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                      available compression methods. This may improve
                      the compression ratio in some cases, but usually
                      the default method gives the best results anyway.</pre>
<p>
</p>
<h2><a name="notes_for_bvmlinuz_i386">NOTES FOR BVMLINUZ/I386</a></h2>
<p>Same as vmlinuz/i386.</p>
<p>
</p>
<h2><a name="notes_for_dos_com">NOTES FOR DOS/COM</a></h2>
<p>Obviously <strong>UPX</strong> won't work with executables that want to read data from
themselves (like some commandline utilities that ship with Win95/98/ME).</p>
<p>Compressed programs only work on a 286+.</p>
<p>Packed programs will be byte-identical to the original after uncompression.</p>
<p>Maximum uncompressed size: ~65100 bytes.</p>
<p>Extra options available for this executable format:</p>
<pre>
  --8086              Create an executable that works on any 8086 CPU.</pre>
<pre>
  --all-methods       Compress the program several times, using all
                      available compression methods. This may improve
                      the compression ratio in some cases, but usually
                      the default method gives the best results anyway.</pre>
<pre>
  --all-filters       Compress the program several times, using all
                      available preprocessing filters. This may improve
                      the compression ratio in some cases, but usually
                      the default filter gives the best results anyway.</pre>
<p>
</p>
<h2><a name="notes_for_dos_exe">NOTES FOR DOS/EXE</a></h2>
<p>dos/exe stands for all ``normal'' 16-bit DOS executables.</p>
<p>Obviously <strong>UPX</strong> won't work with executables that want to read data from
themselves (like some command line utilities that ship with Win95/98/ME).</p>
<p>Compressed programs only work on a 286+.</p>
<p>Extra options available for this executable format:</p>
<pre>
  --8086              Create an executable that works on any 8086 CPU.</pre>
<pre>
  --no-reloc          Use no relocation records in the exe header.</pre>
<pre>
  --all-methods       Compress the program several times, using all
                      available compression methods. This may improve
                      the compression ratio in some cases, but usually
                      the default method gives the best results anyway.</pre>
<p>
</p>
<h2><a name="notes_for_dos_sys">NOTES FOR DOS/SYS</a></h2>
<p>You can only compress plain sys files, sys/exe (two in one)
combos are not supported.</p>
<p>Compressed programs only work on a 286+.</p>
<p>Packed programs will be byte-identical to the original after uncompression.</p>
<p>Maximum uncompressed size: ~65350 bytes.</p>
<p>Extra options available for this executable format:</p>
<pre>
  --8086              Create an executable that works on any 8086 CPU.</pre>
<pre>
  --all-methods       Compress the program several times, using all
                      available compression methods. This may improve
                      the compression ratio in some cases, but usually
                      the default method gives the best results anyway.</pre>
<pre>
  --all-filters       Compress the program several times, using all
                      available preprocessing filters. This may improve
                      the compression ratio in some cases, but usually
                      the default filter gives the best results anyway.</pre>
<p>
</p>
<h2><a name="notes_for_djgpp2_coff">NOTES FOR DJGPP2/COFF</a></h2>
<p>First of all, it is recommended to use <strong>UPX</strong> *instead* of <strong>strip</strong>. strip has
the very bad habit of replacing your stub with its own (outdated) version.
Additionally <strong>UPX</strong> corrects a bug/feature in strip v2.8.x: it
will fix the 4 KByte aligment of the stub.</p>
<p><strong>UPX</strong> includes the full functionality of stubify. This means it will
automatically stubify your COFF files. Use the option <strong>--coff</strong> to
disable this functionality (see below).</p>
<p><strong>UPX</strong> automatically handles Allegro packfiles.</p>
<p>The DLM format (a rather exotic shared library extension) is not supported.</p>
<p>Packed programs will be byte-identical to the original after uncompression.
All debug information and trailing garbage will be stripped, though.</p>
<p>Extra options available for this executable format:</p>
<pre>
  --coff              Produce COFF output instead of EXE. By default
                      UPX keeps your current stub.</pre>
<pre>
  --all-methods       Compress the program several times, using all
                      available compression methods. This may improve
                      the compression ratio in some cases, but usually
                      the default method gives the best results anyway.</pre>
<pre>
  --all-filters       Compress the program several times, using all
                      available preprocessing filters. This may improve
                      the compression ratio in some cases, but usually
                      the default filter gives the best results anyway.</pre>
<p>
</p>
<h2><a name="notes_for_linux__general_">NOTES FOR LINUX [general]</a></h2>
<p>Introduction</p>
<pre>
  Linux/386 support in UPX consists of 3 different executable formats,
  one optimized for ELF excutables (&quot;linux/elf386&quot;), one optimized
  for shell scripts (&quot;linux/sh386&quot;), and one generic format
  (&quot;linux/386&quot;).</pre>
<pre>
  We will start with a general discussion first, but please
  also read the relevant docs for each of the individual formats.</pre>
<pre>
  Also, there is special support for bootable kernels - see the
  description of the vmlinuz/386 format.</pre>
<p>General user's overview</p>
<pre>
  Running a compressed executable program trades less space on a
  ``permanent'' storage medium (such as a hard disk, floppy disk,
  CD-ROM, flash memory, EPROM, etc.) for more space in one or more
  ``temporary'' storage media (such as RAM, swap space, /tmp, etc.).
