       





             [1mGuidelines for writing ksh-93 built-in commands[0m
                              [4mDavid[24m [4mG.[24m [4mKorn[0m







       [4m1.[24m  [4mINTRODUCTION[0m

       A  built-in  command is executed without creating a separate
       process.  Instead, the command is invoked as a C function by
       [1mksh[22m.   If  this  function  has  no side effects in the shell
       process, then the behavior of this built-in is identical  to
       that  of  the  equivalent  stand-alone command.  The primary
       difference in this case is  performance.   The  overhead  of
       process  creation  is  eliminated.   For  commands  of short
       duration, the effect can be dramatic.  For example,  on  SUN
       OS  4.1,  the  time  do run [1mwc [22mon a small file of about 1000
       bytes, runs about 50 times faster as a built-in command.

       In addition, built-in commands that have side effects on the
       shell  environment  can be written.  This is usually done to
       extend the application domain for  shell  programming.   For
       example,  an X-windows extension that makes heavy use of the
       shell variable namespace was added as a group  of  built-ins
       commands  that  are  added  at  run  time.   The result is a
       windowing  shell  that  can  be  used  to  write   X-windows
       applications.

       While  there  are  definite  advantages  to  adding built-in
       commands, there are some disadvantages as well.   Since  the
       built-in  command  and  [1mksh  [22mshare the same address space, a
       coding error in the built-in program may affect the behavior
       of  [1mksh[22m; perhaps causing it to core dump or hang.  Debugging
       is also more complex since your code is  now  a  part  of  a
       larger entity.  The isolation provided by a separate process
       guarantees that all resources used by the  command  will  be
       freed  when  the command completes.  Also, since the address
       space of [1mksh [22mwill be larger, this may increase the  time  it
       takes  [1mksh  [22mto  fork() and exec() a non-builtin command.  It
       makes no sense to add a built-in command that takes  a  long
       time  to run or that is run only once, since the performance
       benefits will  be  negligible.   Built-ins  that  have  side
       effects   in   the   current   shell  environment  have  the
       disadvantage of increasing the coupling between the built-in
       and  [1mksh  [22mmaking  the  overall  system less modular and more
       monolithic.

       Despite these drawbacks, in  many  cases  extending  [1mksh  [22mby
       adding built-in commands makes sense and allows reuse of the
       shell scripting ability in an application  specific  domain.
       This memo describes how to write [1mksh [22mextensions.








       [4m2.[24m  [4mWRITING[24m [4mBUILT-IN[24m [4mCOMMANDS[0m

       There  is a development kit available for writing [1mksh [22mbuilt-
       ins.  The development kit has  three  directories,  [1minclude[22m,
       [1mlib[22m,  and  [1mbin[22m.   The  [1minclude  [22mdirectory  contains  a  sub-
       directory named [1mast [22mthat contains interface  prototypes  for
       functions  that  you  can  call  from  built-ins.   The  [1mlib[0m
       directory contains the [1mast  [22mlibrary1  and  a  library  named
       [1mlibcmd  [22mthat  contains  a version of several of the standard
       POSIX[1] utilities that can be made run time built-ins.   It
       is  best  to  set  the  value  of  the  environment variable
       [1mPACKAGE_ast [22mto the pathname of the directory containing  the
       development  kit.  Users of [1mnmake[22m[2] 2.3 and above will then
       be able to use the rule
            [1m:PACKAGE:       ast[0m
       in their makefiles and not have to specify any  [1m-I  [22mswitches
       to the compiler.

       A  built-in  command has a calling convention similar to the
       [1mmain [22mfunction of a program,
            [1mint main(int argc, char *argv[])[22m.
       However, instead of [1mmain[22m, you must  use  the  function  name
       [1mb_[4m[22mname[24m,  where  [4mname[24m is the name of the built-in you wish to
       define.  The built-in function takes a third [1mvoid*  [22margument
       which  you can define as [1mNULL[22m.  Instead of [1mexit[22m, you need to
       use [1mreturn [22mto terminate your  command.   The  return  value,
       will become the exit status of the command.

