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mono - Mono's ECMA-CLI native code generator (Just-in-Time and Ahead-of-Time)


mono [options] file [arguments...]

mono-sgen [options] file [arguments...]


mono is a runtime implementation of the ECMA Common Language Infrastructure. This can be
used to run ECMA and .NET applications.

The runtime contains a native code generator that transforms the Common Intermediate
Language into native code.

The code generator can operate in two modes: just in time compilation (JIT) or ahead of
time compilation (AOT). Since code can be dynamically loaded, the runtime environment and
the JIT are always present, even if code is compiled ahead of time.

The runtime loads the specified file and optionally passes the arguments to it. The file
is an ECMA assembly. They typically have a .exe or .dll extension.

The runtime provides a number of configuration options for running applications, for
developing and debugging, and for testing and debugging the runtime itself.

The mono command uses the Boehm conservative garbage collector while the mono-sgen command
uses a moving and generational garbage collector.


On Unix-based systems, Mono provides a mechanism to emulate the Windows-style file access,
this includes providing a case insensitive view of the file system, directory separator
mapping (from \ to /) and stripping the drive letters.

This functionality is enabled by setting the MONO_IOMAP environment variable to one of
all, drive and case.

See the description for MONO_IOMAP in the environment variables section for more details.


The following options are available:

--aot, --aot[=options]
This option is used to precompile the CIL code in the specified assembly to native
code. The generated code is stored in a file with the extension .so. This file
will be automatically picked up by the runtime when the assembly is executed.
Ahead-of-Time compilation is most useful if you use it in combination with the
-O=all,-shared flag which enables all of the optimizations in the code generator to
be performed. Some of those optimizations are not practical for Just-in-Time
compilation since they might be very time consuming. Unlike the .NET Framework,
Ahead-of-Time compilation will not generate domain independent code: it generates
the same code that the Just-in-Time compiler would produce. Since most
applications use a single domain, this is fine. If you want to optimize the
generated code for use in multi-domain applications, consider using the -O=shared
flag. This pre-compiles the methods, but the original assembly is still required
to execute as this one contains the metadata and exception information which is not
available on the generated file. When precompiling code, you might want to compile
with all optimizations (-O=all). Pre-compiled code is position independent code.
Pre compilation is just a mechanism to reduce startup time, increase code sharing
across multiple mono processes and avoid just-in-time compilation program startup
costs. The original assembly must still be present, as the metadata is contained
there. AOT code typically can not be moved from one computer to another (CPU-
specific optimizations that are detected at runtime) so you should not try to move
the pre-generated assemblies or package the pre-generated assemblies for
deployment. A few options are available as a parameter to the --aot command line
option. The options are separated by commas, and more than one can be specified:

The AOT compiler will emit a (ELF only) library initializer to automatically
register the aot compiled module with the runtime. This is only useful in
static mode

Instructs the AOT compiler to output assembly code instead of an object

If specified, forces the generated AOT files to be bound to the runtime
version of the compiling Mono. This will prevent the AOT files from being
consumed by a different Mono runtime. full This is currently an
experimental feature as it is not complete. This instructs Mono to
precompile code that has historically not been precompiled with AOT.

When this option is specified, P/Invoke methods are invoked directly instead
of going through the operating system symbol lookup operation.

Same for the llvm tools 'opt' and 'llc'.

Use the GNU style target triple <TRIPLE> to determine some code generation
options, i.e. --mtriple=armv7-linux-gnueabi will generate code that targets
ARMv7. This is currently only supported by the ARM backend. In LLVM mode,
this triple is passed on to the LLVM llc compiler.

When compiling in full aot mode, the IMT trampolines must be precreated in
the AOT image. You can add additional method trampolines with this
argument. Defaults to 128.

Instructs the AOT compiler to not output any debugging information.

This prevents the AOT compiler from generating a direct calls to a method.
The AOT compiler usually generates direct calls for certain methods that do
not require going through the PLT (for example, methods that are known to
not require a hook like a static constructor) or call into simple internal

Instructs the AOT compiler to emit DWARF debugging information. When used
together with the nodebug option, only DWARF debugging information is
emitted, but not the information that can be used at runtime.

When compiling in full aot mode, the generic sharing trampolines must be
precreated in the AOT image. You can add additional method trampolines with
this argument. Defaults to 1024.

When compiling in full aot mode, the method trampolines must be precreated
in the AOT image. You can add additional method trampolines with this
argument. Defaults to 1024.

Instructs the AOT compiler to save the output to the specified file.

If the AOT compiler cannot compile a method for any reason, enabling this
flag will output the skipped methods to the console.

Override the value of a static readonly field. Usually, during JIT
compilation, the static constructor is ran eagerly, so the value of a static
readonly field is known at compilation time and the compiler can do a number
of optimizations based on it. During AOT, instead, the static constructor
can't be ran, so this option can be used to set the value of such a field
and enable the same set of optimizations. Type can be any of i1, i2, i4 for
integers of the respective sizes (in bytes). Note that signed/unsigned
numbers do not matter here, just the storage size. This option can be
specified multiple times and it doesn't prevent the static constructor for
the type defining the field to execute with the usual rules at runtime
(hence possibly computing a different value for the field).

Instructs the AOT compiler to keep temporary files.

This instructs the compiler to generate sequence point checks that allow
Mono's soft debugger to debug applications even on systems where it is not
possible to set breakpoints or to single step (certain hardware
configurations like the cell phones and video gaming consoles).

static Create an ELF object file (.o) or .s file which can be statically linked
into an executable when embedding the mono runtime. When this option is
used, the object file needs to be registered with the embedded runtime using
the mono_aot_register_module function which takes as its argument the
mono_aot_module_<ASSEMBLY NAME>_info global symbol from the object file:

extern void *mono_aot_module_hello_info;

mono_aot_register_module (mono_aot_module_hello_info);

stats Print various stats collected during AOT compilation.

This is an experimental option for the AOT compiler to use multiple threads
when compiling the methods.

