

ACD(1)                    Minix Programmer's Manual                     ACD(1)


NAME
     acd - a compiler driver

SYNOPSIS
     acd -v[n] -vn[n] -name name -descr descr -T dir [arg ...]

DESCRIPTION
     Acd is a compiler driver, a program that calls the  several  passes  that
     are needed to compile a source file.  It keeps track of all the temporary
     files used between the passes.  It also  defines  the  interface  of  the
     compiler, the options the user gets to see.

     This text only describes acd itself, it says nothing about the  different
     options the C-compiler accepts.  (It has nothing to do with any language,
     other than being a tool to give a compiler a user interface.)

OPTIONS
     Acd itself takes five options:

     -v[n]
          Sets the diagnostic level to n (by default 2).  The higher n is, the
          more  output  acd  generates:  -v0 does not produce any output.  -v1
          prints the basenames of the programs called.  -v2 prints  names  and
          arguments  of  the programs called.  -v3 shows the commands executed
          from the description file too.  -v4 shows the program read from  the
          description file too.  Levels 3 and 4 use backspace overstrikes that
          look good when viewing the output with a smart pager.

     -vn[n]
          Like -v except that no command is  executed.   The  driver  is  just
          play-acting.

     -name name
          Acd is normally linked to the name the compiler is to be called with
          by  the user.  The basename of this, say cc, is the call name of the
          driver.  It plays a role in selecting the proper  description  file.
          With  the  -name  option  one can change this.  Acd -name cc has the
          same effect as calling the program as cc.

     -descr descr
          Allows one to choose the pass description file of  the  driver.   By
          default descr is the same as name, the call name of the program.  If
          descr  doesn't  start  with  /,   ./,   or   ../   then   the   file
          /usr/lib/descr/descr  will  be  used  for the description, otherwise
          descr itself.  Thus cc -descr newcc  calls  the  C-compiler  with  a
          different description file without changing the call name.  Finally,
          if descr is "-", standard input is read.  (The default lib directory
          /usr/lib,  may  be  changed to dir at compile time by -DLIB=\"dir\".
          The default descr may  be  set  with  -DDESCR=\"descr\"  for  simple
          installations on a system without symlinks.)

     -T dir
          Temporary files are made in /tmp by default, which may be overridden
          by  the  environment variable TMPDIR, which may be overridden by the
          -T option.



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THE DESCRIPTION FILE
     The description file is a program interpreted  by  the  driver.   It  has
     variables,  lists  of  files,  argument  parsing  commands, and rules for
     transforming input files.

  Syntax
     There are four simple objects:

          Words, Substitutions, Letters, and Operators.

     And there are two ways to group objects:

          Lists, forming sequences of anything but letters,

          Strings, forming sequences of anything but Words and Operators.

     Each object has the following syntax:

     Words
          They are sequences of characters, like cc, -I/usr/include, /lib/cpp.
          No  whitespace  and  no special characters.  The backslash character
          (\)  may  be  used  to  make  special  characters   common,   except
          whitespace.   A  backslash  followed  by  whitespace  is  completely
          removed from the input.  The sequence \n is changed to a newline.

     Substitutions
          A substitution (henceforth called 'subst') is formed with a $,  e.g.
          $opt,  $PATH,  ${lib}, $*.  The variable name after the $ is made of
          letters, digits and  underscores,  or  any  sequence  of  characters
          between parentheses or braces, or a single other character.  A subst
          indicates that the value of the named variable must  be  substituted
          in the list or string when fully evaluated.

     Letters
          Letters are the single characters that would make up a word.

     Operators
          The characters =, +, -, *, <, and > are the  operators.   The  first
          four  must  be  surrounded  by  whitespace if they are to be seen as
          special (they are often used in arguments).  The last two are always
          special.

     Lists
          One line of objects in the  description  file  forms  a  list.   Put
          parentheses  around  it  and  you  have  a  sublist.   The values of
          variables are lists.

     Strings
          Anything that is not yet a word is a string.  All it needs  is  that
          the  substs in it are evaluated, e.g.  $LIBPATH/lib$key.a.  A single
          subst doesn't make a string, it expands to  a  list.   You  need  at
          least one letter or other subst next to it.  Strings (and words) may
          also be formed by enclosing them in double quotes.   Only  \  and  $
          keep their special meaning within quotes.