       The  steps  necessary  to create and add a run time built-in
       are illustrated in the following simple  example.   Suppose,
       you  wish  to  add  a  built-in  command  named  [1mhello [22mwhich
       requires one argument and prints the word hello followed  by
       its  argument.   First,  write  the following program in the
       file [1mhello.c[22m:
            [1m#include     <stdio.h>[0m
            [1mint b_hello(int argc, char *argv[], void *context)[0m
            [1m{[0m
                    [1mif(argc != 2)[0m
                    [1m{[0m
                            [1mfprintf(stderr,"Usage: hello arg\n");[0m
                            [1mreturn(2);[0m
                    [1m}[0m
                    [1mprintf("hello %s\n",argv[1]);[0m
                    [1mreturn(0);[0m
            [1m}[0m

       Next, the program needs to be compiled.  On some systems  it
       is  necessary  to  specify  a  compiler  option  to  produce
       position independent code for dynamic linking.   If  you  do
       not  compile  with  [1mnmake  [22mit  is important to specify the a
       special include directory when compiling built-ins.
            [1mcc -pic -I$PACKAGE_ast/include -c hello.c[0m
       since the special version of [1m<stdio.h>  [22min  the  development
       kit  is  required.   This  command  generates [1mhello.o [22min the
       current directory.


       ____________________

       1. [1mast [22mstands for Advanced Software Technology




       On some systems, you cannot load [1mhello.o [22mdirectly, you  must
       build  a  shared library instead.  Unfortunately, the method
       for generating  a  shared  library  differs  with  operating
       system.   However,  if  you  are building with the ATT [1mnmake[0m
       program you can use the [1m:LIBRARY: [22mrule to specify this in  a
       system  independent  fashion.   In  addition,  if  you  have
       several built-ins, it is desirable to build a shared library
       that contains them all.

       The final step is using the built-in.  This can be done with
       the  [1mksh  [22mcommand  [1mbuiltin[22m.   To  load  the  shared  library
       [1mhello.so [22mand to add the built-in [1mhello[22m, invoke the command,
            [1mbuiltin -f hello hello[0m
       The  suffix  for  the shared library can be omitted in which
       case the shell will add an appropriate suffix for the system
       that  it  is  loading  from.   Once  this  command  has been
       invoked, you can invoke [1mhello [22mas you do any other command.

       It is often desirable to make a command [4mbuilt-in[24m  the  first
       time  that  it  is  referenced.   The  first  time  [1mhello [22mis
       invoked,  [1mksh  [22mshould  load  and  execute  it,  whereas  for
       subsequent invocations [1mksh [22mshould just execute the built-in.
       This can be done by creating a file  named  [1mhello  [22mwith  the
       following contents:
            [1mfunction hello[0m
            [1m{[0m
                    [1munset -f hello[0m
                    [1mbuiltin -f hello hello[0m
                    [1mhello "$@"[0m
            [1m}[0m
       This file [1mhello [22mneeds to be placed in a directory that is in
       your [1mFPATH [22mvariable.  In addition,  the  full  pathname  for
       [1mhello.so  [22mshould be used in this script so that the run time
       loader will be able to find this shared  library  no  matter
       where the command [1mhello [22mis invoked.


       [4m3.[24m  [4mCODING[24m [4mREQUIREMENTS[24m [4mAND[24m [4mCONVENTIONS[0m

       As mentioned above, the entry point for built-ins must be of
       the form [1mb_[4m[22mname[24m.  Your built-ins can call functions from the
       standard  C  library,  the  [1mast [22mlibrary, interface functions
       provided by [1mksh[22m, and your own functions.  You  should  avoid
       using  any  global  symbols beginning with [1msh_[22m, [1mnv_[22m, and [1med_[0m
       since these are used by [1mksh [22mitself.   In  addition,  [1m#define[0m
       constants in [1mksh [22minterface files, use symbols beginning with
       [1mSH_ [22mto that you should avoid using names beginning with [1mSH_[22m.

       [4m3.1[24m  [4mHeader[24m [4mFiles[0m

       The  development  kit provides a portable interface to the C
       library and to libast.  The header files in the  development
       kit are compatible with K&R C[3], ANSI-C[4], and C++[5].

