) ; This is the destructor
     Q
   @end example
   
   The above example demonstrates general class syntax.
   
   @node Constructors
   @subsection Constructors
   
   A @emph{constructor} is an M entry point that is called when a new instance of a class is created.
   
   A constructor must be the first entry point in a class routine, its tag must match the class/routine name, and it must take two arguments, @code{THIS} and @code{INIT}.
   
   @code{THIS} represents the instance of the object being accessed, and @code{INIT} represents an initializer that can be used to assign an initial value to the object when instantiating the class.
   
   A constructor looks like this:
   
   @example
   %FRACTION(THIS,INIT):OBJECT ;
      S THIS("NUMERATOR"):PRIVATE=$P(INIT,"/",1)
      S THIS("DENOMINATOR"):PRIVATE=$P(INIT,"/",2)
      Q
   @end example
   
   @emph{Syntax}
   @example
   @emph{<class-name>}(THIS,INIT)[:@emph{<superclass>}]
   @end example
   
   In the above example, @emph{<superclass>} represents the name of a class from which this class should inherit. In this case, the @code{FRACTION} class inherits from the @code{OBJECT} class. Note that this is not strictly necessary in this case, as all classes in FreeM automatically inherit from @code{OBJECT}.
   
   @node Destructors
   @subsection Destructors
   A @code{destructor} is called when you @code{KILL} an instance variable. Its tag must be @code{DESTROY}, and it must take one argument (@code{THIS}).
   
   The destructor should be used to clean up any resources used by class methods.
   
   A destructor looks like this:
   
   @example
   DESTROY(THIS) ;
     ; free any resources that should be freed at the end of the object's lifetime
     Q
   @end example
   
 @node Inheritance  @node Inheritance
 @subsection Inheritance  @section Inheritance
   
   Every class you create will automatically inherit the methods and functionality of the @code{OBJECT} class, supplied with FreeM.
   
   When attempting to call a method, FreeM will first search the class routine for a matching entry point, and then follow the inheritance chain upwards until a matching entry point is found. If the final class in the chain does not have a matching entry point, FreeM will try to find a matching entry point in the @code{OBJECT} class.
   
   Inheritance is achieved by specifying the name of the superclass in the constructor:
   
   @example
   CLASS(THIS,INIT):SUPERCLASS
   @end example
   
   @node Runtime Polymorphism
   @subsection Runtime Polymorphism
   
   You can achieve runtime polymorphism by subclassing, and defining methods in the subclass that match the names of existing methods in the superclass. Following FreeM inheritance rules, the overridden method in the subclass will be called, and the method in the superclass will not.
   
   Note that the overridden method in the subclass can take a different set or number of arguments than the @emph{formallist} of the superclass method would specify.
   
 @node Methods  @node Methods
 @subsection Methods  @section Methods
   Class methods are defined as tags with @emph{formallist}s in a class routine, and per the typical FreeM object pattern, must take at least one argument, being @code{THIS} (representing a reference to the object instance being accessed).
   
   The following class (@code{MYCLASS}) has a constructor, a destructor, and a method called @code{MYMETHOD}:
   
   @example
   %MYCLASS(THIS,INIT) ;
     Q THIS
   DESTROY(THIS) ;
     Q
   MYMETHOD(THIS) ;
     Q "VALUE"
   @end example
   
   The dot operator is used to invoke class methods:
   
   @example
   DEFAULT.USER> N MYOBJ=$#^%MYCLASS("")
   DEFAULT.USER> W MYOBJ.MYMETHOD()
   VALUE
   @end example
   
   @node Public and Private Variables 
   @section Public and Private Variables
   
   FreeM supports private fields with the @code{:PRIVATE} specifier in the @code{SET} command, enforcing classical object-oriented data encapsulation. The @code{:PUBLIC} specifier is provided for completeness, and is the default.
   
   The below constructor for a @code{FRACTION} class defines two private fields:
   
 @node Public Variables   @example
 @subsection Public Variables  %FRACTION(THIS,INIT):OBJECT ;
      S THIS("NUMERATOR"):PRIVATE=$P(INIT,"/",1)
      S THIS("DENOMINATOR"):PRIVATE=$P(INIT,"/",2)
      Q
   @end example   
   
   Either of the following commands will create a public field:
   
   @example
     S THIS("VARNAM")="Initial Value"
     S THIS("VARNAM"):PUBLIC="Initial Value"
   @end example
   
   Attempting to access private fields from outside of the class will raise error condition @code{ZOBJFLDACCV}. 
   
   @node Instantiating Objects
   @section Instantiating Objects
   
   To instantiate an object (i.e., create an object from a certain class), you will use the @code{NEW} command as follows:
   
   @example
   NEW MYSTR=$#^%STRING("myString")
   @end example
   
   This will create a local variable called MYSTR of type STRING, and initialize it with the value myString. 
   