Prepends <PREFIX> to the name of tools ran by the AOT compiler, i.e.
'as'/'ld'. For example, --tool=prefix=arm-linux-gnueabi- will make the AOT
compiler run

Instructs the AOT compiler to emit debug symbol information.

For more information about AOT, see: http://www.mono-project.com/docs/advanced/aot/

Currently the only option supported by this command line argument is disable which
disables the attach functionality.

--config filename
Load the specified configuration file instead of the default one(s). The default
files are /etc/mono/config and ~/.mono/config or the file specified in the
MONO_CONFIG environment variable, if set. See the mono-config(5) man page for
details on the format of this file.

This instructs the Mono runtime to start a debugging agent inside the Mono runtime
and connect it to a client user interface will control the Mono process. This
option is typically used by IDEs, like the MonoDevelop IDE.

The configuration is specified using one of more of the following options:

Use this option to specify the IP address where your debugger client is
listening to.

Specifies the diagnostics log level for

Used to specify the file where the log will be stored, it defaults to
standard output.

Defaults to no, with the default option Mono will actively connect to the
host/port configured with the address option. If you set it to 'y', it
instructs the Mono runtime to start debugging in server mode, where Mono
actively waits for the debugger front end to connect to the Mono process.
Mono will print out to stdout the IP address and port where it is listening.

If set to yes, Mono will call setpgid(0, 0) on startup, if that function is
available on the system. This is useful for ensuring that signals delivered
to a process that is executing the debuggee are not propagated to the
debuggee, e.g. when Ctrl-C sends SIGINT to the sdb tool.

Defaults to yes, with the default option Mono will suspend the vm on startup
until it connects successfully to a debugger front end. If you set it to
'n', in conjunction with server=y, it instructs the Mono runtime to run as
normal, while caching metadata to send to the debugger front end on

This is used to specify the transport that the debugger will use to
communicate. It must be specified and currently requires this to be

Configures the virtual machine to be better suited for desktop applications.
Currently this sets the GC system to avoid expanding the heap as much as possible
at the expense of slowing down garbage collection a bit.

This is an experimental flag that instructs the Mono runtime to not generate any
code at runtime and depend exclusively on the code generated from using mono
--aot=full previously. This is useful for platforms that do not permit dynamic
code generation. Notice that this feature will abort execution at runtime if a
codepath in your program, or Mono's class libraries attempts to generate code
dynamically. You should test your software upfront and make sure that you do not
use any dynamic features.

--gc=boehm, --gc=sgen
Selects the Garbage Collector engine for Mono to use, Boehm or SGen. Currently
this merely ensures that you are running either the mono or mono-sgen commands.
This flag can be set in the MONO_ENV_OPTIONS environment variable to force all of
your child processes to use one particular kind of GC with the Mono runtime.

--help, -h
Displays usage instructions.

--llvm If the Mono runtime has been compiled with LLVM support (not available in all
configurations), Mono will use the LLVM optimization and code generation engine to
JIT or AOT compile. For more information, consult: http://www.mono-

When using a Mono that has been compiled with LLVM support, it forces Mono to
fallback to its JIT engine and not use the LLVM backend.

--optimize=MODE, -O=MODE
MODE is a comma separated list of optimizations. They also allow optimizations to
be turned off by prefixing the optimization name with a minus sign. In general,
Mono has been tuned to use the default set of flags, before using these flags for a
deployment setting, you might want to actually measure the benefits of using them.
The following optimization flags are implemented in the core engine:
abcrem Array bound checks removal
all Turn on all optimizations
aot Usage of Ahead Of Time compiled code
branch Branch optimizations
cfold Constant folding
cmov Conditional moves [arch-dependency]
deadce Dead code elimination
consprop Constant propagation
copyprop Copy propagation
fcmov Fast x86 FP compares [arch-dependency]
float32 Perform 32-bit float arithmetic using 32-bit operations
gshared Enable generic code sharing.
inline Inline method calls
intrins Intrinsic method implementations
linears Linear scan global reg allocation
leaf Leaf procedures optimizations
loop Loop related optimizations
peephole Peephole postpass
precomp Precompile all methods before executing Main
sched Instruction scheduling
shared Emit per-domain code
sse2 SSE2 instructions on x86 [arch-dependency]
tailc Tail recursion and tail calls
For example, to enable all the optimization but dead code elimination and inlining,
you can use:
The flags that are flagged with [arch-dependency] indicate that the given option if
used in combination with Ahead of Time compilation (--aot flag) would produce pre-
compiled code that will depend on the current CPU and might not be safely moved to
another computer.

The following optimizations are supported

Requests that the runtime performn 32-bit floating point operations using
only 32-bits. By default the Mono runtime tries to use the highest
precision available for floating point operations, but while this might
render better results, the code might run slower. This options also
affects the code generated by the LLVM backend.

inline Controls whether the runtime should attempt to inline (the default), or not
inline methods invocations

Mono supports different runtime versions. The version used depends on the program
that is being run or on its configuration file (named program.exe.config). This
option can be used to override such autodetection, by forcing a different runtime
version to be used. Note that this should only be used to select a later compatible
runtime version than the one the program was compiled against. A typical usage is
for running a 1.1 program on a 2.0 version:
mono --runtime=v2.0.50727 program.exe

--security, --security=mode
Activate the security manager, a currently experimental feature in Mono and it is
OFF by default. The new code verifier can be enabled with this option as well.

Using security without parameters is equivalent as calling it with the "cas"

The following modes are supported:

Enables the core-clr security system, typically used for
Moonlight/Silverlight applications. It provides a much simpler security
system than CAS, see http://www.mono-project.com/docs/web/moonlight/ for
more details and links to the descriptions of this new system.

Enables the new verifier and performs basic verification for code validity.
In this mode, unsafe code and P/Invoke are allowed. This mode provides a
better safety guarantee but it is still possible for managed code to crash

Enables the new verifier and performs full verification of the code being
executed. It only allows verifiable code to be executed. Unsafe code is
not allowed but P/Invoke is. This mode should not allow managed code to
crash mono. The verification is not as strict as ECMA 335 standard in order
to stay compatible with the MS runtime.

