BSD Unix kernel running under the Amiga OS
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README.md

README for ixemul.library and ixnet.library version 48.0

IMPORTANT

Always read the NEWS file for information about new features! The NEWS file is part of the ixemul source archive.

Also look at the ix.h header: ixemul-specific functions are documented in that header.

Starting with ixemul.library version 42.0 the ssystem() function is no longer supported. The only two applications use this function are (to my knowledge) gcc and man. You should either upgrade to the latest gcc (2.7.2 or higher) or replace gcc by gccv (which is part of older gcc distributions). You can find a new ‘man’ version in Geek Gadgets.

Starting with ixemul.library version 43.1 various networking functions have been moved to libc.a. See the NEWS file for details. It only concerns certain somewhat specialized functions, so ‘normal’ clients and daemons shouldn’t be affected.

This library is part of Geek Gadgets. For more information see ftp.ninemoons.com, /pub/geekgadgets.

INTRODUCTION

Originally created in 1991 or 1992 by Markus Wild, the ixemul.library has become the driving force behind the Geek Gadgets project.

Essentially it is a BSD Unix kernel running under the Amiga OS. The code for handling Unix signals is taken almost verbatim from the BSD kernel sources, for example. Multitasking and file I/O is, of course, passed on to the Amiga OS. Because the library resembles BSD Unix so closely, it has made it possible to port almost all Unix programs.

However, because of the conformance to BSD, the library is not too conservative with resources or overly concerned with Amiga standards. For example, command line expansion uses Unix semantics and doesn’t use ReadArgs(). The purpose of ixemul.library is to emulate Unix as well as is technically possible. So given a choice between Amiga behavior or Unix behavior, the last one is chosen.

HISTORY

As I mentioned, the library was originally created by Markus Wild. It allowed him to port Unix programs, most notably gcc and a Unix shell (pdksh), which in turn gave him the opportunity to start porting NetBSD Unix. At some point he stopped working on ixemul.library and continued with the real thing, NetBSD.

Although Markus made some snapshots of his library available on Internet, he was about the only one who could actually compile it. The snapshots where never complete and didn’t use a standard Make tool.

Rafael W. Luebbert managed around June 1994 to actually compile the library using bits and pieces from four different source releases. Even then he had to write some missing code and had to debug a lot before he got it working. Luebbert released versions 40.1 through 40.4. Since this was the only version available that could actually be compiled, Fred Fish switched to version 40.4 for his FreshFish CDROM series.

Leonard Norrgard also started working on 40.4, fixing a variety of problems. So for a time there were two versions of the library, both derived from 40.4. Around March 1995 I began contributing my own fixes for the library to Fred, including a fix for a horrible memory trashing bug which substantially improved the stability of the library. Starting with version 42.0 I became the new maintainer of the library. I merged Leonard’s version of the library into 42.0, the whole source distribution was cleaned up and reorganized and many bugs were fixed. Also new functionality was added such as timezone support.

Thanks to Jeff Shepherd the support for network functions has been totally reorganized for version 43.0. A new ixnet.library was introduced that contains all the network handling. This new library supports both AS225 and AmiTCP. Furthermore, there is no longer any need to maintain two versions of an Internet client or daemon as ixnet.library will do the multiplexing for you. The program itself is completely shielded from the actual networking package in use. Also, should a new networking package appear for the Amiga, then it is relatively easy to add support for that package to ixnet.library. All existing programs will automatically be able to work with the new package too.

Version 43.0 also introduced support for automatic stack extension (provided you compiled your program with the -mstackextend flag). This support was done by Matthias Fleischer.

Last, but not least, I’ve added the ptrace() function, which was essential for porting the GNU debugger, GDB. So it is now possible to use a decent debugger with gcc (and with the GNU C++, fortran and ADA compilers).

Version 43.1 was a bug-fix release, which further improved stability.

Version 44.0 made the library much more (Net)BSD compatible, making it even easier to port Unix programs. Also many bugs were fixed.