       The  best  thing  to  do  is  to  include  the  header  file
       [1m<shell.h>[22m.  This header file causes the [1m<ast.h> [22mheader,  the
       [1m<error.h>  [22mheader  and the [1m<stak.h> [22mheader to be included as
       well as defining prototypes for functions that you can  call
       to  get  shell  services for your builtins.  The header file
       [1m<ast.h> [22mprovides prototypes for many  [1mlibast  [22mfunctions  and
       all  the  symbol  and  function  definitions from the ANSI-C





       headers, [1m<stddef.h>[22m, [1m<stdlib.h>[22m, [1m<stdarg.h>[22m, [1m<limits.h>[22m, and
       [1m<string.h>[22m.    It   also   provides   all  the  symbols  and
       definitions  for   the   POSIX[6]   headers   [1m<sys/types.h>[22m,
       [1m<fcntl.h>[22m,  and  [1m<unistd.h>[22m.   You  should  include  [1m<ast.h>[0m
       instead of one or more  of  these  headers.   The  [1m<error.h>[0m
       header  provides  the  interface  to  the  error  and option
       parsing  routines  defined  below.   The   [1m<stak.h>   [22mheader
       provides  the  interface  to  the memory allocation routines
       described below.

       Programs that want to use the  information  in  [1m<sys/stat.h>[0m
       should  include  the file [1m<ls.h> [22minstead.  This provides the
       complete POSIX interface to [1mstat() [22mrelated functions even on
       non-POSIX systems.

       [4m3.2[24m  [4mInput/Output[0m

       [1mksh  [22muses [1msfio[22m, the Safe/Fast I/O library[7], to perform all
       I/O operations.  The [1msfio [22mlibrary, which is part of  [1mlibast[22m,
       provides  a  superset  of  the functionality provided by the
       standard I/O library defined in  ANSI-C.   If  none  of  the
       additional  functionality  is  required,  and if you are not
       familiar with [1msfio [22mand you do not want  to  spend  the  time
       learning  it,  then  you  can use [1msfio [22mvia the [1mstdio [22mlibrary
       interface.   The  development  kit   contains   the   header
       [1m<stdio.h>  [22mwhich  maps  [1mstdio  [22mcalls to [1msfio [22mcalls.  In most
       instances the mapping is done by macros or inline  functions
       so  that  there  is  no overhead.  The man page for the [1msfio[0m
       library is in an Appendix.

       However, there are some very nice extensions and performance
       improvements  in [1msfio [22mand if you plan any major extensions I
       recommend that you use it natively.

       [4m3.3[24m  [4mError[24m [4mHandling[0m

       For error messages  it  is  best  to  use  the  [1mast  [22mlibrary
       function  [1merrormsg() [22mrather that sending output to [1mstderr [22mor
       the equivalent [1msfstderr  [22mdirectly.   Using  [1merrormsg()  [22mwill
       make   error  message  appear  more  uniform  to  the  user.
       Furthermore, using [1merrormsg() [22mshould make it  easier  to  do
       error  message  translation  for  other  locales  in  future
       versions of [1mksh[22m.

       The first argument to [1merrormsg() [22mspecifies the dictionary in
       which  the  string  will  be  searched for translation.  The
       second argument to [1merrormsg() [22mcontains that error  type  and
       value.   The third argument is a [4mprintf[24m style format and the
       remaining arguments are arguments to be printed as  part  of
       the  message.   A  new-line  is  inserted at the end of each
       message and therefore, should not  appear  as  part  of  the
       format  string.   The  second  argument should be one of the
       following:

       [1mERROR_exit([4m[22mn[24m[1m)[22m: If [4mn[24m is not-zero, the builtin will exit value
            [4mn[24m after printing the message.

       [1mERROR_system([4m[22mn[24m[1m)[22m: Exit   builtin  with  exit  value  [4mn[24m  after
            printing the message.  The  message  will  display  the
            message  corresponding  to [1merrno [22menclosed within [1m[ ] [22mat
            the end of the message.






       [1mERROR_usage([4m[22mn[24m[1m)[22m: Will generate a usage message and exit.   If
            [4mn[24m is non-zero, the exit value will be 2.  Otherwise the
            exit value will be 0.

       [1mERROR_debug([4m[22mn[24m[1m)[22m: Will print a level [4mn[24m debugging  message  and
            will then continue.

       [1mERROR_warn([4m[22mn[24m[1m)[22m: Prints a warning message. [4mn[24m is ignored.