The security system acts on user code: code contained in mscorlib or the global
assembly cache is always trusted.

Configures the virtual machine to be better suited for server operations
(currently, allows a heavier threadpool initialization).

Verifies mscorlib and assemblies in the global assembly cache for valid IL, and all
user code for IL verifiability.

This is different from --security's verifiable or validil in that these options
only check user code and skip mscorlib and assemblies located on the global
assembly cache.

-V, --version
Prints JIT version information (system configuration, release number and branch
names if available).


The following options are used to help when developing a JITed application.

--debug, --debug=OPTIONS
Turns on the debugging mode in the runtime. If an assembly was compiled with
debugging information, it will produce line number information for stack traces.

The optional OPTIONS argument is a comma separated list of debugging options.
These options are turned off by default since they generate much larger and slower
code at runtime.

The following options are supported:

casts Produces a detailed error when throwing a InvalidCastException. This
option needs to be enabled as this generates more verbose code at execution

Disable some JIT optimizations which are usually only disabled when running
inside the debugger. This can be helpful if you want to attach to the
running process with mdb.

gdb Generate and register debugging information with gdb. This is only supported
on some platforms, and only when using gdb 7.0 or later.

Turns on profiling. For more information about profiling applications and code
coverage see the sections "PROFILING" and "CODE COVERAGE" below.

This option can be used multiple times, each time will load an
additional profiler. This allows developers to use modules that extend the JIT
through the Mono profiling interface.

Shows method names as they are invoked. By default all methods are traced. The
trace can be customized to include or exclude methods, classes or assemblies. A
trace expression is a comma separated list of targets, each target can be prefixed
with a minus sign to turn off a particular target. The words `program', `all' and
`disabled' have special meaning. `program' refers to the main program being
executed, and `all' means all the method calls. The `disabled' option is used to
start up with tracing disabled. It can be enabled at a later point in time in the
program by sending the SIGUSR2 signal to the runtime. Assemblies are specified by
their name, for example, to trace all calls in the System assembly, use:

mono --trace=System app.exe

Classes are specified with the T: prefix. For example, to trace all calls to the
System.String class, use:

mono --trace=T:System.String app.exe

And individual methods are referenced with the M: prefix, and the standard method

mono --trace=M:System.Console:WriteLine app.exe

Exceptions can also be traced, it will cause a stack trace to be printed every time
an exception of the specified type is thrown. The exception type can be specified
with or without the namespace, and to trace all exceptions, specify 'all' as the
type name.

mono --trace=E:System.Exception app.exe

As previously noted, various rules can be specified at once:

mono --trace=T:System.String,T:System.Random app.exe

You can exclude pieces, the next example traces calls to System.String except for
the System.String:Concat method.

mono --trace=T:System.String,-M:System.String:Concat

You can trace managed to unmanaged transitions using the wrapper qualifier:

mono --trace=wrapper app.exe

Finally, namespaces can be specified using the N: prefix:

mono --trace=N:System.Xml

Don't align stack frames on the x86 architecture. By default, Mono aligns stack
frames to 16 bytes on x86, so that local floating point and SIMD variables can be
properly aligned. This option turns off the alignment, which usually saves one
intruction per call, but might result in significantly lower floating point and
SIMD performance.

Generate a JIT method map in a /tmp/perf-PID.map file. This file is then used, for
example, by the perf tool included in recent Linux kernels. Each line in the file


Currently this option is only supported on Linux.


The maintainer options are only used by those developing the runtime itself, and not
typically of interest to runtime users or developers.

--break method
Inserts a breakpoint before the method whose name is `method'
(namespace.class:methodname). Use `Main' as method name to insert a breakpoint on
the application's main method. You can use it also with generics, for example

Inserts a breakpoint on exceptions. This allows you to debug your application with
a native debugger when an exception is thrown.

--compile name
This compiles a method (namespace.name:methodname), this is used for testing the
compiler performance or to examine the output of the code generator.

Compiles all the methods in an assembly. This is used to test the compiler
performance or to examine the output of the code generator

This generates a postscript file with a graph with the details about the specified
method (namespace.name:methodname). This requires `dot' and ghostview to be
installed (it expects Ghostview to be called "gv"). The following graphs are
cfg Control Flow Graph (CFG)
dtree Dominator Tree
code CFG showing code
ssa CFG showing code after SSA translation
optcode CFG showing code after IR optimizations
Some graphs will only be available if certain optimizations are turned on.

Instruct the runtime on the number of times that the method specified by --compile
(or all the methods if --compileall is used) to be compiled. This is used for
testing the code generator performance.

Displays information about the work done by the runtime during the execution of an

Perform maintenance of the process shared data. semdel will delete the global
semaphore. hps will list the currently used handles.

-v, --verbose
Increases the verbosity level, each time it is listed, increases the verbosity
level to include more information (including, for example, a disassembly of the
native code produced, code selector info etc.).


The Mono runtime allows external processes to attach to a running process and load
assemblies into the running program. To attach to the process, a special protocol is
implemented in the Mono.Management assembly.

With this support it is possible to load assemblies that have an entry point (they are
created with -target:exe or -target:winexe) to be loaded and executed in the Mono process.

The code is loaded into the root domain, and it starts execution on the special runtime
attach thread. The attached program should create its own threads and return after

This support allows for example debugging applications by having the csharp shell attach
to running processes.


The mono runtime includes a profiler that can be used to explore various performance
related problems in your application. The profiler is activated by passing the --profile
command line argument to the Mono runtime, the format is:


Mono has a built-in profiler called 'default' (and is also the default if no arguments are
specified), but developers can write custom profilers, see the section "CUSTOM PROFILERS"
for more details.

