Compress

This is the default operation, eg. upx yourfile.exe will compress the file specified on the command line.

Decompress

All UPX supported file formats can be unpacked using the -d switch, eg. upx -d yourfile.exe will uncompress the file you've just compressed.

Test

The -t command tests the integrity of the compressed and uncompressed data, eg. upx -t yourfile.exe check whether your file can be safely decompressed. Note, that this command doesn't check the whole file, only the part that will be uncompressed during program execution. This means that you should not use this command instead of a virus checker.

List

The -l command prints out some information about the compressed files specified on the command line as parameters, eg upx -l yourfile.exe shows the compressed / uncompressed size and the compression ratio of yourfile.exe.

NOTES FOR ATARI/TOS

This is the executable format used by the Atari ST/TT, a Motorola 68000 based personal computer which was popular in the late '80s. Support of this format is only because of nostalgic feelings of one of the authors and serves no practical purpose :-). See https://freemint.github.io for more info.

Packed programs will be byte-identical to the original after uncompression. All debug information will be stripped, though.

Extra options available for this executable format:

--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.

NOTES FOR BVMLINUZ/I386

Same as vmlinuz/i386.

NOTES FOR DOS/COM

Obviously UPX won't work with executables that want to read data from themselves (like some commandline utilities that ship with Win95/98/ME).

Compressed programs only work on a 286+.

Packed programs will be byte-identical to the original after uncompression.

Maximum uncompressed size: ~65100 bytes.

Extra options available for this executable format:

--8086              Create an executable that works on any 8086 CPU.
--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.
--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.

NOTES FOR DOS/EXE

dos/exe stands for all "normal" 16-bit DOS executables.

Obviously UPX won't work with executables that want to read data from themselves (like some command line utilities that ship with Win95/98/ME).

Compressed programs only work on a 286+.

Extra options available for this executable format:

--8086              Create an executable that works on any 8086 CPU.
--no-reloc          Use no relocation records in the exe header.
--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.

NOTES FOR DOS/SYS

Compressed programs only work on a 286+.

Packed programs will be byte-identical to the original after uncompression.

Maximum uncompressed size: ~65350 bytes.

Extra options available for this executable format:

--8086              Create an executable that works on any 8086 CPU.
--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.
--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.

NOTES FOR DJGPP2/COFF

First of all, it is recommended to use UPX *instead* of strip. strip has the very bad habit of replacing your stub with its own (outdated) version. Additionally UPX corrects a bug/feature in strip v2.8.x: it will fix the 4 KiB alignment of the stub.

UPX includes the full functionality of stubify. This means it will automatically stubify your COFF files. Use the option --coff to disable this functionality (see below).

UPX automatically handles Allegro packfiles.

The DLM format (a rather exotic shared library extension) is not supported.

Packed programs will be byte-identical to the original after uncompression. All debug information and trailing garbage will be stripped, though.

Extra options available for this executable format:

--coff              Produce COFF output instead of EXE. By default
                    UPX keeps your current stub.
--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.
--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.

NOTES FOR LINUX [general]

Introduction

Linux/386 support in UPX consists of 3 different executable formats,
one optimized for ELF executables ("linux/elf386"), one optimized
for shell scripts ("linux/sh386"), and one generic format
("linux/386").
We will start with a general discussion first, but please
also read the relevant docs for each of the individual formats.
Also, there is special support for bootable kernels - see the
description of the vmlinuz/386 format.

General user's overview

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.
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.
Depending on the statistics of usage and access, and the relative
speeds of CPU, RAM, swap space, /tmp, and file system storage, then
invoking and running a compressed executable can be faster than
directly running the corresponding uncompressed program.
The operating system might perform 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 file system.
``Medium-sized'' programs which access about 1/3 to 1/2 of their
stored program bytes can do particularly 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.
Currently, executables compressed by UPX do not share RAM at runtime
in the way that executables mapped from a file system 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.
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''.

General benefits:

- 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.
- Compressed programs are completely self-contained. No need for
  any external program.
- 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. ]
- As the stub only uses syscalls and isn't linked against libc it
  should run under any Linux configuration that can run ELF
  binaries.
- For the same reason compressed executables should run under
  FreeBSD and other systems which can run Linux binaries.
  [ Please send feedback on this topic ]

General drawbacks:

- 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.
- `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.

General notes:

- As UPX leaves your original program untouched it is advantageous
  to strip it before compression.
- If you compress a script you will lose platform independence -
  this could be a problem if you are using NFS mounted disks.
- Compression of suid, guid and sticky-bit programs is rejected
  because of possible security implications.
- For the same reason there is no sense in making any compressed
  program suid.
- 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.
- 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.

NOTES FOR LINUX/ELF386

Please read the general Linux description first.

