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 @settitle The FreeM Manual  @settitle The FreeM Manual
   
 @copying  @copying
 This manual is for FreeM, (version 0.64.0-rc1), 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 software@footnote{FreeM subscribes to the software licensing philosophy described in @emph{Free Software, Free Society:
   Selected Essays of Richard M. Stallman}.} implementation of the M programming language.
   
   Print-optimized versions of this book are typeset in @emph{Computer Modern} by the author using the @emph{GNU Texinfo} tools.
   
 Copyright @copyright{} 2014-2025 Coherent Logic Development LLC  Copyright @copyright{} 2014-2025 Coherent Logic Development LLC
   
Line 42  This is the official manual for the Free Line 44  This is the official manual for the Free
   
 @menu  @menu
 * Introduction::                        About the FreeM Project, its history, and its goals.  * Introduction::                        About the FreeM Project, its history, and its goals.
   * Document Conventions::                Conventions used in this manual.
   * A Note on Standards::                 FreeM positioning regarding the M Development Committee.
 * FreeM Invocation::                    How to invoke FreeM from the command line.  * FreeM Invocation::                    How to invoke FreeM from the command line.
 * The FreeM Environment Daemon::        Managing shared resources in the FreeM environment.  * The FreeM Environment Daemon::        Managing shared resources in the FreeM environment.
 * The FreeM Direct-Mode Environment::   Executing M programs interactively.  * The FreeM Direct-Mode Environment::   Executing M programs interactively.
   * Debugging::                           The program development cycle of FreeM.
   
 * Directives::                          Per-Routine Language Directives.  * Directives::                          Per-Routine Language Directives.
 * Intrinsic Special Variables::         Intrinsic Special Variables.  * Intrinsic Special Variables::         Intrinsic Special Variables.
Line 91  This is the official manual for the Free Line 96  This is the official manual for the Free
 * Interrupt Handling::                  How FreeM handles interrupts.  * Interrupt Handling::                  How FreeM handles interrupts.
 * Error Processing::                    How to handle errors in M program code.  * Error Processing::                    How to handle errors in M program code.
 * FreeM Error Codes::                   Explanations of each FreeM error code.  * FreeM Error Codes::                   Explanations of each FreeM error code.
 * Debugging::                           The program development cycle of FreeM.  
   
 * System Configuration::                Configuring your FreeM installation.  * System Configuration::                Configuring your FreeM installation.
 * Accessing FreeM from C Programs::     How to use the mlib interface.  * Accessing FreeM from C Programs::     How to use the mlib interface.
Line 109  This is the official manual for the Free Line 113  This is the official manual for the Free
 @node Introduction  @node Introduction
 @unnumbered Introduction  @unnumbered Introduction
   
 FreeM started its life as @emph{FreeMUMPS}, written for MS-DOS and ported to SCO UNIX by a mysterious individual going by the name of "Shalom ha-Ashkenaz". It was released to MUG Deutschland in 1998. In 1999, Ronald L. Fox ported FreeM to the Red Hat Linux 5 of the GNU/Linux operating system. Thereafter, maintenance was taken over by the Generic Universal M Project, which changed its name first to Public Standard MUMPS and then by popular request to FreeM.  FreeM started its life as @emph{FreeMUMPS}, written for MS-DOS and ported to SCO UNIX by a mysterious individual going by the name of "Shalom ha-Ashkenaz". It was released to MUG Deutschland in 1998. In 1999, Ronald L. Fox ported FreeM to the Red Hat Linux 5 distribution of the GNU/Linux operating system. Thereafter, maintenance was taken over by the Generic Universal M Project, which changed its name first to Public Standard MUMPS and then by popular request to FreeM.
   
 When GT.M was open-sourced in late 1999, FreeM and GUMP were essentially abandoned. L.D. Landis, the owner of the original GUMP SourceForge project, and one of FreeM's significant contributors, passed maintenance of FreeM and ownership of its SourceForge project to Serena Willis in 2014. At this point, FreeM would not compile or run on modern Linux systems, so steps were taken to remedy the most pressing issues in the codebase. Limitations on the terminal size (previously hard-coded to 80x25) were lifted, and new @code{$VIEW} functions were added to retrieve the terminal size information. @code{$X} and @code{$Y} intrinsic special variables were updated to support arbitrary terminal sizes, and FreeM was once again able to build and run.  When GT.M was open-sourced in late 1999, FreeM and GUMP were essentially abandoned. L.D. Landis, the owner of the original GUMP SourceForge project, and one of FreeM's significant contributors, passed maintenance of FreeM and ownership of its SourceForge project to Serena Willis in 2014. At this point, FreeM would not compile or run on modern Linux systems, so steps were taken to remedy the most pressing issues in the codebase. Limitations on the terminal size (previously hard-coded to 80x25) were lifted, and new @code{$VIEW} functions were added to retrieve the terminal size information. @code{$X} and @code{$Y} intrinsic special variables were updated to support arbitrary terminal sizes, and FreeM was once again able to build and run.
   