If a profiler is not specified, the default profiler is used. The profiler_args is a
profiler-specific string of options for the profiler itself. The default profiler accepts
the following options 'alloc' to profile memory consumption by the application; 'time' to
profile the time spent on each routine; 'jit' to collect time spent JIT-compiling methods
and 'stat' to perform sample statistical profiling. If no options are provided the
default is 'alloc,time,jit'.

By default the profile data is printed to stdout: to change this, use the 'file=filename'
option to output the data to filename. For example:

mono --profile program.exe

That will run the program with the default profiler and will do time and allocation

mono --profile=default:stat,alloc,file=prof.out program.exe

Will do sample statistical profiling and allocation profiling on program.exe. The profile
data is put in prof.out. Note that the statistical profiler has a very low overhead and
should be the preferred profiler to use (for better output use the full path to the mono
binary when running and make sure you have installed the addr2line utility that comes from
the binutils package).


This is the most advanced profiler.

The Mono log profiler can be used to collect a lot of information about a program running
in the Mono runtime. This data can be used (both while the process is running and later)
to do analyses of the program behaviour, determine resource usage, performance issues or
even look for particular execution patterns.

This is accomplished by logging the events provided by the Mono runtime through the
profiling interface and periodically writing them to a file which can be later inspected
with the mprof-report(1) tool.

More information about how to use the log profiler is available on the mprof-report(1)


Mono provides a mechanism for loading other profiling modules which in the form of shared
libraries. These profiling modules can hook up to various parts of the Mono runtime to
gather information about the code being executed.

To use a third party profiler you must pass the name of the profiler to Mono, like this:

mono --profile=custom program.exe

In the above sample Mono will load the user defined profiler from the shared library
`mono-profiler-custom.so'. This profiler module must be on your dynamic linker library

A list of other third party profilers is available from Mono's web site (www.mono-

Custom profiles are written as shared libraries. The shared library must be called `mono-
profiler-NAME.so' where `NAME' is the name of your profiler.

For a sample of how to write your own custom profiler look in the Mono source tree for in
the samples/profiler.c.


Mono ships with a code coverage module. This module is activated by using the Mono
--profile=cov option. The format is: --profile=cov[:assembly-name[/namespace]] test-

By default code coverage will default to all the assemblies loaded, you can limit this by
specifying the assembly name, for example to perform code coverage in the routines of your
program use, for example the following command line limits the code coverage to routines
in the "demo" assembly:

mono --profile=cov:demo demo.exe

Notice that the assembly-name does not include the extension.

You can further restrict the code coverage output by specifying a namespace:

mono --profile=cov:demo/My.Utilities demo.exe

Which will only perform code coverage in the given assembly and namespace.

Typical output looks like this:

Not covered: Class:.ctor ()
Not covered: Class:A ()
Not covered: Driver:.ctor ()
Not covered: Driver:method ()
Partial coverage: Driver:Main ()
offset 0x000a

The offsets displayed are IL offsets.

A more powerful coverage tool is available in the module `monocov'. See the monocov(1)
man page for details.


To debug managed applications, you can use the mdb command, a command line debugger.

It is possible to obtain a stack trace of all the active threads in Mono by sending the
QUIT signal to Mono, you can do this from the command line, like this:

kill -QUIT pid

Where pid is the Process ID of the Mono process you want to examine. The process will
continue running afterwards, but its state is not guaranteed.

Important: this is a last-resort mechanism for debugging applications and should not be
used to monitor or probe a production application. The integrity of the runtime after
sending this signal is not guaranteed and the application might crash or terminate at any
given point afterwards.

The --debug=casts option can be used to get more detailed information for Invalid Cast
operations, it will provide information about the types involved.

You can use the MONO_LOG_LEVEL and MONO_LOG_MASK environment variables to get verbose
debugging output about the execution of your application within Mono.

The MONO_LOG_LEVEL environment variable if set, the logging level is changed to the set
value. Possible values are "error", "critical", "warning", "message", "info", "debug". The
default value is "error". Messages with a logging level greater then or equal to the log
level will be printed to stdout/stderr.

Use "info" to track the dynamic loading of assemblies.

Use the MONO_LOG_MASK environment variable to limit the extent of the messages you get: If
set, the log mask is changed to the set value. Possible values are "asm" (assembly
loader), "type", "dll" (native library loader), "gc" (garbage collector), "cfg" (config
file loader), "aot" (precompiler), "security" (e.g. Moonlight CoreCLR support) and "all".
The default value is "all". Changing the mask value allows you to display only messages
for a certain component. You can use multiple masks by comma separating them. For example
to see config file messages and assembly loader messages set you mask to "asm,cfg".

The following is a common use to track down problems with P/Invoke:

$ MONO_LOG_LEVEL="debug" MONO_LOG_MASK="dll" mono glue.exe


If you are using LLDB, you can use the mono.py script to print some internal data
structures with it. To use this, add this to your $HOME/.lldbinit file:
command script import $PREFIX/lib/mono/lldb/mono.py

Where $PREFIX is the prefix value that you used when you configured Mono (typically /usr).

Once this is done, then you can inspect some Mono Runtime data structures, for example:
(lldb) p method

(MonoMethod *) $0 = 0x05026ac0 [mscorlib]System.OutOfMemoryException:.ctor()


Mono's XML serialization engine by default will use a reflection-based approach to
serialize which might be slow for continuous processing (web service applications). The
serialization engine will determine when a class must use a hand-tuned serializer based on
a few parameters and if needed it will produce a customized C# serializer for your types
at runtime. This customized serializer then gets dynamically loaded into your

You can control this with the MONO_XMLSERIALIZER_THS environment variable.

The possible values are `no' to disable the use of a C# customized serializer, or an
integer that is the minimum number of uses before the runtime will produce a custom
serializer (0 will produce a custom serializer on the first access, 50 will produce a
serializer on the 50th use). Mono will fallback to an interpreted serializer if the
serializer generation somehow fails. This behavior can be disabled by setting the option
`nofallback' (for example: MONO_XMLSERIALIZER_THS=0,nofallback).


