Mono(Mono 3.0) Mono(Mono 3.0)
mono - Mono's ECMA-CLI native code generator (Just-in-Time and Ahead-
mono [options] file [arguments...]
mono-sgen [options] file [arguments...]
mono is a runtime implementation of the ECMA Common Language Infra‐
structure. 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 dynami‐
cally 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 argu‐
ments 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 debug‐
ging 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 Win‐
dows-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:
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 inde‐
pendent code. Pre compilation is just a mechanism to reduce
startup time, increase code sharing across multiple mono pro‐
cesses 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-gen‐
erated 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 com‐
mas, and more than one can be specified:
The AOT compiler will emit a (ELF only) library initial‐
izer 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 file.
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 precom‐
piled 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
Instructs the AOT compiler to emit DWARF debugging infor‐
mation. 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 addi‐
tional method trampolines with this argument. Defaults
Instructs the AOT compiler to save the output to the
If the AOT compiler cannot compile a method for any rea‐
son, 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 applica‐
tions even on systems where it is not possible to set
breakpoints or to single step (certain hardware configu‐
rations 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;
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 informa‐
For more information about AOT, see: http://www.mono-
Currently the only option supported by this command line argu‐
ment is disable which disables the attach functionality.
Load the specified configuration file instead of the default
one(s). The default files are /etc/mono/config and ~/.mono/con‐
fig or the file specified in the MONO_CONFIG environment vari‐
able, 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 inter‐
face 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
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 use‐
ful 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 sus‐
pend the vm on startup until it connects successfully to
a debugger front end. If you set it to 'n', in conjunc‐
tion with server=y, it instructs the Mono runtime to run
as normal, while caching metadata to send to the debugger
front end on connection..
This is used to specify the transport that the debugger
will use to communicate. It must be specified and cur‐
rently requires this to be 'dt_socket'.
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.
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 run‐
Displays usage instructions.
--llvm If the Mono runtime has been compiled with LLVM support (not
available in all configurations), Mono will use the LLVM opti‐
mization and code generation engine to JIT or AOT compile. For
more information, consult: http://www.mono-project.com/Mono_LLVM
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
MODE is a comma separated list of optimizations. They also
allow optimizations to be turned off by prefixing the optimiza‐
tion 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 bene‐
fits of using them. The following optimizations are imple‐
all Turn on all optimizations
peephole Peephole postpass
branch Branch optimizations
inline Inline method calls
cfold Constant folding
consprop Constant propagation
copyprop Copy propagation
deadce Dead code elimination
linears Linear scan global reg allocation
cmov Conditional moves [arch-dependency]
shared Emit per-domain code
sched Instruction scheduling
intrins Intrinsic method implementations
tailc Tail recursion and tail calls
loop Loop related optimizations
fcmov Fast x86 FP compares [arch-dependency]
leaf Leaf procedures optimizations
aot Usage of Ahead Of Time compiled code
precomp Precompile all methods before executing Main
abcrem Array bound checks removal
ssapre SSA based Partial Redundancy Elimination
sse2 SSE2 instructions on x86 [arch-dependency]
gshared Enable generic code sharing.
For example, to enable all the optimization but dead code elimi‐
nation 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 com‐
pilation (--aot flag) would produce pre-compiled code that will
depend on the current CPU and might not be safely moved to
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 ver‐
sion 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
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" parameter.
The following modes are supported:
cas This allows mono to support declarative security
attributes, e.g. execution of Code Access Security (CAS)
or non-CAS demands.
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/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
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 run‐
The security system acts on user code: code contained in mscor‐
lib or the global assembly cache is always trusted.
Configures the virtual machine to be better suited for server
operations (currently, allows a heavier threadpool initializa‐
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.
Prints JIT version information (system configuration, release
number and branch names if available).
The following options are used to help when developing a JITed applica‐
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 InvalidCastEx‐
ception. This option needs to be enabled as this gener‐
ates more verbose code at execution time.
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
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 appli‐
cations and code coverage see the sections "PROFILING" and "CODE
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 `pro‐
gram', `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 run‐
time. 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 notation:
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 names‐
pace, and to trace all exceptions, specify 'all' as the type
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 Sys‐
tem.String except for the System.String:Concat method.
You can trace managed to unmanaged transitions using the wrapper
mono --trace=wrapper app.exe
Finally, namespaces can be specified using the N: prefix:
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 intruc‐
tion 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 has:
HEXADDR HEXSIZE methodname
Currently this option is only supported on Linux.
JIT MAINTAINER OPTIONS
The maintainer options are only used by those developing the runtime
itself, and not typically of interest to runtime users or developers.
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 "System.Collec‐
Inserts a breakpoint on exceptions. This allows you to debug
your application with a native debugger when an exception is
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
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 Ghost‐
view to be called "gv"). The following graphs are available:
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
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 genera‐
Displays information about the work done by the runtime during
the execution of an application.