 In February of 2020, work began in earnest to build a development and support infrastructure for FreeM and begin the careful process of refining it into a more stable and robust product.  In February of 2020, work began in earnest to build a development and support infrastructure for FreeM and begin the careful process of refining it into a more stable and robust application.
   
   For more information on FreeM history, see @code{@emph{$PREFIX}/share/freem/doc/freem_history.*} (distributed in PostScript, PDF, and plain text formats).
   
 @section Production Readiness  @section Production Readiness
   
Line 224  Serena Willis (Current Maintainer and Pr Line 230  Serena Willis (Current Maintainer and Pr
 Steve Zeck (Code)  Steve Zeck (Code)
   
 @end itemize  @end itemize
   
   @node Document Conventions
   @chapter Document Conventions
   @cindex document conventions
   @cindex conventions
   
   @section Formatting Conventions
   
   This manual uses the following formatting conventions:
   @itemize
   @item Code examples, filesystem paths, and commands are presented in @code{monospace}
   @item Placeholders where the reader is expected to supply a replacement value are presented in @code{@emph{monospace italics}}, and depending on context, may be surrounded by angle brackets
   @item New terminology is introduced in @emph{proportional italics}
   @end itemize
   
   @section Definitions
   
   FreeM uses abbreviations for common language elements:
   
   @table @asis
   @item @emph{$PREFIX}
   Refers to the base filesystem location within which FreeM is installed. For most distribution methods of FreeM, @emph{$PREFIX} represents either @code{/} or @code{/usr/local}.
   @item @emph{dlabel}
   Refers to a label in an M routine, beginning in the first column of the line. Can be a @emph{name} or an @emph{intlit}.
   @item @emph{entryref}
   Refers to an M routine entry point, denoted in the format @code{@emph{dlabel} [+@emph{intexpr}][^@emph{routine}]}.
   @item @emph{expr}
   Refers to any expression. Often presented in the format @emph{expr V <type>}, where @emph{V} means @emph{giving}; e.g., @emph{expr V lvn} means @emph{expression giving local variable name}.
   @item @emph{glvn}
   Refers to the name of an M global, local, or structured system variable.
   @item @emph{gvn}
   Refers to the name of an M global variable.
   @item @emph{intexpr}
   Refers to an integer expression.
   @item @emph{intlit}
   Refers to an integer literal.
   @item @emph{ISV}, @emph{isv}
   Refers to an M intrinsic special variable; @code{$JOB} and @code{$IO} are examples of ISVs.
   @item @emph{L}
   Indicates a @emph{list} of the following item, e.g., @emph{L gvn} means @emph{list of global variable names}.
   @item @emph{lvn}
   Refers to the name of an M local variable.
   @item @emph{strlit}
   Refers to an M string literal.
   @item @emph{ssvn}
   Refers to the name of an M structured system variable.
   @item @emph{tvexpr}
   Refers to a truth-valued expression.
   @end table
   
   @node A Note on Standards
   @chapter A Note on Standards
   @cindex MDC
   @cindex M Development Committee
   @cindex ANSI X11.1
   @cindex standards, ANSI
   @cindex standards, MDC
   
   FreeM attempts to implement as many features as possible from the M Development Committee's unpublished @emph{Millennium Draft Standard}, as well as its predecessors.
   
   The maintainer of FreeM (who is also the author of this book) is largely in favor of standardization efforts, and hopes that the MDC will resume activities, and will happily participate if it does so in an open, public, transparent, and democratic manner. Until then, however, FreeM will attempt to improve the M language, in cooperation with other free software M implementers where possible. Any breaking changes introduced in future MDC releases of the @emph{Standard} (such as the rumored @emph{M5}) which prove incompatible with FreeM will be handled via the @code{$DIALECT} special variable (to be changed to @code{$ZDIALECT} in a coming release).
   
   The conformance document required per the @emph{Standard} should be installed as a @code{man} page on any computer system where FreeM is made available. Simply type @code{man freem_conformance} to access.
   
 @node FreeM Invocation  @node FreeM Invocation
 @chapter FreeM Invocation  @chapter FreeM Invocation
 @cindex invocation, command-line  @cindex invocation, command-line
Line 240  When @code{-r} or @code{--routine} are p Line 310  When @code{-r} or @code{--routine} are p
   
 @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.
   