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  Evaluation
     One thing has to be carefully understood: Substitutions are delayed until
     the  last  possible moment, and description files make heavy use of this.
     Only if a subst is tainted,  either  because  its  variable  is  declared
     local,  or  because  a  subst  in  its variable's value is tainted, is it
     immediately substituted.  So if a list is assigned  to  a  variable  then
     this  list is only checked for tainted substs.  Those substs are replaced
     by the value of their variable.  This is called partial evaluation.

     Full evaluation expands all substs,  the  list  is  flattened,  i.e.  all
     parentheses are removed from sublists.

     Implosive evaluation is the last that has to be done to a list before  it
     can  be  used  as  a command to execute.  The substs within a string have
     been evaluated to lists after full expansion, but a string must be turned
     into  a  single word, not a list.  To make this happen, a string is first
     exploded to all possible combinations of words choosing one member of the
     lists within the string.  These words are tried one by one to see if they
     exist as a file.  The first one that exists is taken, if none exists than
     the  first  choice  is  used.  As an example, assume LIBPATH equals (/lib
     /usr/lib), key is (c) and key happens to be local.  Then we have:

          "$LIBPATH/lib$key.a"

     before evaluation,

          "$LIBPATH/lib(c).a"

     after partial evaluation,

          "(/lib/libc.a /usr/lib/libc.a)"

     after full evaluation, and finally

          /usr/lib/libc.a

     after implosion, if the file exists.

  Operators
     The operators modify the way evaluation is done  and  perform  a  special
     function on a list:

     *    Forces full evaluation on all the list elements following  it.   Use
          it  to  force substitution of the current value of a variable.  This
          is the only operator that forces immediate evaluation.

     +    When a + exists in a list that is  fully  evaluated,  then  all  the
          elements  before the + are imploded and all elements after the + are
          imploded and added to the list if they are not already in the  list.
          So  this  operator  can be used either for set addition, or to force
          implosive expansion within a sublist.

     -    Like +, except that elements after the - are removed from the list.





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     The set operators can be used to gather options that exclude  each  other
     or for their side effect of implosive expansion.  You may want to write:

          cpp -I$LIBPATH/include

     to call cpp with an extra include directory,  but  $LIBPATH  is  expanded
     using  a  filename  starting  with -I so this won't work.  Given that any
     problem in Computer  Science  can  be  solved  with  an  extra  level  of
     indirection, use this instead:

          cpp -I$INCLUDE
          INCLUDE = $LIBPATH/include +

  Special Variables
     There are three special variables used in a description  file:   $*,  $<,
     and  $>.   These  variables  are always local and mostly read-only.  They
     will be explained later.

  A Program
     The lists in a description file form a program that is executed from  the
     first  to the last list.  The first word in a list may be recognized as a
     builtin command (only if the first list element is indeed simply a word.)
     If  it  is  not a builtin command then the list is imploded and used as a
     UNIX command with arguments.

     Indentation (by tabs or spaces) is not just makeup for a program, but are
     used  to group lines together.  Some builtin commands need a body.  These
     bodies are simply lines at a deeper indentation.

     Empty lines are not ignored either, they have the same indentation  level
     as  the line before it.  Comments (starting with a # and ending at end of
     line) have an indentation of their own and can be used as null commands.

     Acd will complain about unexpected indentation shifts and  empty  bodies.
     Commands  can share the same body by placing them at the same indentation
     level before the indented body.  They are then "guards" to the same body,
     and  are  tried  one  by  one until one succeeds, after which the body is
     executed.

     Semicolons may be used to separate commands  instead  of  newlines.   The
     commands are then all at the indentation level of the first.

  Execution phases
     The driver runs in three phases: Initialization, Argument  scanning,  and
     Compilation.   Not  all  commands  work  in  all phases.  This is further
     explained below.