       [4m3.4[24m  [4mOption[24m [4mParsing[0m

       The  first  thing  that a built-in should do is to check the
       arguments for correctness and to print any usage messages on
       standard error.  For consistency with the rest of [1mksh[22m, it is
       best to use the [1mlibast [22mfunctions [1moptget() [22mand  [1moptusage()[22mfor
       this  purpose.  The header [1m<error.h> [22mincluded prototypes for
       these functions.  The [1moptget() [22mfunction is  similar  to  the
       System  V  C  library  function  [1mgetopt()[22m, but provides some
       additional  capabilities.   Built-ins  that   use   [1moptget()[0m
       provide a more consistent user interface.

       The [1moptget() [22mfunction is invoked as
            [1mint optget(char *argv[], const char *optstring)[0m
       where  [1margv  [22mis  the argument list and [1moptstring [22mis a string
       that  specifies  the  allowable  arguments  and   additional
       information  that is used to format [4musage[24m messages.  In fact
       a complete man page in [1mtroff [22mor [1mhtml  [22mcan  be  generated  by
       passing  a usage string as described by the [1mgetopts [22mcommand.
       Like [1mgetopt()[22m, single letter options are represented by  the
       letter  itself,  and options that take a string argument are
       followed by  the  [1m:  [22mcharacter.   Option  strings  have  the
       following special characters:

       [1m:    [22mUsed  after a letter option to indicate that the option
            takes an option argument.   The  variable  [1mopt_info.arg[0m
            will  point  to  this value after the given argument is
            encountered.

       [1m#    [22mUsed after a letter option to indicate that the  option
            can   only   take  a  numerical  value.   The  variable
            [1mopt_info.num [22mwill contain this value  after  the  given
            argument is encountered.

       [1m?    [22mUsed  after  a  [1m:  [22mor  [1m# [22m(and after the optional [1m?[22m)  to
            indicate the  the  preceding  option  argument  is  not
            required.

       [1m[[22m...[1m] [22mAfter  a  [1m:  [22mor [1m#[22m, the characters contained inside the
            brackets are used to identify the option argument  when
            generating a [4musage[24m message.

       [4mspace[24m The  remainder  of  the  string will only be used when
            generating usage messages.

       The [1moptget() [22mfunction returns the matching option letter  if
       one  of  the  legal  option is matched.  Otherwise, [1moptget()[0m
       returns

       [1m':'  [22mIf there is  an  error.   In  this  case  the  variable
            [1mopt_info.arg [22mcontains the error string.






       [1m0    [22mIndicates   the   end   of   options.    The   variable
            [1mopt_info.index  [22mcontains  the   number   of   arguments
            processed.

       [1m'?'  [22mA  usage  message has been required.  You normally call
            [1moptusage() [22mto generate and display the usage message.

       The following is an example of the option parsing portion of
       the [1mwc [22mutility.
            [1m#include <shell.h>[0m
            [1mwhile(1) switch(n=optget(argv,"xf:[file]"))[0m
            [1m{[0m
                    [1mcase 'f':[0m
                            [1mfile = opt_info.arg;[0m
                            [1mbreak;[0m
                    [1mcase ':':[0m
                            [1merror(ERROR_exit(0), opt_info.arg);[0m
                            [1mbreak;[0m
                    [1mcase '?':[0m
                            [1merror(ERROR_usage(2), opt_info.arg);[0m
                            [1mbreak;[0m
            [1m}[0m

       [4m3.5[24m  [4mStorage[24m [4mManagement[0m

       It  is  important  that  any memory used by your built-in be
       returned.  Otherwise, if your built-in is called frequently,
       [1mksh  [22mwill  eventually  run  out of memory.  You should avoid
       using  [1mmalloc()  [22mfor  memory  that  must  be  freed   before
       returning  from  you  built-in, because by default, [1mksh [22mwill
       terminate you built-in in the event of an interrupt and  the
       memory will not be freed.

       The  best  way  to  to  allocate  variable  sized storage is
       through calls to the  [1mstak  [22mlibrary  which  is  included  in
       [1mlibast [22mand which is used extensively by [1mksh [22mitself.  Objects
       allocated with the [1mstakalloc() [22mfunction are freed  when  you
       function  completes  or aborts.  The [1mstak [22mlibrary provides a
       convenient way to build variable length  strings  and  other
       objects  dynamically.   The man page for the [1mstak [22mlibrary is
       contained in the Appendix.