Line 522  DEFAULT.USER> Line 592  DEFAULT.USER>
   
 Such expressions will be immediately evaluated, and the result printed on @code{@ref{$IO}}.  Such expressions will be immediately evaluated, and the result printed on @code{@ref{$IO}}.
   
   @node Debugging
   @chapter Debugging
   @cindex debugging
   
   @section Debugging Synopsis
   FreeM includes an interactive debugger, entered using the @code{BREAK "DEBUG"} command. The debugger is also entered if @code{Ctrl-C} is pressed, @code{Ctrl-C} handling is enabled, and you are in direct mode.
   
   If you would like to enter the debugger automatically each time an error is encountered, add the following to your @code{LCLINIT} routine:
   
   @example
   S $ETR="B ""DEBUG"""
   @end example
   
   @section Debugging Commands
   The debugger uses its own unique command language, where M commands are unavailable. Commands are as follows:
   
   @table @asis
   @item @code{exit}, @code{quit}
   Exits the debugger and returns to direct mode or normal program execution.
   @item @code{e} @emph{glvn}, @code{examine} @emph{glvn}
   Prints the value of @emph{glvn} to the terminal.
   @item @code{t}, @code{trace}
   Toggles @emph{trace mode} on and off. When trace mode is on, FreeM will display information about each @code{DO} or @code{GOTO} command encountered, including the routine which invoked the branch, which type of branch was invoked, and the target of the branch.
   @item @code{s}, @code{step}
   Single-steps through FreeM code command-by-command.
   @item @code{n}, @code{next}
   Single-steps through FreeM code line-by-line.
   @item @code{c}, @code{cont}, @code{continue}
   Resumes normal program execution, disabling single-step mode.
   @item @code{bt}, @code{backtrace}
   Produces a stack trace.
   @item @code{h}, @code{halt}
   Halts the process being debugged and returns control to the operating system.
   @item @code{w [[+|-|?]@emph{<glvn>}]}, @code{watch [[+|-|?]@emph{<glvn>}]}
   With no arguments, toggles watchpoints on and off. With @code{+}, adds @emph{<glvn>} to the watchlist. With @code{-}, removes @emph{<glvn>} from the watchlist. With @code{?}, queries the watch status of @emph{<glvn>}.
   @end table
   
 @node Directives  @node Directives
 @chapter Directives  @chapter Directives
 @cindex directives  @cindex directives
Line 2315  ELSE[@emph{:postcondition}] Line 2422  ELSE[@emph{:postcondition}]
 @quotation  @quotation
 @emph{Non-Standard Behavior}  @emph{Non-Standard Behavior}
   
 FreeM allows a @emph{postcondition} on @code{ELSE}. While explicitly forbidden in the @emph{standard}--and for good reason--it was decided that FreeM should allow postconditions everywhere, both for the sake of foolish consistency (the likes of which Emerson warned against), and for the benefit of entrants to a hypothetical future obfuscated M contest, and those with a Machiavellian predisposition to wicked perversions and undue cleverness.  FreeM allows a @emph{postcondition} on @code{ELSE}. While explicitly forbidden in the @emph{Standard}, it was decided that FreeM should allow postconditions everywhere, both for the sake of foolish consistency (the likes of which Emerson warned against), and for the benefit of entrants to a hypothetical future obfuscated M contest, and those with a Machiavellian predisposition to wicked perversions and undue cleverness.
   
 Using postconditions on @code{ELSE} should be strictly avoided in production code, as they have no practical use, and may contribute to technical debt, hardening of the arteries, hobgoblins, a small mind, a surfeit of logic, climate change, @emph{Daily WTF} rants, or the meltdown of global financial markets.  Using postconditions on @code{ELSE} should be strictly avoided in production code, as they have no practical use, and may contribute to technical debt, hardening of the arteries, hobgoblins, a small mind, a surfeit of logic, climate change, @emph{Daily WTF} rants, or the meltdown of global financial markets.
 @end quotation  @end quotation
Line 2470  IF[@emph{:postcondition}] @emph{tvexpr}[ Line 2577  IF[@emph{:postcondition}] @emph{tvexpr}[
   
 In its argumentless form, allows the remainder of the line of code following @code{IF} to execute only if @code{$TEST} evaluates to @emph{1}, provided the optional @emph{postcondition} evaluates to @emph{true} or is omitted.  In its argumentless form, allows the remainder of the line of code following @code{IF} to execute only if @code{$TEST} evaluates to @emph{1}, provided the optional @emph{postcondition} evaluates to @emph{true} or is omitted.
   