Version 45.0 fixed a few bugs, improved stack extension (see the new ixstack utility) and further improved Ctrl-C handling by adding ‘sessions’, something that was needed for the Geek Gadgets X port.

Version 45.1 was a bug-fix release, and also improved uid/gid handling.

Version 46.0 adds shared memory (shm*) and basic p.OS support.

Version 46.1 renames the __os variable to ix_os.

Version 47.0 completes the p.OS support and adds Unix semaphore and message support. Also fixes several bugs.

Version 47.1 fixes a minor but very irritating bug in the ixstack utility.

Version 47.2 is a bug-fix release.

Version 47.3 is a bug-fix release.

Version 48.0 is a bug-fix release and removes the p.OS support. p.OS didn’t become a success, so ixemul no longer supports it.

USAGE

The Aminet distribution of ixemul consists of several archives: one for each flavor (CPU/FPU combination) of the library, one for the timezone support, an archive for documentation, an archive for various utilities and an SDK archive, containing headers, libraries and C-objects that provide the startup code. There is also an ixemul source archive. If you don’t mind recompiling everything, and if you have a decent Geek Gadgets environment, then all you have to do is retrieve this archive, as all the other archives are generated from this source distribution.

Besides the library itself, there are also some utilities and other goodies. First of all, there is a special trace version of the library. Together with the ixtrace tool it allows you to see which library functions are called by the program you want to debug. If you compile the library yourself you will also get, as part of the compile process, a debug version of the library. If you get an Enforcer hit in the library, and if you have SegTracker installed, then you can track down in which source and at which line the hit took place using the gccfindhit tool (available from Aminet and Geek Gadgets) that is similar to the FindHit tool which is part of the Enforcer package. I’ve used this with great success in the past.

Various settings that influence the behavior of the library can be set using the ixprefs utility. A small tool ixrun allowing you to run AmigaOS scripts from within the Unix shell is also provided. An ixemul-specific pipe-handler allows you to set up a pipe between an AmigaOS utility and an Unix program. A pipe between two Unix programs is handled by the ixemul.library itself, but since AmigaOS programs are not under the control of the library, this handler is used instead.

A recent addition is the ixstack tool, which can list and set the minimum stack size an ixemul program needs. It can also show the actual stack usage of ixemul programs if you start it with ‘-s’.

Finally, the tools zic (for manipulating the timezone databases) and ixtimezone are also part of ixemul. The ixtimezone tool can be used to automatically adjust the Amiga time based on the currently selected timezone. The best way to use this tool is to install the timezone databases (in etc:zoneinfo), set the TZ environment variable correctly (in my case “Europe/Amsterdam”), add the line “ixtimezone -patch-resource >nil:” to your user-startup and set the Amiga clock to Greenwich Mean Time (or Universal Time, as it is now called). Now you will never have to worry about things like Daylight Saving Time as each time ixtimezone is called, this tool checks the current timezone and will patch the Amiga clock to the right time. This tool is also very useful if you also run NetBSD or Linux, as these OSes expect that the internal Amiga clock adheres to GMT.

I advise that you set the ixemul “Network support” to the correct method using the ixprefs program. It will improve performance somewhat if ixemul doesn’t have to guess which network library is available, if any.

MEMORY REQUIREMENTS

The library itself needs about 200 Kb, but if you want to do some useful work, such as compiling programs, I advise at least 4 Mb. More importantly is the stack size: set it to at least 50 Kb. If you have problems with certain programs try to increase the stack size.

The reason for these large stacks is that the Unix operating system automatically extends the stack if it is too small. Therefore some Unix programs aren’t exactly conservative with their stack space.

We can do the same on AmigaOS, however, by compiling programs with the -mstackextend flag. Or you can set the minimum stack size of a program using the ixstack tool. If the current stack is too small, the program will automatically extend the stack on startup.