       Before [1mksh [22mcalls each built-in command, it saves the current
       stack  location and restores it after it returns.  It is not
       necessary to save and restore the stack location in  the  [1mb_[0m
       entry function, but you may want to write functions that use
       this stack are restore it when leaving  the  function.   The
       following  coding  convention  will  do this in an efficient
       manner:
            [4myourfunction[24m[1m()[0m
            [1m{[0m
                    [1mchar    *savebase;[0m
                    [1mint     saveoffset;[0m
                    [1mif(saveoffset=staktell())[0m
                            [1msavebase = stakfreeze(0);[0m
                    ...
                    [1mif(saveoffset)[0m
                            [1mstakset(savebase,saveoffset);[0m
                    [1melse[0m
                            [1mstakseek(0);[0m





            [1m}[0m


       [4m4.[24m  [4mCALLING[24m [1mksh [4m[22mSERVICES[0m

       Some of the more interesting  applications  are  those  that
       extend  the  functionality  of  [1mksh  [22min application specific
       directions.  A  prime  example  of  this  is  the  X-windows
       extension  which adds builtins to create and delete widgets.
       The [1mnval [22mlibrary is used to interface with  the  shell  name
       space.   The  [1mshell  [22mlibrary  is  used to access other shell
       services.

       [4m4.1[24m  [4mThe[24m [4mnval[24m [4mlibrary[0m

       A great deal of power is derived from  the  ability  to  use
       portions  of  the  hierarchal variable namespace provided by
       [1mksh-93 [22mand turn these names into active objects.

       The [1mnval [22mlibrary is used to interface with shell  variables.
       A  man  page  for this file is provided in an Appendix.  You
       need to include the header [1m<nval.h> [22mto access the  functions
       defined  in the [1mnval [22mlibrary.  All the functions provided by
       the [1mnval [22mlibrary begin with  the  prefix  [1mnv_[22m.   Each  shell
       variable  is  an  object  in  an  associative  table that is
       referenced by name.  The type  [1mNamval_t*  [22mis  pointer  to  a
       shell  variable.   To operate on a shell variable, you first
       get a handle to the variable with the [1mnv_open() [22mfunction and
       then supply the handle returned as the first argument of the
       function that provides an operation on  the  variable.   You
       must call [1mnv_close() [22mwhen you are finished using this handle
       so that the space can be freed once the value is unset.  The
       two  most  frequent  operations  are to get the value of the
       variable,  and  to  assign  value  to  the  variable.    The
       [1mnv_getval() [22mreturns a pointer the the value of the variable.
       In some cases the pointer returned is to a region that  will
       be  overwritten  by the next [1mnv_getval() [22mcall so that if the
       value isn't used immediately, it  should  be  copied.   Many
       variables   can   also   generate   a  numeric  value.   The
       [1mnv_getnum() [22mfunction returns a numeric value for  the  given
       variable   pointer,  calling  the  arithmetic  evaluator  if
       necessary.

       The [1mnv_putval() [22mfunction is used to assign a new value to  a
       given  variable.   The  second  argument  to [1mputval() [22mis the
       value to be assigned and the third argument is a [4mflag[24m  which
       is used in interpreting the second argument.

       Each  shell  variable  can have one or more attributes.  The
       [1mnv_isattr() [22mis used to test for the existence of one or more
       attributes.   See  the  appendix  for  a  complete  list  of
       attributes.

       By default, each shell variable passively stores the  string
       you  give  with with [1mnv_putval()[22m, and returns the value with
       [1mgetval()[22m.  However, it is possible to turn any node into  an
       active  entity  by  assigning  functions  to it that will be
       called whenever [1mnv_putval() [22mand/or  [1mnv_getval()  [22mis  called.
       In  fact  there are up to five functions that can associated
       with each variable to override  the  default  actions.   The
       type [1mNamfun_t [22mis used to define these functions.  Only those





       that are non-[1mNULL [22moverride the default actions.  To override
       the  default  actions,  you  must  allocate  an  instance of
       [1mNamfun_t[22m, and then assign the functions  that  you  wish  to
       override.    The   [1mputval()   [22mfunction   is  called  by  the
       [1mnv_putval()  [22mfunction.   A  [1mNULL  [22mfor  the  [4mvalue[24m   argument
       indicates  a  request  to  unset  the  variable.   The  [4mtype[0m
       argument might contain the [1mNV_INTEGER [22mbit so you  should  be
       prepared  to  do  a  conversion  if necessary.  The [1mgetval()[0m
       function is called by [1mnv_getval() [22mvalue and  must  return  a
       string.    The   [1mgetnum()  [22mfunction  is  called  by  by  the
       arithmetic evaluator and must return  double.   If  omitted,
       then  it  will  call [1mnv_getval() [22mand convert the result to a
       number.