   
 @emph{Syntax}  @emph{Syntax}
 @example  @example
 IF[@emph{:postcondition}]  IF[@emph{:postcondition}]  @emph{command...}
 @end example  @end example
   
   @cartouche
   @quotation
   @emph{Style Recommendation}
   
   In the interest of readability and maintainability, we recommend avoiding the argumentless form of @code{IF} in new code. It is an obsolete relic of an era when routine sizes were severely limited, and can be difficult to spot, as the use of whitespace (@code{IF  @emph{command}}) makes the intent of its use non-obvious at a glance. It is also far too easy to inadvertently delete the extra space, leading to program errors easily avoided otherwise.
   
   We recommend explicitly checking the value of @code{$TEST} instead, as in @code{IF $TEST @emph{command}} or @code{@emph{command}:$TEST ...}, as this makes the intent immediately clear both to M newcomers and seasoned experts, and sacrifices nothing of value, even on the oldest computer systems where FreeM can be used today.
   @end quotation
   @end cartouche
   
 @node JOB  @node JOB
 @section JOB  @section JOB
 @cindex JOB  @cindex JOB
Line 4430  A @code{REAL} is a numeric interpretatio Line 4548  A @code{REAL} is a numeric interpretatio
 @cindex data types, STRING  @cindex data types, STRING
 @cindex types, STRING  @cindex types, STRING
   
 A @code{STRING} is any data in FreeM.  The @code{STRING} is the fundamental FreeM data type. Other types are inferred from the context of their usage.
   
   @subsection String Rules
   
   The following rules apply to all FreeM strings:
   
   @itemize
   @item Must not exceed 255 characters
   @item Must not contain @code{$C(0)}, @code{$C(201)}, or @code{$C(202)}
   @end itemize
   
   @subsection String Quoting Rules
   Strings in FreeM must be surrounded in double quotes:
   
   @example
   SET MYSTRING="This is a string literal"
   @end example
   
   If you want to include double quotes inside of a string, simply double them:
   
   @example
   SET MYSTRING="This is a ""string literal"" with embedded double quotes"
   @end example
   
   
 @node Custom Types (Classes)  @node Custom Types (Classes)
 @section Custom Types (Classes)  @section Custom Types (Classes)
Line 4469  Optionally, one or more comma-delimited Line 4610  Optionally, one or more comma-delimited
 A value of up to 255 characters in length  A value of up to 255 characters in length
 @end itemize  @end itemize
   
 The percent sign will force the named global into the @code{SYSTEM} namespace of the current FreeM environment.  A leading percent sign in the global name will force the named global into the @code{SYSTEM} namespace of the current FreeM environment.
   
 @node Creating Globals  @node Creating Globals
 @section Creating Globals  @section Creating Globals
Line 4560  Transaction processing allows applicatio Line 4701  Transaction processing allows applicatio
 @section Transaction Processing  @section Transaction Processing
 @cindex 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).  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 concurrent operation is not interrupted until the last possible moment), or in serial mode (where writes are guaranteed to be atomic, consistent, isolated, and durable).
   
 @subsection Theory of Operation  @subsection Theory of Operation
   
Line 4639  The effect of this is that the operation Line 4780  The effect of this is that the operation
 @cindex variables, local  @cindex variables, local
 @cindex local variables  @cindex local variables
   
   @section Local Variables Overview
   
   FreeM @emph{local variables} have the same data structure as global variables, but are scoped to a single FreeM process, and stored in memory.
   
   Each local comprises three elements:
   
   @itemize @bullet
   @item
   An alphabetic name beginning with a letter or 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
   
   @node Creating Local Variables
   @section Creating Local Variables
   @cindex local variables, creating
   
   To create a local variable, use the @code{SET} command:
   
   @example
   SET MYLOCAL("foo","bar")="this is the data value"
   @end example
   
   @node Removing Local Variables
   @section Removing Local Variables
   @cindex local variables, removing
   
   To remove an entire local variable, you can use the @code{KILL} command with the unsubscripted name of the variable:
   
   @example
   KILL MYLOCAL
   @end example
   
   If you only want to remove part of a local variable, i.e., beginning at a certain subscript level, use the @code{KILL} command with a subscripted name:
   
   @example
   KILL MYLOCAL("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 MYLOCAL("foo")
   @end example
   
 @node Scoping  @node Scoping
 @chapter Scoping  @chapter Scoping
 @cindex scoping  @cindex scoping
   
   By default, FreeM local variables and their values are scoped to the entire process, meaning that any function or subroutine can access and modify their values. This can lead to pernicious bugs.
   