Turns off the garbage collection in Mono. This should be only used for debugging

When Mono is compiled with LLVM support, this instructs the runtime to stop using
LLVM after the specified number of methods are JITed. This is a tool used in
diagnostics to help isolate problems in the code generation backend. For example
LLVM_COUNT=10 would only compile 10 methods with LLVM and then switch to the Mono
JIT engine. LLVM_COUNT=0 would disable the LLVM engine altogether.

If set, this variable will instruct Mono to ahead-of-time compile new assemblies on
demand and store the result into a cache in ~/.mono/aot-cache.

Mono contains a feature which allows modifying settings in the .config files
shipped with Mono by using config section mappers. The mappers and the mapping
rules are defined in the $prefix/etc/mono/2.0/settings.map file and, optionally, in
the settings.map file found in the top-level directory of your ASP.NET application.
Both files are read by System.Web on application startup, if they are found at the
above locations. If you don't want the mapping to be performed you can set this
variable in your environment before starting the application and no action will be

Mono has a cache of ConfigSection objects for speeding up WebConfigurationManager
queries. Its default size is 100 items, and when more items are needed, cache
evictions start happening. If evictions are too frequent this could impose
unnecessary overhead, which could be avoided by using this environment variable to
set up a higher cache size (or to lower memory requirements by decreasing it).

If set, causes Mono.Cairo to collect stack traces when objects are allocated, so
that the finalization/Dispose warnings include information about the instance's

If set, this variable overrides the default system configuration directory
($PREFIX/etc). It's used to locate machine.config file.

Sets the style of COM interop. If the value of this variable is "MS" Mono will use
string marhsalling routines from the liboleaut32 for the BSTR type library, any
other values will use the mono-builtin BSTR string marshalling.

If set, this variable overrides the default runtime configuration file
($PREFIX/etc/mono/config). The --config command line options overrides the
environment variable.

Override the automatic cpu detection mechanism. Currently used only on arm. The
format of the value is as follows:

"armvV [thumb[2]]"

where V is the architecture number 4, 5, 6, 7 and the options can be currently be
"thumb" or "thumb2". Example:

MONO_CPU_ARCH="armv4 thumb" mono ...

When Mono is built with a soft float fallback on ARM and this variable is set to
"1", Mono will always emit soft float code, even if a VFP unit is detected.

If set, tells mono NOT to attempt using native asynchronous I/O services. In that
case, a default select/poll implementation is used. Currently only epoll() is

If this environment variable is `yes', the runtime uses unmanaged collation (which
actually means no culture-sensitive collation). It internally disables managed
collation functionality invoked via the members of System.Globalization.CompareInfo
class. Collation is enabled by default.

Unix only: If set, disables the shared memory files used for cross-process handles:
process have only private handles. This means that process and thread handles are
not available to other processes, and named mutexes, named events and named
semaphores are not visible between processes. This is can also be enabled by
default by passing the "--disable-shared-handles" option to configure. This is the
default from mono 2.8 onwards.

Unix only: If set, disable usage of shared memory for exposing performance
counters. This means it will not be possible to both externally read performance
counters from this processes or read those of external processes.

When set, enables the use of a fully managed DNS resolver instead of the regular
libc functions. This resolver performs much better when multiple queries are run in

Note that /etc/nsswitch.conf will be ignored.

For platforms that do not otherwise have a way of obtaining random bytes this can
be set to the name of a file system socket on which an egd or prngd daemon is

Unix only: Enable support for cross-process handles. Cross-process handles are
used to expose process handles, thread handles, named mutexes, named events and
named semaphores across Unix processes.

This environment variable allows you to pass command line arguments to a Mono
process through the environment. This is useful for example to force all of your
Mono processes to use LLVM or SGEN without having to modify any launch scripts.

Used to pass extra options to the debugger agent in the runtime, as they were
passed using --debugger-agent=.

Sets the type of event log provider to use (for System.Diagnostics.EventLog).
Possible values are:

Persists event logs and entries to the local file system. The directory in
which to persist the event logs, event sources and entries can be specified
as part of the value. If the path is not explicitly set, it defaults to
"/var/lib/mono/eventlog" on unix and "%APPDATA%no\ventlog" on Windows.

win32 Uses the native win32 API to write events and registers event logs and event
sources in the registry. This is only available on Windows. On Unix, the
directory permission for individual event log and event source directories
is set to 777 (with +t bit) allowing everyone to read and write event log
entries while only allowing entries to be deleted by the user(s) that
created them.

null Silently discards any events.

The default is "null" on Unix (and versions of Windows before NT), and "win32" on
Windows NT (and higher).

If set, contains a colon-separated list of text encodings to try when turning
externally-generated text (e.g. command-line arguments or filenames) into Unicode.
The encoding names come from the list provided by iconv, and the special case
"default_locale" which refers to the current locale's default encoding.

When reading externally-generated text strings UTF-8 is tried first, and then this
list is tried in order with the first successful conversion ending the search.
When writing external text (e.g. new filenames or arguments to new processes) the
first item in this list is used, or UTF-8 if the environment variable is not set.

The problem with using MONO_EXTERNAL_ENCODINGS to process your files is that it
results in a problem: although its possible to get the right file name it is not
necessarily possible to open the file. In general if you have problems with
encodings in your filenames you should use the "convmv" program.

When using Mono with the SGen garbage collector this variable controls several
parameters of the collector. The variable's value is a comma separated list of

Sets the size of the nursery. The size is specified in bytes and must be a
power of two. The suffixes `k', `m' and `g' can be used to specify kilo-,
mega- and gigabytes, respectively. The nursery is the first generation (of
two). A larger nursery will usually speed up the program but will obviously
use more memory. The default nursery size 4 MB.

major=collector Specifies which major collector to use.
Options are `marksweep' for the Mark&Sweep collector, and `marksweep-conc'
for concurrent Mark&Sweep. The non-concurrent Mark&Sweep collector is the

Once the heap size gets larger than this size, ignore what the default major
collection trigger metric says and only allow four nursery size's of major
heap growth between major collections.