Perform maintenance of the process shared data. semdel will
delete the global semaphore. hps will list the currently used
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 selec‐
tor 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
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 invocation.
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 vari‐
ous 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 profil‐
ers, see the section "CUSTOM PROFILERS" for more details.
If a profiler is not specified, the default profiler is used. The pro‐
filer_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 exam‐
mono --profile program.exe
That will run the program with the default profiler and will do time
and allocation profiling.
mono --profile=default:stat,alloc,file=prof.out program.exe
Will do sample statistical profiling and allocation profiling on pro‐
gram.exe. The profile data is put in prof.out. Note that the statisti‐
cal profiler has a very low overhead and should be the preferred pro‐
filer 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
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
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 path.
A list of other third party profilers is available from Mono's web site
Custom profiles are written as shared libraries. The shared library
must be called `mono-profiler-NAME.so' where `NAME' is the name of your
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: --pro‐
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 fol‐
lowing command line limits the code coverage to routines in the "demo"
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
mono --profile=cov:demo/My.Utilities demo.exe
Which will only perform code coverage in the given assembly and names‐
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 ()
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
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 com‐
mand 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
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 guaran‐
teed and the application might crash or terminate at any given point
The --debug=casts option can be used to get more detailed information
for Invalid Cast operations, it will provide information about the
You can use the MONO_LOG_LEVEL and MONO_LOG_MASK environment variables
to get verbose debugging output about the execution of your application
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
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 application.
You can control this with the MONO_XMLSERIALIZER_THS environment vari‐
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 seri‐
alizer 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 purposes
When Mono is compiled with LLVM support, this instructs the run‐
time 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 com‐
pile new assemblies on demand and store the result into a 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 $pre‐
fix/etc/mono/2.0/settings.map file and, optionally, in the set‐
tings.map file found in the top-level directory of your ASP.NET
application. Both files are read by System.Web on application
startup, if they are found at the above locations. If you don't
want the mapping to be performed you can set this variable in
your environment before starting the application and no action
will be taken.
Mono has a cache of ConfigSection objects for speeding up Web‐
ConfigurationManager queries. Its default size is 100 items, and
when more items are needed, cache evictions start happening. If
evictions are too frequent this could impose unnecessary over‐
head, which could be avoided by using this environment variable
to set up a higher cache size (or to lower memory requirements
by decreasing it).
If set, causes Mono.Cairo to collect stack traces when objects
are allocated, so that the finalization/Dispose warnings include
information about the instance's origin.
If set, this variable overrides the default system configuration
directory ($PREFIX/etc). It's used to locate machine.config
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 configura‐
tion 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:
where V is the architecture number 4, 5, 6, 7 and the options
can be currently be "thunb". Example:
MONO_CPU_ARCH="armv4 thumb" mono ...
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 supported.
If this environment variable is `yes', the runtime uses unman‐
aged collation (which actually means no culture-sensitive colla‐
tion). 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 parallel.
Note that /etc/nsswitch.conf will be ignored.
For platforms that do not otherwise have a way of obtaining ran‐
dom bytes this can be set to the name of a file system socket on
which an egd or prngd daemon is listening.
Unix only: Enable support for cross-process handles. Cross-
process handles are used to expose process handles, thread han‐
dles, named mutexes, named events and named semaphores across
This environment variable allows you to pass command line argu‐
ments to a Mono process through the environment. This is use‐
ful 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.Diagnos‐
tics.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\vent‐
log" 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 per‐
mission for individual event log and event source direc‐
tories 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 argu‐
ments 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
When reading externally-generated text strings UTF-8 is tried
first, and then this list is tried in order with the first suc‐
cessful 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 words.
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 giga‐
bytes, 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.
Specifies which major collector to use. Options are
`marksweep' for the Mark&Sweep collector, `marksweep-
conc' for concurrent Mark&Sweep, `marksweep-par' for par‐
allel Mark&Sweep, `marksweep-fixed' for Mark&Sweep with a
fixed heap, and `marksweep-fixed-par' for parallel
Mark&Sweep with a fixed heap. The Mark&Sweep collector is
Sets the size of the major heap (not including the large
object space) for the fixed-heap Mark&Sweep collector
(i.e. `marksweep-fixed' and `marksweep-fixed-par'). The
size is in bytes, with optional suffixes `k', `m' and `g'
to specify kilo-, mega- and gigabytes, respectively. The
default is 512 megabytes.
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 collec‐
tion 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 collec‐
tor. If enabled, the sweep phase of the garbage collec‐
tion is done piecemeal whenever the need arises, typi‐
cally during nursery collections. Lazy 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 ref‐
erences 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,
Specifies the target save ratio for the major collector.
The collector lets a given amount of memory to be pro‐
moted 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 cal‐
culated 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 nurs‐
ery 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 EXPERIMEN‐
TAL, so it might disappear in later versions of mono.