  The Commands
     The commands accept arguments that are usually generic  expressions  that
     implode  to  a  word  or  a list of words.  When var is specified, then a
     single word or subst needs to be given, so an assignment  can  be  either
     name = value, or $name = value.

     var = expr ...
          The partially evaluated list of  expressions  is  assigned  to  var.
          During  the  evaluation  is  var  marked  as  local,  and  after the


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          assignment set from undefined to defined.

     unset var
          Var is set to null and is marked as undefined.

     import var
          If var is defined in the environment of acd then it is  assigned  to
          var.  The environment variable is split into words at whitespace and
          colons.  Empty space between two colons (::)  is changed to a dot.

     mktemp var [suffix]
          Assigns to var the name of a new temporary file,  usually  something
          like  /tmp/acd12345x.  If suffix is present then it will be added to
          the temporary file's name.  (Use it because  some  programs  require
          it,  or  just  because it looks good.)  Acd remembers this file, and
          will delete it as soon as you stop referencing it.

     temporary word
          Mark the file named by word as a temporary file.  You have  to  make
          sure  that the name is stored in some list in imploded form, and not
          just temporarily created when word is  evaluated,  because  then  it
          will be immediately removed and forgotten.

     stop suffix
          Sets the target suffix for the compilation  phase.   Something  like
          stop  .o  means  that  the  source  files must be compiled to object
          files.  At least one  stop  command  must  be  executed  before  the
          compilation  phase  begins.   It  may  not  be  changed  during  the
          compilation phase.  (Note: There is no  restriction  on  suffix,  it
          need not start with a dot.)

     treat file suffix
          Marks the file as having the given suffix  for  the  compile  phase.
          Useful for sending a -l option directly to the loader by treating it
          as having the .a suffix.

     numeric arg
          Checks if arg is a number.  If not then acd will exit  with  a  nice
          error message.

     error expr ...
          Makes the driver print the error message expr ... and exit.

     if expr = expr
          If tests if the two expressions are equal using set comparison, i.e.
          each   expression   should  contain  all  the  words  in  the  other
          expression.  If the test succeeds then the if-body is executed.

     ifdef var
          Executes the ifdef-body if var is defined.

     ifndef var
          Executes the ifndef-body if var is undefined.





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     iftemp arg
          Executes the iftemp-body if arg is a temporary file.  Use it when  a
          command  has the same file as input and output and you don't want to
          clobber the source file:

               transform .o .o
                       iftemp $*
                               $> = $*
                       else
                               cp $* $>
                       optimize $>

     ifhash arg
          Executes the ifhash-body if arg is an existing file with  a  '#'  as
          the very first character.  This usually indicates that the file must
          be pre-processed:

               transform .s .o
                       ifhash $*
                               mktemp ASM .s
                               $CPP $* > $ASM
                       else
                               ASM = $*
                       $AS -o $> $ASM
                       unset ASM

     else Executes the else-body if  the  last  executed  if,  ifdef,  ifndef,
          iftemp,  or  ifhash  was  unsuccessful.   Note  that  else  need not
          immediately follow an if, but you are advised not  to  make  use  of
          this.  It is a "feature" that may not last.

     apply suffix1 suffix2
          Executed inside a transform rule body to transform  the  input  file
          according  to  another  transform  rule that has the given input and
          output suffixes.  The file under $* will  be  replaced  by  the  new
          file.   So if there is a .c .i preprocessor rule then the example of
          ifhash can be replaced by:

               transform .s .o
                       ifhash $*
                               apply .c .i
                       $AS -o $> $*

     include descr
          Reads another description file and replaces  the  include  with  it.
          Execution  continues  with  the  first list in the new program.  The
          search for descr is the same as used for  the  -descr  option.   Use
          include to switch in different front ends or back ends, or to call a
          shared description file with a different initialization.  Note  that
          descr is only evaluated the first time the include is called.  After
          that the include has been replaced with  the  included  program,  so
          changing its argument won't get you a different file.

     arg string ...
          Arg may be executed in the initialization and scanning phase to post
          an argument scanning rule, that's all the command itself does.  Like


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          an if that fails it allows more guards to share the same body.

     transform suffix1 suffix2
          Transform, like arg, only posts a rule to transform a file with  the
          suffix suffix1 into a file with the suffix suffix2.

     prefer suffix1 suffix2
          Tells that the transformation rule from suffix1 to suffix2 is to  be
          preferred when looking for a transformation path to the stop suffix.
          Normally the shortest route to the stop suffix is used.   Prefer  is
          ignored  on  a  combine, because the special nature of combines does
          not allow ambiguity.