Sets the evacuation threshold in percent. This option is only available on
the Mark&Sweep major collectors. The value must be an integer in the range
0 to 100. The default is 66. If the sweep phase of the collection finds
that the occupancy of a specific heap block type is less than this
percentage, it will do a copying collection for that block type in the next
major collection, thereby restoring occupancy to close to 100 percent. A
value of 0 turns evacuation off.

Enables or disables lazy sweep for the Mark&Sweep collector. If enabled,
the sweeping of individual major heap blocks is done piecemeal whenever the
need arises, typically during nursery collections. Lazy sweeping is enabled
by default.

Enables or disables concurrent sweep for the Mark&Sweep collector. If
enabled, the iteration of all major blocks to determine which ones can be
freed and which ones have to be kept and swept, is done concurrently with
the running program. Concurrent sweeping is enabled by default.

Specifies how application threads should be scanned. Options are `precise`
and `conservative`. Precise marking allow the collector to know what values
on stack are references and what are not. Conservative marking threats all
values as potentially references and leave them untouched. Precise marking
reduces floating garbage and can speed up nursery collection and allocation
rate, it has the downside of requiring a significant extra memory per
compiled method. The right option, unfortunately, requires experimentation.

Specifies the target save ratio for the major collector. The collector lets
a given amount of memory to be promoted from the nursery due to minor
collections before it triggers a major collection. This amount is based on
how much memory it expects to free. It is represented as a ratio of the size
of the heap after a major collection. Valid values are between 0.1 and 2.0.
The default is 0.5. Smaller values will keep the major heap size smaller
but will trigger more major collections. Likewise, bigger values will use
more memory and result in less frequent major collections. This option is
EXPERIMENTAL, so it might disappear in later versions of mono.

Specifies the default allocation allowance when the calculated size is too
small. The allocation allowance is how much memory the collector let be
promoted before triggered a major collection. It is a ratio of the nursery
size. Valid values are between 1.0 and 10.0. The default is 4.0. Smaller
values lead to smaller heaps and more frequent major collections. Likewise,
bigger values will allow the heap to grow faster but use more memory when it
reaches a stable size. This option is EXPERIMENTAL, so it might disappear
in later versions of mono.

Specifies which minor collector to use. Options are 'simple' which promotes
all objects from the nursery directly to the old generation and 'split'
which lets object stay longer on the nursery before promoting.

Specifies the ratio of memory from the nursery to be use by the alloc space.
This only can only be used with the split minor collector. Valid values are
integers between 1 and 100. Default is 60.

Specifies the required age of an object must reach inside the nursery before
been promoted to the old generation. This only can only be used with the
split minor collector. Valid values are integers between 1 and 14. Default
is 2.

Enables or disables cementing. This can dramatically shorten nursery
collection times on some benchmarks where pinned objects are referred to
from the major heap.

When using Mono with the SGen garbage collector this environment variable can be
used to turn on various debugging features of the collector. The value of this
variable is a comma separated list of words. Do not use these options in

number Sets the debug level to the specified number.

After each major collection prints memory consumption for before and after
the collection and the allowance for the minor collector, i.e. how much the
heap is allowed to grow from minor collections before the next major
collection is triggered.

Gathers statistics on the classes whose objects are pinned in the nursery
and for which global remset entries are added. Prints those statistics when
shutting down.


This performs a consistency check on minor collections and also clears the
nursery at collection time, instead of the default, when buffers are
allocated (clear-at-gc). The consistency check ensures that there are no
major to minor references that are not on the remembered sets.

Checks that the mod-union cardtable is consistent before each finishing
major collection pause. This check is only applicable to concurrent major

Checks that mark bits in the major heap are consistent at the end of each
major collection. Consistent mark bits mean that if an object is marked,
all objects that it had references to must also be marked.

After nursery collections, and before starting concurrent collections, check
whether all nursery objects are pinned, or not pinned - depending on
context. Does nothing when the split nursery collector is used.

Performs a check to make sure that no references are left to an unloaded

Clears the nursery incrementally when the thread local allocation buffers
(TLAB) are created. The default setting clears the whole nursery at GC

Clears the nursery incrementally when the thread local allocation buffers
(TLAB) are created, but at GC time fills it with the byte `0xff`, which
should result in a crash more quickly if `clear-at-tlab-creation` doesn't
work properly.

This clears the nursery at GC time instead of doing it when the thread local
allocation buffer (TLAB) is created. The default is to clear the nursery at
TLAB creation time.

Don't do minor collections. If the nursery is full, a major collection is
triggered instead, unless it, too, is disabled.

Don't do major collections.

Forces the GC to scan the stack conservatively, even if precise scanning is

Disables the managed allocator.

If set, does a plausibility check on the scan_starts before and after each

If set, does a complete object walk of the nursery at the start of each
minor collection.

If set, dumps the contents of the nursery at the start of each minor
collection. Requires verify-nursery-at-minor-gc to be set.

Dumps the heap contents to the specified file. To visualize the
information, use the mono-heapviz tool.

Outputs the debugging output to the specified file. For this to work, Mono
needs to be compiled with the BINARY_PROTOCOL define on sgen-gc.c. You can
then use this command to explore the output
sgen-grep-binprot 0x1234 0x5678 < file

If set, objects allocated in the nursery are suffixed with a canary (guard)
word, which is checked on each minor collection. Can be used to detect/debug
heap corruption issues.

If enabled, finalizers will not be run. Everything else will be unaffected:
finalizable objects will still be put into the finalization queue where they
survive until they're scheduled to finalize. Once they're not in the queue
anymore they will be collected regularly.

Log verbosely around the finalization process to aid debugging.