  Running a compressed executable also requires some additional CPU
  cycles to generate the compressed executable in the first place,
  and to decompress it at each invocation.</pre>
<pre>
  How much space is traded?  It depends on the executable, but many
  programs save 30% to 50% of permanent disk space.  How much CPU
  overhead is there?  Again, it depends on the executable, but
  decompression speed generally is at least many megabytes per second,
  and frequently is limited by the speed of the underlying disk
  or network I/O.</pre>
<pre>
  Depending on the statistics of usage and access, and the relative
  speeds of CPU, RAM, swap space, /tmp, and filesystem storage, then
  invoking and running a compressed executable can be faster than
  directly running the corresponding uncompressed program.
  The operating system might perfrom fewer expensive I/O operations
  to invoke the compressed program.  Paging to or from swap space
  or /tmp might be faster than paging from the general filesystem.
  ``Medium-sized'' programs which access about 1/3 to 1/2 of their
  stored program bytes can do particulary well with compression.
  Small programs tend not to benefit as much because the absolute
  savings is less.  Big programs tend not to benefit proportionally
  because each invocation may use only a small fraction of the program,
  yet UPX decompresses the entire program before invoking it.
  But in environments where disk or flash memory storage is limited,
  then compression may win anyway.</pre>
<pre>
  Currently, executables compressed by UPX do not share RAM at runtime
  in the way that executables mapped from a filesystem do.  As a
  result, if the same program is run simultaneously by more than one
  process, then using the compressed version will require more RAM and/or
  swap space.  So, shell programs (bash, csh, etc.)  and ``make''
  might not be good candidates for compression.</pre>
<pre>
  UPX recognizes three executable formats for Linux: Linux/elf386,
  Linux/sh386, and Linux/386.  Linux/386 is the most generic format;
  it accommodates any file that can be executed.  At runtime, the UPX
  decompression stub re-creates in /tmp a copy of the original file,
  and then the copy is (re-)executed with the same arguments.
  ELF binary executables prefer the Linux/elf386 format by default,
  because UPX decompresses them directly into RAM, uses only one
  exec, does not use space in /tmp, and does not use /proc.
  Shell scripts where the underlying shell accepts a ``-c'' argument
  can use the Linux/sh386 format.  UPX decompresses the shell script
  into low memory, then maps the shell and passes the entire text of the
  script as an argument with a leading ``-c''.</pre>
<p>General benefits:</p>
<pre>
  - UPX can compress all executables, be it AOUT, ELF, libc4, libc5,
    libc6, Shell/Perl/Python/... scripts, standalone Java .class
    binaries, or whatever...
    All scripts and programs will work just as before.</pre>
<pre>
  - Compressed programs are completely self-contained. No need for
    any external program.</pre>
<pre>
  - UPX keeps your original program untouched. This means that
    after decompression you will have a byte-identical version,
    and you can use UPX as a file compressor just like gzip.
    [ Note that UPX maintains a checksum of the file internally,
      so it is indeed a reliable alternative. ]</pre>
<pre>
  - As the stub only uses syscalls and isn't linked against libc it
    should run under any Linux configuration that can run ELF
    binaries.</pre>
<pre>
  - For the same reason compressed executables should run under
    FreeBSD and other systems which can run Linux binaries.