   @node Determining Object Class
   @section Determining Object Class
   
   To determine the class of any FreeM local variable, you will use the @code{$$TYPE()} method:
   
   @example
   USER> W MYSTR.$$TYPE()
   ^%STRING
   @end example
   
 @node Private Variables  The @code{$$TYPE()} method is a member of the @code{OBJECT} class. 
 @subsection Private Variables  
   
 @node Libraries  @node Libraries
 @chapter Libraries  @chapter Libraries
Line 4569  To remove the above mapping, any of the Line 4933  To remove the above mapping, any of the
   KILL ^$SYSTEM("MAPPINGS","GLOBAL","^FOO")    KILL ^$SYSTEM("MAPPINGS","GLOBAL","^FOO")
 @end example  @end example
   
 @node Transaction Processing  
 @chapter Transaction Processing  
 @cindex transaction processing  
   
 FreeM implements a significant subset of the transaction processing features from @emph{ANSI X11.1-1995}. This allows a series of global operations to be conducted all at once, either in batch mode (where concurrency is not disturbed), or in serial mode (where writes are guaranteed to be atomic, consistent, isolated, and durable).  
   
 @section Theory of Operation  
   
 FreeM uses a pessimistic concurrency control mechanism for @code{SERIAL} transactions, meaning that any @code{TSTART} command that includes the @code{SERIAL} transaction parameter will cause the process to acquire the transaction processing mutex, which prevents any process but the one holding the mutex from performing any data access (read or write) until either @code{TCOMMIT} or @code{TROLLBACK} is called, either committing or rolling back the transaction, respectively.  
   
 Any transaction in between its @code{TSTART} and @code{TCOMMIT}/@code{TROLLBACK} is said to be @emph{in-flight}. During the in-flight stage, pending global operations are held only in memory and after-image journals.  
   
 FreeM maintains a list of all globals affected during a transaction in-flight. When a @code{TCOMMIT} is reached, FreeM will generate a @emph{checkpoint} of each global data file to be changed by the transaction. These checkpoints allow all FreeM globals to be restored to their pre-transaction state if a @code{TCOMMIT} should fail part of the way through its operation.   
   
 Checkpoints can have one of two modes:  
   
 @table @asis  
   
 @item @code{CP_REMOVE}  
 Used for globals that did not exist prior to the beginning of this transaction. Simply marks the entire global data file for deletion in case of @code{TCOMMIT} failure.  
   
 @item @code{CP_RESTORE}  
 Used for globals that @emph{did} exist prior to the beginning of this transaction. In this case, the entire global data file is copied to a new file with a @code{.chk} extension. In cases of @code{TCOMMIT} failure, @code{CP_RESTORE} checkpoint files will be restored over the partially-modified live data file.  
   
 @end table  
   
 The below example shows a few global operations and checkpoints for a transaction in-flight using the @code{trantab} direct-mode command:  
   
 @verbatim  
 TL1:DEFAULT.USER> trantab  
  $TLEVEL 1*  
   Operations for Transaction ID: 6ea14aad-b8f1-47f9-9f52-4f513f892bc0 [RESTARTABLE SERIAL]  
   
    OP. NO.   ACTION         KEY/DATA  
    -------   ------         --------  
    1         SET            ^FOO=3  
    2         KILL           ^FOO  
    3         SET            ^jpw=10  
    4         SET            ^BRANDNEW=6  
   
   Global checkpoints:  
   
    GLOBAL                        MODE                FILES  
    ------                        ----                -----  
    ^BRANDNEW                     CP_REMOVE           IN:   /usr/local/var/freem/USER/globals/^BRANDNEW  
    ^jpw                          CP_RESTORE          IN:   /usr/local/var/freem/USER/globals/^jpw  
                                                      OUT:  /usr/local/var/freem/USER/globals/^jpw.23390.1.chk  
    ^FOO                          CP_RESTORE          IN:   /usr/local/var/freem/USER/globals/^FOO  
                                                      OUT:  /usr/local/var/freem/USER/globals/^FOO.23390.1.chk  
 @end verbatim  
   
 In the above example, @code{IN} files are the live data file that will be overwritten or removed, and @code{OUT} files are the checkpoints themselves. Note that @code{OUT} files are only used for @code{CP_RESTORE} checkpoints.  
   
   
 @node Asynchronous Event Handling  @node Asynchronous Event Handling
Line 4834  You can also set up a trigger that appli Line 5146  You can also set up a trigger that appli
   
 This routine is the implementation of the @code{$ZCOLUMNS} intrinsic special variable.  This routine is the implementation of the @code{$ZCOLUMNS} intrinsic special variable.
   
 @section ^%SYSINIT  @section %SYSINIT
 @cindex ^%SYSINIT  @cindex %SYSINIT
 @cindex system library routines, ^%SYSINIT  @cindex system library routines, %SYSINIT
   
 This routine is the default startup routine for FreeM running in direct mode.  This routine is the default startup routine for FreeM running in direct mode.
   
Line 6059  Module headers should adhere to the foll Line 6371  Module headers should adhere to the foll
  *    binding library   *    binding library
  *   *
  *     *  
  *   Author: Serena Willis <jpw@coherent-logic.com>   *   Author: Serena Willis <snw@coherent-logic.com>
  *    Copyright (C) 1998 MUG Deutschland   *    Copyright (C) 1998 MUG Deutschland
  *    Copyright (C) <Year> Coherent Logic Development LLC   *    Copyright (C) <Year> Coherent Logic Development LLC
  *   *
Removed from v.1.27  
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  Added in v.1.33

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