Provides a prefix the runtime uses to look for Global Assembly Caches. Directories
are separated by the platform path separator (colons on unix). MONO_GAC_PREFIX
should point to the top directory of a prefixed install. Or to the directory
provided in the gacutil /gacdir command. Example:

Enables some filename rewriting support to assist badly-written applications that
hard-code Windows paths. Set to a colon-separated list of "drive" to strip drive
letters, or "case" to do case-insensitive file matching in every directory in a
path. "all" enables all rewriting methods. (Backslashes are always mapped to
slashes if this variable is set to a valid option).
For example, this would work from the shell:


If you are using mod_mono to host your web applications, you can use the MonoIOMAP
directive instead, like this:

MonoIOMAP <appalias> all

See mod_mono(8) for more details.

Additionally. Mono includes a profiler module which allows one to track what
adjustements to file paths IOMAP code needs to do. The tracking code reports the
managed location (full stack trace) from which the IOMAP-ed call was made and, on
process exit, the locations where all the IOMAP-ed strings were created in managed
code. The latter report is only approximate as it is not always possible to
estimate the actual location where the string was created. The code uses simple
heuristics - it analyzes stack trace leading back to the string allocation location
and ignores all the managed code which lives in assemblies installed in GAC as well
as in the class libraries shipped with Mono (since they are assumed to be free of
case-sensitivity issues). It then reports the first location in the user's code -
in most cases this will be the place where the string is allocated or very close to
the location. The reporting code is implemented as a custom profiler module (see
the "PROFILING" section) and can be loaded in the following way:

mono --profile=iomap yourapplication.exe

Note, however, that Mono currently supports only one profiler module at a time.

When Mono is using the LLVM code generation backend you can use this environment
variable to pass code generation options to the LLVM compiler.

If set to "disabled", System.IO.FileSystemWatcher will use a file watcher
implementation which silently ignores all the watching requests. If set to any
other value, System.IO.FileSystemWatcher will use the default managed
implementation (slow). If unset, mono will try to use inotify, FAM, Gamin, kevent
under Unix systems and native API calls on Windows, falling back to the managed
implementation on error.

Mono supports a plugin model for its implementation of System.Messaging making it
possible to support a variety of messaging implementations (e.g. AMQP, ActiveMQ).
To specify which messaging implementation is to be used the evironement variable
needs to be set to the full class name for the provider. E.g. to use the RabbitMQ
based AMQP implementation the variable should be set to:


If set causes the mono process to be bound to a single processor. This may be
useful when debugging or working around race conditions.

Disable inlining of thread local accesses. Try setting this if you get a segfault
early on in the execution of mono.

Provides a search path to the runtime where to look for library
files. This is a tool convenient for debugging applications, but
should not be used by deployed applications as it breaks the assembly
loader in subtle ways.
Directories are separated by the platform path separator (colons on unix). Example:
Relative paths are resolved based on the launch-time current directory.
Alternative solutions to MONO_PATH include: installing libraries into
the Global Assembly Cache (see gacutil(1)) or having the dependent
libraries side-by-side with the main executable.
For a complete description of recommended practices for application
deployment, see

Experimental RTC support in the statistical profiler: if the user has
the permission, more accurate statistics are gathered. The MONO_RTC
value must be restricted to what the Linux rtc allows: power of two
from 64 to 8192 Hz. To enable higher frequencies like 4096 Hz, run as root:

echo 4096 > /proc/sys/dev/rtc/max-user-freq

For example:

MONO_RTC=4096 mono --profiler=default:stat program.exe

If set its the directory where the ".wapi" handle state is stored. This is the
directory where the Windows I/O Emulation layer stores its shared state data
(files, events, mutexes, pipes). By default Mono will store the ".wapi" directory
in the users's home directory.

Uses the string value of this variable as a replacement for the host name when
creating file names in the ".wapi" directory. This helps if the host name of your
machine is likely to be changed when a mono application is running or if you have a
.wapi directory shared among several different computers. Mono typically uses the
hostname to create the files that are used to share state across multiple Mono
processes. This is done to support home directories that might be shared over the

If set, extra checks are made during IO operations. Currently, this includes only
advisory locks around file writes.

The name of the theme to be used by Windows.Forms. Available themes today include
"clearlooks", "nice" and "win32". The default is "win32".

The time, in seconds, that the SSL/TLS session cache will keep it's entry to avoid
a new negotiation between the client and a server. Negotiation are very CPU
intensive so an application-specific custom value may prove useful for small
embedded systems. The default is 180 seconds.

The minimum number of threads in the general threadpool will be
MONO_THREADS_PER_CPU * number of CPUs. The default value for this variable is 1.

Controls the threshold for the XmlSerializer to produce a custom serializer for a
given class instead of using the Reflection-based interpreter. The possible values
are `no' to disable the use of a custom serializer or a number to indicate when the
XmlSerializer should start serializing. The default value is 50, which means that
the a custom serializer will be produced on the 50th use.

Sets the revocation mode used when validating a X509 certificate chain (https,
ftps, smtps...). The default is 'nocheck', which performs no revocation check at
all. The other possible values are 'offline', which performs CRL check (not
implemented yet) and 'online' which uses OCSP and CRL to verify the revocation
status (not implemented yet).


If set to any value, temporary source files generated by ASP.NET support classes
will not be removed. They will be kept in the user's temporary directory.

If set, enables some features of the runtime useful for debugging. This variable
should contain a comma separated list of debugging options. Currently, the
following options are supported:

If this variable is set, when the Mono VM runs into a verification problem,
instead of throwing an exception it will break into the debugger. This is
useful when debugging verifier problems

casts This option can be used to get more detailed information from InvalidCast
exceptions, it will provide information about the types involved.

Collects information about pagefaults. This is used internally to track
the number of page faults produced to load metadata. To display this
information you must use this option with "--stats" command line option.

This is an Optimization for multi-AppDomain applications (most commonly
ASP.NET applications). Due to internal limitations Mono, Mono by default
does not use typed allocations on multi-appDomain applications as they could
leak memory when a domain is unloaded. Although this is a fine default, for
applications that use more than on AppDomain heavily (for example, ASP.NET
applications) it is worth trading off the small leaks for the increased
performance (additionally, since ASP.NET applications are not likely going
to unload the application domains on production systems, it is worth using
this feature).