Specifies which minor collector to use. Options are 'sim‐
ple' 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 collec‐
tor. Valid values are integers between 1 and 14. Default
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 production.
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 collec‐
tion 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 collectors.
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 noth‐
ing when the split nursery collector is used.
Performs a check to make sure that no references are left
to an unloaded AppDomain.
This clears the nursery at GC time instead of doing it
when the thread local allocation buffer (TLAB) is cre‐
ated. The default is to clear the nursery at TLAB cre‐
Don't do minor collections. If the nursery is full, a
major collection is triggered instead, unless it, too, is
Don't do major collections.
Forces the GC to scan the stack conservatively, even if
precise scanning is available.
Disables the managed allocator.
If set, does a plausibility check on the scan_starts
before and after each collection
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 visu‐
alize 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
Provides a prefix the runtime uses to look for Global Assembly
Caches. Directories are separated by the platform path separa‐
tor (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: /home/user‐
Enables some filename rewriting support to assist badly-written
applications that hard-code Windows paths. Set to a colon-sepa‐
rated 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 ana‐
lyzes 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 allo‐
cated 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
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, Sys‐
tem.IO.FileSystemWatcher will use the default managed implemen‐
tation (slow). If unset, mono will try to use inotify, FAM,
Gamin, kevent under Unix systems and native API calls on Win‐
dows, falling back to the managed implementation on error.
Mono supports a plugin model for its implementation of Sys‐
tem.Messaging making it possible to support a variety of messag‐
ing 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:
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
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
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 network.
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 sys‐
tems. The default is 180 seconds.
The maximum number of threads in the general threadpool will be
20 + (MONO_THREADS_PER_CPU * number of CPUs). The default value
for this variable is 10.
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).
ENVIRONMENT VARIABLES FOR DEBUGGING
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 debug‐
ging. This variable should contain a comma separated list of
debugging options. Currently, the following options are sup‐
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 debug‐
ging 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 symbols.
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 dis‐
plays 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 infor‐
mation 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 dele‐
gate throws an exception. Mono will normally try to
unwind the stack to the first (managed) exception han‐
dler, 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.
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 mes‐
sages set you mask to "asm,cfg".
Used for runtime tracing of method calls. The format of the
comma separated trace options is:
disabled Trace output off upon start.
You can toggle trace output on/off sending a SIGUSR2 signal to
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 stan‐
dard error, respectively. If it's set to Console.Out or Con‐
sole.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 cer‐
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 vari‐
able 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 JIT‐
ted code is emitted into a shared library, loadable into gdb.
This enables, for example, to see managed frame names on gdb
Enables the maximum JIT verbosity for the specified method. This
is very helpfull to diagnose a miscompilation problems of a spe‐
If set, makes the JIT output information about detected CPU fea‐
tures (such as SSE, CMOV, FCMOV, etc) to stdout.
If you want to use Valgrind, you will find the file `mono.supp' useful,
it contains the suppressions for the GC which trigger incorrect warn‐
ings. 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'.
Begin and end of runtime initialization.
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 compilation.
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 cre‐
ate 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
On Unix assemblies are loaded from the installation lib directory. If
you set `prefix' to /usr, the assemblies will be located in /usr/lib.
On Windows, the assemblies are loaded from the directory where mono and
The directory for the ahead-of-time compiler demand creation
assemblies are located.
Mono runtime configuration file. See the mono-config(5) manual
page for more information.
Contains Mono certificate stores for users / machine. See the
certmgr(1) manual page for more information on managing certifi‐
cate stores and the mozroots(1) page for information on how to
import the Mozilla root certificates into the Mono certificate
Files in this directory allow a user to customize the configura‐
tion for a given system assembly, the format is the one
described in the mono-config(5) page.
Contains Mono cryptographic keypairs for users / machine. They
can be accessed by using a CspParameters object with DSACryp‐
toServiceProvider and RSACryptoServiceProvider classes.
Contains Mono isolated storage for non-roaming users, roaming
users and local machine. Isolated storage can be accessed using
the classes from the System.IO.IsolatedStorage namespace.
Configuration information for individual assemblies is loaded by
the runtime from side-by-side files with the .config files, see
the http://www.mono-project.com/Config for more information.
ASP.NET applications are configured through these files, the
configuration is done on a per-directory basis. For more infor‐
mation on this subject see the http://www.mono-project.com/Con‐
Mailing lists are listed at the http://www.mono-project.com/Mail‐
SEE ALSOcertmgr(1), csharp(1), mcs(1), mdb(1), monocov(1), monodis(1), mono-
config(5), mozroots(1), mprof-report(1), pdb2mdb(1), xsp(1),
For more information on AOT: http://www.mono-project.com/AOT
For ASP.NET-related documentation, see the xsp(1) manual page