The linux/elf386 format decompresses directly into RAM, uses only one exec, does not use space in /tmp, and does not use /proc.

Linux/elf386 is automatically selected for Linux ELF executables.

Packed programs will be byte-identical to the original after uncompression.

How it works:

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.
The UPX stub is about 1700 bytes long, partly written in assembler
and only uses kernel syscalls. It is not linked against any libc.

Specific drawbacks:

- 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 "out of memory"
  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.

Extra options available for this executable format:

(none)

NOTES FOR LINUX/SH386

Please read the general Linux description first.

Shell scripts where the underling 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''. It does not use space in /tmp, and does not use /proc.

Linux/sh386 is automatically selected for shell scripts that use a known shell.

Packed programs will be byte-identical to the original after uncompression.

How it works:

For shell script executables (files beginning with "#!/" or "#! /")
where the shell is known to accept "-c <command>", 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 "-c".  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: "#! /bin/sh option3\n".)
The UPX stub is about 1700 bytes long, partly written in assembler
and only uses kernel syscalls. It is not linked against any libc.

Specific drawbacks:

- 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 "out of memory"
  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.

Extra options available for this executable format:

(none)

NOTES FOR LINUX/386

Please read the general Linux description first.

The generic linux/386 format decompresses to /tmp and needs /proc file system support. It starts the decompressed program via the execve() syscall.

Linux/386 is only selected if the specialized linux/elf386 and linux/sh386 won't recognize a file.

Packed programs will be byte-identical to the original after uncompression.

How it works:

For files which are not ELF and not a script for a known "-c" shell,
UPX uses kernel execve(), which first requires decompressing to a
temporary file in the file system.  Interestingly -
because of the good memory management of the Linux kernel - this
often does not introduce a noticeable delay, and in fact there
will be no disk access at all if you have enough free memory as
the entire process takes places within the file system buffers.
A compressed executable consists of the UPX stub and an overlay
which contains the original program in a compressed form.
The UPX stub is a statically linked ELF executable and does
the following at program startup:
  1) decompress the overlay to a temporary location in /tmp
  2) open the temporary file for reading
  3) try to delete the temporary file and start (execve)
     the uncompressed program in /tmp using /proc/<pid>/fd/X as
     attained by step 2)
  4) if that fails, fork off a subprocess to clean up and
     start the program in /tmp in the meantime
The UPX stub is about 1700 bytes long, partly written in assembler
and only uses kernel syscalls. It is not linked against any libc.

Specific drawbacks:

- You need additional free disk space for the uncompressed program
  in your /tmp directory. This program is deleted immediately after
  decompression, but you still need it for the full execution time
  of the program.
- You must have /proc file system support as the stub wants to open
  /proc/<pid>/exe and needs /proc/<pid>/fd/X. This also means that you
  cannot compress programs that are used during the boot sequence
  before /proc is mounted.
- Utilities like `top' will display numerical values in the process
  name field. This is because Linux computes the process name from
  the first argument of the last execve syscall (which is typically
  something like /proc/<pid>/fd/3).
- Because of temporary decompression to disk the decompression speed
  is not as fast as with the other executable formats. Still, I can see
  no noticeable delay when starting programs like my ~3 MiB emacs (which
  is less than 1 MiB when compressed :-).

Extra options available for this executable format:

--force-execve      Force the use of the generic linux/386 "execve"
                    format, i.e. do not try the linux/elf386 and
                    linux/sh386 formats.

NOTES FOR PS1/EXE

This is the executable format used by the Sony PlayStation (PSone), a MIPS R3000 based gaming console which is popular since the late '90s. Support of this format is very similar to the Atari one, because of nostalgic feelings of one of the authors.

Packed programs will be byte-identical to the original after uncompression, until further notice.

Maximum uncompressed size: ~1.89 / ~7.60 MiB.

Notes:

- UPX creates as default a suitable executable for CD-Mastering
  and console transfer. For a CD-Master main executable you could also try
  the special option "--boot-only" as described below.
  It has been reported that upx packed executables are fully compatible with
  the Sony PlayStation 2 (PS2, PStwo) and Sony PlayStation Portable (PSP) in
  Sony PlayStation (PSone) emulation mode.
- Normally the packed files use the same memory areas like the uncompressed
  versions, so they will not override other memory areas while unpacking.
  If this isn't possible UPX will abort showing a 'packed data overlap'
  error. With the "--force" option UPX will relocate the loading address
  for the packed file, but this isn't a real problem if it is a single or
  the main executable.

Extra options available for this executable format:

--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.
--8-bit             Uses 8 bit size compression [default: 32 bit]
--8mib-ram          PSone has 8 MiB ram available [default: 2 MiB]
--boot-only         This format is for main exes and CD-Mastering only !
                    It may slightly improve the compression ratio,
                    decompression routines are faster than default ones.
                    But it cannot be used for console transfer !
--no-align          This option disables CD mode 2 data sector format
                    alignment. May slightly improves the compression ratio,
                    but the compressed executable will not boot from a CD.
                    Use it for console transfer only !