Instructs the runtime to try to use a generic runtime-invoke wrapper instead
of creating one invoke wrapper.

gdb Equivalent to setting the MONO_XDEBUG variable, this emits symbols into a
shared library as the code is JITed that can be loaded into GDB to inspect

Automatically generates sequence points where the IL stack is empty. These
are places where the debugger can set a breakpoint.

Makes the JIT generate an explicit NULL check on variable dereferences
instead of depending on the operating system to raise a SIGSEGV or another
form of trap event when an invalid memory location is accessed.

Captures the interrupt signal (Control-C) and displays a stack trace when
pressed. Useful to find out where the program is executing at a given
point. This only displays the stack trace of a single thread.

Instructs the runtime to initialize the stack with some known values (0x2a
on x86-64) at the start of a method to assist in debuggin the JIT engine.

This option will leak delegate trampolines that are no longer referenced as
to present the user with more information about a delegate misuse.
Basically a delegate instance might be created, passed to unmanaged code,
and no references kept in managed code, which will garbage collect the code.
With this option it is possible to track down the source of the problems.

This option will cause mono to abort with a descriptive message when during
stack unwinding after an exception it reaches a native stack frame. This
happens when a managed delegate is passed to native code, and the managed
delegate throws an exception. Mono will normally try to unwind the stack to
the first (managed) exception handler, and it will skip any native stack
frames in the process. This leads to undefined behaviour (since mono doesn't
know how to process native frames), leaks, and possibly crashes too.

This option will disable the GDB backtrace emitted by the runtime after a
SIGSEGV or SIGABRT in unmanaged code.

This option will suspend the program when a native SIGSEGV is received.
This is useful for debugging crashes which do not happen under gdb, since a
live process contains more information than a core file.

This option causes the runtime to check for calling convention mismatches
when using pinvoke, i.e. mixing cdecl/stdcall. It only works on windows. If
a mismatch is detected, an ExecutionEngineException is thrown.

The logging level, possible values are `error', `critical', `warning', `message',
`info' and `debug'. See the DEBUGGING section for more details.

Controls the domain of the Mono runtime that logging will apply to. If set, the
log mask is changed to the set value. Possible values are "asm" (assembly loader),
"type", "dll" (native library loader), "gc" (garbage collector), "cfg" (config file
loader), "aot" (precompiler), "security" (e.g. Moonlight CoreCLR support) and
"all". The default value is "all". Changing the mask value allows you to display
only messages for a certain component. You can use multiple masks by comma
separating them. For example to see config file messages and assembly loader
messages set you mask to "asm,cfg".

Used for runtime tracing of method calls. The format of the comma separated trace
options is:

[-]M:method name
[-]T:class name
disabled Trace output off upon start.

You can toggle trace output on/off sending a SIGUSR2 signal to the program.

If set, enables the System.Diagnostics.DefaultTraceListener, which will print the
output of the System.Diagnostics Trace and Debug classes. It can be set to a
filename, and to Console.Out or Console.Error to display output to standard output
or standard error, respectively. If it's set to Console.Out or Console.Error you
can append an optional prefix that will be used when writing messages like this:
Console.Error:MyProgramName. See the System.Diagnostics.DefaultTraceListener
documentation for more information.

This eases WCF diagnostics functionality by simply outputs all log messages from
WCF engine to "stdout", "stderr" or any file passed to this environment variable.
The log format is the same as usual diagnostic output.

This throws an exception when a X11 error is encountered; by default a message is
displayed but execution continues

Set this value to 1 to prevent the serializer from removing the temporary files
that are created for fast serialization; This might be useful when debugging.

This is used in the System.Windows.Forms implementation when running with the X11
backend. This is used to debug problems in Windows.Forms as it forces all of the
commands send to X11 server to be done synchronously. The default mode of
operation is asynchronous which makes it hard to isolate the root of certain

This environment variable controls the kind of generic sharing used. This variable
is used by internal JIT developers and should not be changed in production. Do not
use it. The variable controls which classes will have generic code sharing
enabled. Permissible values are:

all All generated code can be shared.

Only the classes in System.Collections.Generic will have its code shared
(this is the default value).

corlib Only code in corlib will have its code shared.

none No generic code sharing will be performed.
Generic code sharing by default only applies to collections. The Mono JIT by default
turns this on.

When the the MONO_XDEBUG env var is set, debugging info for JITted code is emitted
into a shared library, loadable into gdb. This enables, for example, to see managed
frame names on gdb backtraces.

Enables the maximum JIT verbosity for the specified method. This is very helpfull
to diagnose a miscompilation problems of a specific method.

If set, makes the JIT output information about detected CPU features (such as SSE,
CMOV, FCMOV, etc) to stdout.

If set, the JIT will not perform any hardware capability detection. This may be
useful to pinpoint the cause of JIT issues. This is the default when Mono is built
as an AOT cross compiler, so that the generated code will run on most hardware.


If you want to use Valgrind, you will find the file `mono.supp' useful, it contains the
suppressions for the GC which trigger incorrect warnings. Use it like this:
valgrind --suppressions=mono.supp mono ...


On some platforms, Mono can expose a set of DTrace probes (also known as user-land
statically defined, USDT Probes).

They are defined in the file `mono.d'.

ves-init-begin, ves-init-end
Begin and end of runtime initialization.

method-compile-begin, method-compile-end
Begin and end of method compilation. The probe arguments are class name, method
name and signature, and in case of method-compile-end success or failure of

gc-begin, gc-end
Begin and end of Garbage Collection.

To verify the availability of the probes, run:
dtrace -P mono'$target' -l -c mono


Mono's Ping implementation for detecting network reachability can create the ICMP packets
itself without requiring the system ping command to do the work. If you want to enable
this on Linux for non-root users, you need to give the Mono binary special permissions.

As root, run this command:
# setcap cap_net_raw=+ep /usr/bin/mono

Use mono online using onworks.net services

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