       The functionality of a variable can further be increased  by
       adding  discipline functions that can be associated with the
       variable.  A discipline function allows a script  that  uses
       your   variable   to   define   functions   whose   name  is
       [4mvarname[24m[1m.[4m[22mdiscname[24m where [4mvarname[24m is the name of the  variable,
       and  [4mdiscname[24m  is the name of the discipline.  When the user
       defines such a function,  the  [1msettrap()  [22mfunction  will  be
       called  with the name of the discipline and a pointer to the
       parse tree corresponding to the  discipline  function.   The
       application  determines  when  these  functions are actually
       executed.  By default, [1mksh [22mdefines [1mget[22m, [1mset[22m,  and  [1munset  [22mas
       discipline functions.

       In addition, it is possible to provide a data area that will
       be passed as an argument to each of these functions whenever
       any  of  these  functions are called.  To have private data,
       you need to define and allocate a structure that looks like
            [1mstruct [4m[22myours[0m
            [1m{[0m
                    [1mNamfun_t        fun;[0m
                    [4myour_data_fields[24m[1m;[0m
            [1m};[0m

       [4m4.2[24m  [4mThe[24m [4mshell[24m [4mlibrary[0m

       There are several functions that are used by [1mksh [22mitself that
       can also be called from built-in commands.  The man page for
       these routines are in the Appendix.

       The [1msh_addbuiltin() [22mfunction can be used to  add  or  delete
       builtin  commands.   It  takes the name of the built-in, the
       address of the function that implements the built-in, and  a
       [1mvoid*  [22mpointer  that  will be passed to this function as the
       third agument whenever  it  is  invoked.   If  the  function
       address  is  [1mNULL[22m,  the  specified built-in will be deleted.
       However, special built-in functions  cannot  be  deleted  or
       modified.

       The  [1msh_fmtq()  [22mfunction takes a string and returns a string
       that is quoted as necessary so that it can be used as  shell
       input.   This function is used to implement the [1m%q [22moption of
       the shell built-in [1mprintf [22mcommand.

       The [1msh_parse() [22mfunction returns a parse  tree  corresponding
       to  a  give  file  stream.   The  tree  can  be  executed by
       supplying it as the first argument to the [1msh_trap() [22mfunction
       and   giving   a   value   of  [1m1  [22mas  the  second  argument.





       Alternatively, the [1msh_trap() [22mfunction can parse and  execute
       a  string  by  passing  the string as the first argument and
       giving [1m0 [22mas the second argument.

       The [1msh_isoption() [22mfunction can be used to set to see whether
       one or more of the option settings is enabled.




























































                                [4mREFERENCES[0m

         1. [4mPOSIX[24m   [4m-[24m   [4mPart[24m  [4m2:[24m  [4mShell[24m  [4mand[24m  [4mUtilities,[24m  IEEE  Std
            1003.2-1992, ISO/IEC 9945-2:1993.

         2. Glenn Fowler, Nmake reference needed

         3. Brian W. Kernighan and Dennis M. Ritchie,  [4mThe[24m  [4mC[24m  [4mPro-[0m
            [4mgramming[24m [4mLanguage[24m, Prentice Hall, 1978.

         4. American  National  Standard  for Information Systems -
            Programming Language - C, ANSI X3.159-1989.

         5. Bjarne Stroustroup, [4mC++[24m, Addison Wesley, xxxx

         6. [4mPOSIX[24m [4m-[24m [4mPart[24m [4m1:[24m [4mSystem[24m [4mApplication[24m  [4mProgram[24m  [4mInterface,[0m
            IEEE Std 1003.1-1990, ISO/IEC 9945-1:1990.

         7. David  Korn  and  Kiem-Phong Vo, [4mSFIO[24m [4m-[24m [4mA[24m [4mSafe/Fast[24m [4mIn-[0m
            [4mput/Output[24m [4mlibrary,[24m Proceedings of the  Summer  Usenix,
            pp. , 1991.













