PROGRAMMING WITH IXEMUL

There is really very little to do. If you have a decent Geek Gadgets setup with the necessary compilers and tools, and if you have installed the ixemul SDK, then you are ready to go. There is no need to do anything special, the communication between your program and ixemul.library is handled by the startup code (the *crt0.o files in the /gg/lib directory) and the standard C library (libc.a, also in /gg/lib) which are automatically linked with your program. Note that ixemul.library together with the compile tools also provides profiling support (compile and link with -pg), and base-relative and resident support (compile with -fbaserel or -resident). Compile and link with -g to add debug information so that you can debug your program with gdb.

If you start gdb with the -enforcer option, then the program you are debugging will automatically stop and drop into the debugger as soon as an Enforcer hit occurs. This is obviously very useful.

The startup code will automatically expand the command-line for you (e.g. ‘echo *’ will expand to ‘echo ’). If you want to disable this in your program, then you should add the line:

int ix_expand_cmd_line = 0;

to your source. This global variable will tell the system not to expand the command-line.

Code like you would on a Unix system, so DON’T USE any information private to the library!

PORTING UNIX PROGRAMS

Most programs compile out-of-the-box. There are a few exceptions to this rule. First of all, programs like linkers and the like that have to be able to read or write the standard Amiga hunk format obviously need a lot of work.

Secondly, there is no virtual memory support, and therefore no real fork() function. In most cases the fork() function is only used to spawn a new program, and in such cases it is possible to replace fork() by vfork(), which is a light-weight fork() replacement that was originally created for Unix to reduce the overhead a real fork() introduced.

A vfork() doesn’t create a copy of itself as fork() does, but uses the parent’s code and data. Since the child will quickly call execve() to spawn another program, this sharing of the code and data is no problem and saves a lot of time.

There is one restriction, however: since the child process is running on the stack of the parent until an exit() or execve() call the child must be careful not to clobber the parent’s stack. So this will fail:

  main()
  {
    switch (vfork())
    {
      case -1:      // Error
      case 0:       // Child
        return 0;   // Return from main()

      default:      // Parent
        wait(0);
        break;
    }

    return 0;
  }

It fails because the child just returns, and this return will clobber the stack for when the parent returns. But this is correct:

  main()
  {
    switch (vfork())
    {
      case -1:              // Error
      case 0:               // Child
        exit(0);            // Exit
        break;

      default:              // Parent
        wait(0);
        break;
    }

    return 0;
  }

Here the child calls exit or execve and therefor leaves the underlying stack intact for the parent. This restriction can cause very hard to find bugs (I know, I spent quite a few hours researching this problem).

In some cases, such as a Unix shell (pdksh for example), you really want to be able to port a program that uses a fork() that cannot be replaced with vfork(). There is a way to do this, although it is a lot of work. First of all, the program has to be compiled with -resident. Now you replace the fork() by a ix_vfork() call, and in the child code you call vfork_setup_child() (new for 44.0! Replaces the ix_resident()/ix_get_vars2() pair) to copy the original data hunk to a new location. Next you have to copy all the parent’s data structures to the child. In other words, you have to copy the complete state of the parent process to the child process. This can be a lot of work. Finally, you call vfork_resume() which unblocks the parent so that you now have two processes running separately from each other.

It is important to realize that you should never exit() from the parent before all vfork()‘ed children have died. Since exiting from the parent causes the parents code and data segments to be deallocated, the child would find itself without code space to run on, and would probably cause a severe machine crash!

So always call at least `wait(0)’ before returning from the parent.

For an example of how this works, see jobs.c from the pdksh source distribution. It’s a kind of poor-man’s fork().

The third case I’ve come across that couldn’t easily be ported were programs that dump their state to a new file. Emacs does this, as does GNU Common Lisp. The idea is that such a program will read lots of packages, and then dump itself to a new file. That new file can in turn be executed, and you will no longer have to load all those packages. All this assumes that each time you load a program, all the code and data ends up at the same memory addresses as before. Something that is true for Unix, but not for the Amiga due to the lack of virtual memory.

