Mono(Mono 2.5) Mono(Mono 2.5)
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 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 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 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 collec-
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:
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 com-
pilation 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 multi-
ple 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 deploy-
ment. 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:
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.
Instructs the AOT compiler to save the output to the specified file.
Instructs the AOT compiler to emit debug symbol information.
Instructs the AOT compiler to keep temporary files.
This is an experimental option for the AOT compiler to use multiple threads when compiling the methods.
Instructs the AOT compiler to not output any debugging information.
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.
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 IMT trampolines must be precreated in the AOT image. You can add additional method
trampolines with this argument. Defaults to 128.
If the AOT compiler cannot compile a method for any reason, enabling this flag will output the skipped methods to the con-
The AOT compiler will emit a (ELF only) library initializer to automatically register the aot compiled module with the run-
time. This is only useful in static mode
Instructs the AOT compiler to output assembly code instead of an object file.
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) 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;
For more information about AOT, see: http://www.mono-project.com/AOT
Currently the only option supported by this command line argument 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/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 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:
This is used to specify the transport that the debugger will use to communicate. It must be specified and currently
requires this to be 'dt_socket'.
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 debug-
ger front end to connect to the Mono process. Mono will print out to stdout the IP address and port where it is listening.
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 pro-
cesses to use one particular kind of GC with the Mono runtime.
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-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 back-
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 set-
ting, you might want to actually measure the benefits of using them. The following optimizations are implemented:
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 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 compila-
tion (--aot flag) would produce pre-compiled code that will depend on the current CPU and might not be safely moved to another com-
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
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
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 Mono.
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 verifica-
tion 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, a no-op).
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 assem-
blies 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 application.
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 ver-
bose 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 mdb.
gdb Generate and register debugging information with gdb. This is only supported on some platforms, and only when using gdb 7.0
Turns on profiling. For more information about profiling applications and code coverage see the sections "PROFILING" and "CODE COV-
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 meth-
ods, 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 pro-
gram 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 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 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.
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 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 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.
Inserts a breakpoint on exceptions. This allows you to debug your application with a native debugger when an exception is thrown.
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 genera-
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 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 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 application.
Perform maintenance of the process shared data. semdel will delete the global semaphore. hps will list the currently used handles.
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 cre-
ate 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 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 pro-
filers, 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 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 program.exe. The profile data is put in prof.out. Note that the statis-
tical 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) page.
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 path.
A list of other third party profilers is available from Mono's web site (www.mono-project.com/Performance_Tips)
Custom profiles are written as shared libraries. The shared library must be called `mono-profiler-NAME.so' where `NAME' is the name of
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 per-
form code coverage in the routines of your program use, for example the following command line limits the code coverage to routines in the
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 ()
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 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 guaranteed 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 applica-
tion 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 mes-
sages 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 parame-
ters 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 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 purposes
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 alto-
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 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 $prefix/etc/mono/2.0/settings.map file and, optionally, in the settings.map file
found in the top-level directory of your ASP.NET application. Both files are read by System.Web on application startup, if they are
found at the above locations. If you don't want the mapping to be performed you can set this variable in your environment before
starting the application and no action will be taken.
If set, this variable overrides the default system configuration directory ($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:
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 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.
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 listening.
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.
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/event-
log" on unix and "%APPDATA%noventlog" on Windows.
win32 Uses the native win32 API to write events and registers event logs and event sources in the registry. This is only avail-
able 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
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 file-
names 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 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.
Specifies which major collector to use. Options are `marksweep' for the Mark&Sweep collector, `marksweep-par' for parallel
Mark&Sweep, `marksweep-fixed' for Mark&Sweep with a fixed heap, `marksweep-fixed-par' for parallel Mark&Sweep with a fixed
heap and `copying' for the copying collector. The Mark&Sweep collector is the default.
Sets the size of the major heap (not including the large object space) for the fixed-heap Mark&Sweep collector (i.e. `mark-
sweep-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.
Specifies which write barrier to use. Options are `cardtable' and `remset'. The card table barrier is faster but less pre-
cise, and only supported for the Mark&Sweep major collector on 32 bit platforms. The default is `cardtable' if it is sup-
ported, otherwise `remset'. The cardtable write barrier is faster and has a more stable and usually smaller memory footprint.
If the program causes too much pinning during thread scan, it might be faster to enable remset.
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 concurrent sweep for the Mark&Sweep collector. If enabled, the sweep phase of the garbage collection is
done in a thread concurrently with the application. Concurrent sweep is disabled by default.
Specifies how application threads should be scanned. Options are `precise` and `conservative`. Precise marking allow the col-
lector 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 alloca-
tion rate, it has the downside of requiring a significant extra memory per compiled method. The right option, unfortunately,
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.
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 refer-
ences that are not on the remembered sets.
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 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 available.
If set, does a plausibility check on the scan_starts before and after each collection
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
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 gacu-
til /gacdir command. Example: /home/username/.mono:/usr/local/mono/
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 loca-
tions where all the IOMAP-ed strings were created in managed code. The latter report is only approximate as it is not always possi-
ble 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
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 imple-
mentation on error.
Mono supports a plugin model for its implementation of System.Messaging making it possible to support a variety of messaging imple-
mentations (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
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
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 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 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
Collects information about pagefaults. This is used internally to track the number of page faults produced to load meta-
data. 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 break-
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 excep-
tion. 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.
The logging level, possible values are `error', `critical', `warning', `message', `info' and `debug'. See the DEBUGGING section for
Controls the domain of the Mono runtime that logging will apply to. If set, the log mask is changed to the set value. Possible val-
ues 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:
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 Win-
dows.Forms as it forces all of the commands send to X11 server to be done synchronously. The default mode of operation is asyn-
chronous which makes it hard to isolate the root of certain problems.
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. Permis-
sible 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 spe-
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'.
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 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
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 mint live.
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 certificate
stores and the mozroots(1) page for information on how to import the Mozilla root certificates into the Mono certificate store.
Files in this directory allow a user to customize the configuration 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 DSACryptoServi-
ceProvider and RSACryptoServiceProvider classes.
~/.config/.isolatedstorage, ~/.local/share/.isolatedstorage, /usr/share/.isolatedstorage
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 information
on this subject see the http://www.mono-project.com/Config_system.web page.
Mailing lists are listed at the http://www.mono-project.com/Mailing_Lists
certmgr(1), csharp(1), mcs(1), mdb(1), monocov(1), monodis(1), mono-config(5), mozroots(1), pdb2mdb(1), xsp(1), mod_mono(8).
For more information on AOT: http://www.mono-project.com/AOT
For ASP.NET-related documentation, see the xsp(1) manual page