          The two suffixes on a transform or prefer may be the same, giving  a
          rule that is only executed when preferred.

     combine suffix-list suffix
          Combine is like transform except that it  allows  a  list  of  input
          suffixes to match several types of input files that must be combined
          into one.

     scan The scanning phase may be run early from  the  initialization  phase
          with  the scan command.  Use it if you need to make choices based on
          the  arguments  before  posting  the  transformation  rules.   After
          running this, scan and arg become no-ops.

     compile
          Move on to the compilation phase early, so that you have a chance to
          run  a  few  extra  commands before exiting.  This command implies a
          scan.

     Any other command is seen as a UNIX command.  This is where the <  and  >
     operators  come  into  play.   They  redirect standard input and standard
     output to the file mentioned after them, just like the shell.   Acd  will
     stop with an error if the command is not successful.

  The Initialization Phase
     The driver starts by executing the program once from  top  to  bottom  to
     initialize variables and post argument scanning and transformation rules.

  The Scanning Phase
     In this phase the driver makes a pass over the command line arguments  to
     process  options.   Each  arg  rule is tried one by one in the order they
     were posted against the front of the argument list.  If a match  is  made
     then  the  matched  arguments  are removed from the argument list and the
     arg-body is executed.  If no match can be made then the first argument is
     moved  to  the  list  of  files waiting to be transformed and the scan is
     restarted.

     The match is done as follows: Each of the strings after  arg  must  match
     one  argument at the front of the argument list.  A character in a string
     must match a character in an argument word, a subst in a string may match
     1  to  all  remaining  characters in the argument, preferring the longest
     possible match.  The hyphen in a argument starting with a  hyphen  cannot
     be matched by a subst.  Therefore:



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          arg -i

     matches only the argument -i.

          arg -O$n

     matches any argument that starts with -O and is at least three characters
     long.  Lastly,

          arg -o $out

     matches -o and the argument following it,  unless  that  argument  starts
     with a hyphen.

     The variable $* is set to all the matched arguments before  the  arg-body
     is executed.  All the substs in the arg strings are set to the characters
     they match.  The variable $> is set to  null.   All  the  values  of  the
     variables are saved and the variables marked local.  All variables except
     $> are marked read-only.  After the arg-body is executed is the value  of
     $>  concatenated  to  the  file list.  This allows one to stuff new files
     into the transformation phase.  These added names are not evaluated until
     the start of the next phase.

  The Compilation Phase
     The files gathered in the  file  list  in  the  scanning  phase  are  now
     transformed  one by one using the transformation rules.  The shortest, or
     preferred route is computed for each file all the way to the stop suffix.
     Each  file  is  transformed  until  it  lands at the stop suffix, or at a
     combine rule.  After a while all files are either fully transformed or at
     a combine rule.

     The driver chooses a combine rule that is not  on  a  path  from  another
     combine  rule and executes it.  The file that results is then transformed
     until it again lands  at  a  combine  rule  or  the  stop  suffix.   This
     continues until all files are at the stop suffix and the program exits.

     The paths through transform rules may be ambiguous and have cycles,  they
     will  be  resolved.   But  paths  through  combines  must be unambiguous,
     because of the many paths from the different files that  meet  there.   A
     description  file will usually have only one combine rule for the loader.
     However if you do have a combine conflict then put a no-op transform rule
     in front of one to resolve the problem.

     If a file matches a long and a short  suffix  then  the  long  suffix  is
     preferred.   By  putting a null input suffix ("") in a rule one can match
     any file that no other rule matches.  You can send unknown files  to  the
     loader this way.

     The variable $* is set to the file to be transformed or the files  to  be
     combined  before the transform or combine-body is executed.  $> is set to
     the output file name, it may  again  be  modified.   $<  is  set  to  the
     original  name  of  the first file of $* with the leading directories and
     the suffix removed.  $* will be made up  of  temporary  files  after  the
     first  rule.  $> will be another temporary file or the name of the target
     file ($< plus the stop suffix), if the stop suffix is reached.