    [ Please send feedback on this topic ]</pre>
<p>General drawbacks:</p>
<pre>
  - It is not advisable to compress programs which usually have many
    instances running (like `sh' or `make') because the common segments of
    compressed programs won't be shared any longer between different
    processes.</pre>
<pre>
  - `ldd' and `size' won't show anything useful because all they
    see is the statically linked stub.  Since version 0.82 the section
    headers are stripped from the UPX stub and `size' doesn't even
    recognize the file format.  The file patches/patch-elfcode.h has a
    patch to fix this bug in `size' and other programs which use GNU BFD.</pre>
<p>General notes:</p>
<pre>
  - As UPX leaves your original program untouched it is advantageous
    to strip it before compression.</pre>
<pre>
  - If you compress a script you will lose platform independence -
    this could be a problem if you are using NFS mounted disks.</pre>
<pre>
  - Compression of suid, guid and sticky-bit programs is rejected
    because of possible security implications.</pre>
<pre>
  - For the same reason there is no sense in making any compressed
    program suid.</pre>
<pre>
  - Obviously UPX won't work with executables that want to read data
    from themselves. E.g., this might be a problem for Perl scripts
    which access their __DATA__ lines.</pre>
<pre>
  - In case of internal errors the stub will abort with exitcode 127.
    Typical reasons for this to happen are that the program has somehow
    been modified after compression.
    Running `strace -o strace.log compressed_file' will tell you more.</pre>
<p>
</p>
<h2><a name="notes_for_linux_elf386">NOTES FOR LINUX/ELF386</a></h2>
<p>Please read the general Linux description first.</p>
<p>The linux/elf386 format decompresses directly into RAM,
uses only one exec, does not use space in /tmp,
and does not use /proc.</p>
<p>Linux/elf386 is automatically selected for Linux ELF exectuables.</p>
<p>Packed programs will be byte-identical to the original after uncompression.</p>
<p>How it works:</p>
<pre>
  For ELF executables, UPX decompresses directly to memory, simulating
  the mapping that the operating system kernel uses during exec(),
  including the PT_INTERP program interpreter (if any).
  The brk() is set by a special PT_LOAD segment in the compressed
  executable itself.  UPX then wipes the stack clean except for
  arguments, environment variables, and Elf_auxv entries (this is
  required by bugs in the startup code of /lib/ld-linux.so as of
  May 2000), and transfers control to the program interpreter or
  the e_entry address of the original executable.</pre>
<pre>
  The UPX stub is about 1700 bytes long, partly written in assembler
  and only uses kernel syscalls. It is not linked against any libc.</pre>
<p>Specific drawbacks:</p>
<pre>
  - For linux/elf386 and linux/sh386 formats, you will be relying on
    RAM and swap space to hold all of the decompressed program during
    the lifetime of the process.  If you already use most of your swap
    space, then you may run out.  A system that is &quot;out of memory&quot;
    can become fragile.  Many programs do not react gracefully when
    malloc() returns 0.  With newer Linux kernels, the kernel
    may decide to kill some processes to regain memory, and you
    may not like the kernel's choice of which to kill.  Running
    /usr/bin/top is one way to check on the usage of swap space.</pre>
<p>Extra options available for this executable format:</p>
<pre>
  (none)</pre>
<p>
</p>
<h2><a name="notes_for_linux_sh386">NOTES FOR LINUX/SH386</a></h2>
<p>Please read the general Linux description first.</p>
<p>Shell scripts where the underling shell accepts a ``-c'' argument
can use the Linux/sh386 format.  <strong>UPX</strong> decompresses the shell script
into low memory, then maps the shell and passes the entire text of the
script as an argument with a leading ``-c''.
It does not use space in /tmp, and does not use /proc.</p>
<p>Linux/sh386 is automatically selected for shell scripts that
use a known shell.</p>
<p>Packed programs will be byte-identical to the original after uncompression.</p>
<p>How it works:</p>
<pre>
  For shell script executables (files beginning with &quot;#!/&quot; or &quot;#! /&quot;)
  where the shell is known to accept &quot;-c &lt;command&gt;&quot;, UPX decompresses
  the file into low memory, then maps the shell (and its PT_INTERP),
  and passes control to the shell with the entire decompressed file
  as the argument after &quot;-c&quot;.  Known shells are sh, ash, bash, bsh, csh,
  ksh, tcsh, pdksh.  Restriction: UPX cannot use this method
  for shell scripts which use the one optional string argument after
  the shell name in the script (example: &quot;#! /bin/sh option3\n&quot;.)</pre>
<pre>
  The UPX stub is about 1700 bytes long, partly written in assembler
  and only uses kernel syscalls. It is not linked against any libc.</pre>
<p>Specific drawbacks:</p>
<pre>
  - For linux/elf386 and linux/sh386 formats, you will be relying on
    RAM and swap space to hold all of the decompressed program during
    the lifetime of the process.  If you already use most of your swap
    space, then you may run out.  A system that is &quot;out of memory&quot;
    can become fragile.  Many programs do not react gracefully when
    malloc() returns 0.  With newer Linux kernels, the kernel
    may decide to kill some processes to regain memory, and you