NOTES FOR RTM32/PE and ARM/PE

Same as win32/pe.

NOTES FOR TMT/ADAM

This format is used by the TMT Pascal compiler - see http://www.tmt.com/ .

Extra options available for this executable format:

--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.
--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.

NOTES FOR VMLINUZ/386

The vmlinuz/386 and bvmlinuz/386 formats take a gzip-compressed bootable Linux kernel image ("vmlinuz", "zImage", "bzImage"), gzip-decompress it and re-compress it with the UPX compression method.

vmlinuz/386 is completely unrelated to the other Linux executable formats, and it does not share any of their drawbacks.

Notes:

- Be sure that "vmlinuz/386" or "bvmlinuz/386" is displayed
during compression - otherwise a wrong executable format
may have been used, and the kernel won't boot.

Benefits:

- Better compression (but note that the kernel was already compressed,
so the improvement is not as large as with other formats).
Still, the bytes saved may be essential for special needs like
boot disks.
   For example, this is what I get for my 2.2.16 kernel:
      1589708  vmlinux
       641073  bzImage        [original]
       560755  bzImage.upx    [compressed by "upx -9"]
- Much faster decompression at kernel boot time (but kernel
  decompression speed is not really an issue these days).

Drawbacks:

(none)

Extra options available for this executable format:

--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.
--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.

NOTES FOR WATCOM/LE

UPX has been successfully tested with the following extenders:
DOS4G, DOS4GW, PMODE/W, DOS32a, CauseWay.
The WDOS/X extender is partly supported (for details
see the file bugs BUGS).

DLLs and the LX format are not supported.

Extra options available for this executable format:

--le                Produce an unbound LE output instead of
                    keeping the current stub.

NOTES FOR WIN32/PE

The PE support in UPX is quite stable now, but probably there are still some incompatibilities with some files.

Because of the way UPX (and other packers for this format) works, you can see increased memory usage of your compressed files because the whole program is loaded into memory at startup. If you start several instances of huge compressed programs you're wasting memory because the common segments of the program won't get shared across the instances. On the other hand if you're compressing only smaller programs, or running only one instance of larger programs, then this penalty is smaller, but it's still there.

If you're running executables from network, then compressed programs will load faster, and require less bandwidth during execution.

DLLs are supported. But UPX compressed DLLs can not share common data and code when they got used by multiple applications. So compressing msvcrt.dll is a waste of memory, but compressing the dll plugins of a particular application may be a better idea.

Screensavers are supported, with the restriction that the filename must end with ".scr" (as screensavers are handled slightly different than normal exe files).

UPX compressed PE files have some minor memory overhead (usually in the 10 - 30 KiB range) which can be seen by specifying the "-i" command line switch during compression.

Extra options available for this executable format:

--compress-exports=0 Don't compress the export section.
                     Use this if you plan to run the compressed
                     program under Wine.
--compress-exports=1 Compress the export section. [DEFAULT]
                     Compression of the export section can improve the
                     compression ratio quite a bit but may not work
                     with all programs (like winword.exe).
                     UPX never compresses the export section of a DLL
                     regardless of this option.
 --compress-icons=0  Don't compress any icons.
 --compress-icons=1  Compress all but the first icon.
 --compress-icons=2  Compress all icons which are not in the
                     first icon directory. [DEFAULT]
 --compress-icons=3  Compress all icons.
 --compress-resources=0  Don't compress any resources at all.
 --keep-resource=list Don't compress resources specified by the list.
                     The members of the list are separated by commas.
                     A list member has the following format: I<type[/name]>.
                     I<Type> is the type of the resource. Standard types
                     must be specified as decimal numbers, user types can be
                     specified by decimal IDs or strings. I<Name> is the
                     identifier of the resource. It can be a decimal number
                     or a string. For example:
                     --keep-resource=2/MYBITMAP,5,6/12345
                     UPX won't compress the named bitmap resource "MYBITMAP",
                     it leaves every dialog (5) resource uncompressed, and
                     it won't touch the string table resource with identifier
                     12345.
 --force             Force compression even when there is an
                     unexpected value in a header field.
                     Use with care.
 --strip-relocs=0    Don't strip relocation records.
 --strip-relocs=1    Strip relocation records. [DEFAULT]
                     This option only works on executables with base
                     address greater or equal to 0x400000. Usually the
                     compressed files becomes smaller, but some files
                     may become larger. Note that the resulting file will
                     not work under Windows 3.x (Win32s).
                     UPX never strips relocations from a DLL
                     regardless of this option.
 --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.
 --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.