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     $> is passed to the next rule; it is imploded and checked to be a  single
     word.   This  driver  does  not  store  intermediate  object files in the
     current directory like most other compilers, but keeps them in /tmp  too.
     (Who  knows  if  the current directory can have files created in?)  As an
     example, here is how you can express the "normal" method:

          transform .s .o
                  if $> = $<.o
                          # Stop suffix is .o
                  else
                          $> = $<.o
                          temporary $>
                  $AS -o $> $*

     Note that temporary is not called if the target  is  already  the  object
     file,  or  you would lose the intended result!  $> is known to be a word,
     because $< is local.  (Any string whose substs are all  expanded  changes
     to a word.)

  Predefined Variables
     The driver has three variables predefined:  PROGRAM, set to the call name
     of the driver, VERSION, the driver's version number, and ARCH, set to the
     name of the default output architecture.  The  latter  is  optional,  and
     only defined if acd was compiled with -DARCH=\"arch-name\".

EXAMPLE
     As an example a description file for a C compiler is  given.   It  has  a
     front  end (ccom), an intermediate code optimizer (opt), a code generator
     (cg), an assembler (as), and  a  loader  (ld).   The  compiler  can  pre-
     process, but there is also a separate cpp.  If the -D and options like it
     are changed to look like -o then this example  is  even  as  required  by
     POSIX.

          # The compiler support search path.
          C =     /lib /usr/lib /usr/local/lib

          # Compiler passes.
          CPP =   $C/cpp $CPP_F
          CCOM =  $C/ccom $CPP_F
          OPT =   $C/opt
          CG =    $C/cg
          AS =    $C/as
          LD =    $C/ld

          # Predefined symbols.
          CPP_F = -D__EXAMPLE_CC__

          # Library path.
          LIBPATH = $USERLIBPATH $C

          # Default transformation target.
          stop .out

          # Preprocessor directives.
          arg -D$name
          arg -U$name


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          arg -I$dir
                  CPP_F = $CPP_F $*

          # Stop suffix.
          arg -c
                  stop .o

          arg -E
                  stop .E

          # Optimization.
          arg -O
                  prefer .m .m
                  OPT = $OPT -O1

          arg -O$n
                  numeric $n
                  prefer .m .m
                  OPT = $OPT $*

          # Add debug info to the executable.
          arg -g
                  CCOM = $CCOM -g

          # Add directories to the library path.
          arg -L$dir
                  USERLIBPATH = $USERLIBPATH $dir

          # -llib must be searched in $LIBPATH later.
          arg -l$lib
                  $> = $LIBPATH/lib$lib.a

          # Change output file.
          arg -o$out
          arg -o $out
                  OUT = $out

          # Complain about a missing argument.
          arg -o
                  error "argument expected after '$*'"

          # Any other option (like -s) are for the loader.
          arg -$any
                  LD = $LD $*

          # Preprocess C-source.
          transform .c .i
                  $CPP $* > $>

          # Preprocess C-source and send it to standard output or $OUT.
          transform .c .E
                  ifndef OUT
                          $CPP $*
                  else
                          $CPP $* > $OUT



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          # Compile C-source to intermediate code.
          transform .c .m
          transform .i .m
                  $CCOM $* $>

          # Intermediate code optimizer.
          transform .m .m
                  $OPT $* > $>

          # Intermediate to assembly.
          transform .m .s
                  $CG $* > $>

          # Assembler to object code.
          transform .s .o
                  if $> = $<.o
                          ifdef OUT
                                  $> = $OUT
                  $AS -o $> $*

          # Combine object files and libraries to an executable.
          combine (.o .a) .out
                  ifndef OUT
                          OUT = a.out
                  $LD -o $OUT $C/crtso.o $* $C/libc.a

FILES

     /usr/lib/descr/descr     - compiler driver description file.

SEE ALSO
     cc(1).

ACKNOWLEDGEMENTS
     Even though the end result doesn't look much like  it,  many  ideas  were
     nevertheless derived from the ACK compiler driver by Ed Keizer.

BUGS
     POSIX requires that if compiling one source file to an object file  fails
     then the compiler should continue with the next source file.  There is no
     way acd can do this, it always stops after error.  It doesn't  even  know
     what an object file is!  (The requirement is stupid anyhow.)

     If you don't think that tabs are 8 spaces wide, then don't mix them  with
     spaces for indentation.

AUTHOR
     Kees J. Bot (kjb@cs.vu.nl)










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