However, if someone wants to do a port of such a program, please contact me as I have developed a technique to implement this. At least, I’ve made this work with a small test program. I’ve tried to use it with GNU Common Lisp, but time constraints prevented me from developing this further.

The last problematic category I’ve seen is GNU Emacs. This program assumes that all the data it allocates will always end up in a continuous memory block, and that the upper 8 bits of each memory address are always the same. The Amiga, however, can have multiple memory blocks positioned at various places in memory. While there is a GNU Emacs port, this port does assume this limitation and if you have an Amiga with many memory blocks (as I had) GNU Emacs may easily crash. No easy solution exists.

AMIGA PROGRAMMING & IXEMUL

If you use Amiga specific resources like Windows and Screens from Intuition, make sure to add an atexit() handler to close those resources, if the user should decide to interrupt your program. Before the program is left, the chain of registered atexit-handlers is called in exit(). So PLEASE NEVER EVER call _exit() if you have registered any custom atexit() handlers. It is a bad habit anyway, but normally you may call _exit() without resource lossage (stdio won’t flush its buffers, but that’s about all).

Ixemul provides a new unique Amiga specific signal called SIGMSG. If you set up a handler for this signal, then the default mapping from SIGBREAKB_CTRL_C into SIGINT will no longer occur, and your handler is called with the following arguments:

signal_handler(SIGMSG, new_exec_signal_mask)

In this case, you have to deal with Exec signals yourself, so don’t forget to clear those signals that you want to receive notification about again later. Thus if you’d want to handle SIGBREAKB_CTRL_C yourself, don’t forget to

SetSignal(0, SIGBREAKF_CTRL_C)

at the end of the handler, or you’ll never get notification about that signal again.

Most of the original BSD signals are implemented. SIGSTOP is currently not implemented, although it would be relatively easy to add. The mechanisms are already in place.

HANDLING OF UID/GID IN IXEMUL.LIBRARY

The following information was provided by Norbert Pueschel:

Here is a list of functions that are concerned with uid/gid management and access to user/group information:

a) uid/gid management: get(e)uid, set(®e)uid, get(e)gid, set(®e)gid, (init|set|get)groups

b) access to user/group informations: set(pass|group)ent, (set|get|end)pwent, get(pw|gr)nam, getpwuid, getgrgid, (_)getlogin, setlogin

c) other functions: setsid, crypt

Ixemul.library has several options how to implement these functions:

1) When MultiUser is installed, ixemul.library will use it to implement the functions named above.

2) When MultiUser is not installed, ixemul.library will use its own uid/gid handling: uids and gids will be inherited by child processes and files and directories created by ixemul using programs will have the current user and group set as long as the filesystem supports that. However, for file accesses only the setting of the flags for the file owner is used (as for any other AmigaOS program).

Processes that are not started by other ixemul using programs use environment variables to determine their (e)uid and (e)gid:

They will first check the variables LOGNAME and USER for a valid user name. If they find this user in the passwd file, they will set up (e)uid and (e)gid respectively. Then the environment variables UID, EUID, GID and EGID will be queried, and, if present, their values will be used.

If none of the above variables is found, the process will be owned by nobody/nogroup (uid/gid -2).

Ixemul.library has three different methods to get user/group informations:

a) If networking software (AmiTCP or Inet) is installed, it will use their user/group databases.

b) If no networking is available, it will try to read the files etc:passwd and etc:group.

c) If both a) and b) fails, it will use a builtin user/group database. This database only knows the nobody/nogroup (uid/gid -2), root/wheel (uid/gid 0) and a user/group combination described by the environment variables LOGNAME or USER, GROUP, UID and GID.

                  Hans Verkuil (hans [__AT__] wyst.hobby.nl, July 14, 1998)

(Parts of this README are from the original README by Markus Wild).