   M provides the @code{NEW} command to work around these issues. When @code{NEW} is called with a local variable as its argument, FreeM will scope the variable to the process stack frame in which the @code{NEW} command occured. When exiting the stack frame (i.e. with the @code{QUIT} command), FreeM will restore the variable to its value prior to being @code{NEW}ed.
   
   @emph{Example}
   
   @example
   MYRTN ;
     S J=1 ; set local variable J to 1
     W J,! ; this will output "1"
     D X   ; execute subroutine X
     W J,! ; this will output "1", as the value of J was restored
     Q
     ;;
   X ;
     N J   ; stack J
     S J=6 ; set its value to 6
     W J,! ; this will output "6"
     Q     ; quit from the subroutine, destroying its stack frame
     ;;
   @end example
   
   @section Scoping Considerations for $TEST
   
   In M, the truth value of comparisons, logic operations, and certain forms of @code{LOCK} is stored in the @code{$TEST} intrinsic special variable, which follows the same rules as any M local variable.
   
   This is probably the most significant design flaw of the language, as the side effects of logic on @code{$TEST} lead to incredibly difficult bugs. However, M allows @code{$TEST} to be @code{NEW}ed, and FreeM provides the @code{THEN} command@footnote{From MDC Type A extension X11/1998-31} to help in the case of conditionals. @code{THEN} stacks @code{$TEST} to the end of the line.
   
   When writing new M code in FreeM, we strongly suggest using @code{THEN} as follows:
   
   @example
   MYRTN ;
     IF MYVAR=1 THEN DO SUBRT
   @end example
   
   This is instead of the traditional form:
   
   @example
   MYRTN ;
     IF MYVAR=1 DO SUBR
   @end example
   
   @cartouche
   @quotation
   @emph{Style Recommendation}
   
   Note that @code{THEN} is not in any currently published version of the @emph{Standard}, but is part of MDC Type A extension X11/1998-31. However, we recommend using @code{THEN} instead of favoring portability, as there is no defensible reason for this incredibly simple feature @emph{not} to be ubiquitous.
   
   If you use other M implementations, you should bug the implementers to implement @code{THEN}, as it at least partially mitigates an inexcusable flaw in the design of M.
   @end quotation
   @end cartouche
   
 @node Decision Constructs  @node Decision Constructs
 @chapter Decision Constructs  @chapter Decision Constructs
 @cindex decision constructs  @cindex decision constructs
Line 5621  Raised when an attempt is made to use a Line 5863  Raised when an attempt is made to use a
   
 @end table  @end table
   
 @node Debugging  
 @chapter Debugging  
 @cindex debugging  
   
 FreeM includes an interactive debugger, entered using the @code{BREAK "DEBUG"} command. The debugger is also entered if @code{Ctrl-C} is pressed, @code{Ctrl-C} handling is enabled, and you are in direct mode.  
   
 The debugger uses its own unique command language, where M commands are unavailable. Commands are as follows:  
   
 @table @asis  
 @item @code{exit}, @code{quit}  
 Exits the debugger and returns to direct mode or normal program execution.  
 @item @code{e} @emph{glvn}, @code{examine} @emph{glvn}  
 Prints the value of @emph{glvn} to the terminal.  
 @item @code{t}, @code{trace}  
 Toggles @emph{trace mode} on and off. When trace mode is on, FreeM will display information about each @code{DO} or @code{GOTO} command encountered, including the routine which invoked the branch, which type of branch was invoked, and the target of the branch.  
 @item @code{s}, @code{step}  
 Single-steps through FreeM code command-by-command.  
 @item @code{n}, @code{next}  
 Single-steps through FreeM code line-by-line.  
 @item @code{c}, @code{cont}, @code{continue}  
 Resumes normal program execution, disabling single-step mode.  
 @item @code{bt}, @code{backtrace}  
 Produces a stack trace.  
 @item @code{h}, @code{halt}  
 Halts the process being debugged and returns control to the operating system.  
 @item @code{w [[+|-|?]@emph{<glvn>}]}, @code{watch [[+|-|?]@emph{<glvn>}]}  
 With no arguments, toggles watchpoints on and off. With @code{+}, adds @emph{<glvn>} to the watchlist. With @code{-}, removes @emph{<glvn>} from the watchlist. With @code{?}, queries the watch status of @emph{<glvn>}.  
 @end table  
   
 @node System Configuration  @node System Configuration
 @chapter System Configuration  @chapter System Configuration
Removed from v.1.35  
changed lines
  Added in v.1.40

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