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


mono [options] file [arguments...]


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

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

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

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

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


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

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

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


The following options are available:
--aot, --aot[=options]
 This option is used to precompile the CIL code in the specified assembly to native code. The generated code is stored in a file with the extension .so. This file will be automatically picked up by the runtime when the assembly is executed.
.Sp 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.
.Sp 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.
.Sp 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.
.Sp Pre compilation is just a mechanism to reduce startup time, increase code sharing across multiple mono processes and avoid just-in-time compilation program startup costs. The original assembly must still be present, as the metadata is contained there.
.Sp AOT code typically can not be moved from one computer to another (CPU-specific optimizations that are detected at runtime) so you should not try to move the pre-generated assemblies or package the pre-generated assemblies for deployment.
.Sp 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:
.Sp 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 emit debug symbol information.
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;

mono_aot_register_module (mono_aot_module_hello_info);

For more information about AOT, see:
 Currently the only option supported by this command line argument is disable which disables the attach functionality.
 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.
.Sp 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.
--config filename
 Load the specified configuration file instead of the default one(s). The default files are /etc/mono/config and ~/.mono/config or the file specified in the MONO_CONFIG environment variable, if set. See the mono-config(5) man page for details on the format of this file.
 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.
--help, -h
 Displays usage instructions.
--optimize=MODE, -O=MODE
 MODE is a comma separated list of optimizations. They also allow optimizations to be turned off by prefixing the optimization name with a minus sign.
.Sp In general, Mono has been tuned to use the default set of flags, before using these flags for a deployment setting, you might want to actually measure the benefits of using them.
.Sp 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.
.Sp For example, to enable all the optimization but dead code elimination and inlining, you can use:         -O=all,-deadce,-inline
.Sp The flags that are flagged with [arch-dependency] indicate that the given option if used in combination with Ahead of Time compilation (--aot flag) would produce pre-compiled code that will depend on the current CPU and might not be safely moved to another computer.
 Mono supports different runtime versions. The version used depends on the program that is being run or on its configuration file (named program.exe.config). This option can be used to override such autodetection, by forcing a different runtime version to be used. Note that this should only be used to select a later compatible runtime version than the one the program was compiled against. A typical usage is for running a 1.1 program on a 2.0 version: mono --runtime=v2.0.50727 program.exe
--security, --security=mode
 Activate the security manager, a currently experimental feature in Mono and it is OFF by default. The new code verifier can be enabled with this option as well.

Using security without parameters is equivalent as calling it with the "cas" 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 for more details and links to the descriptions of this new system.
 Enables the new verifier and performs basic verification for code validity. In this mode, unsafe code and P/Invoke are allowed. This mode provides a better safety guarantee but it is still possible for managed code to crash 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 verification is not as strict as ECMA 335 standard in order to stay compatible with the MS runtime.
The security system acts on user code: code contained in mscorlib or the global assembly cache is always trusted.

 Configures the virtual machine to be better suited for server operations (currently, 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 assemblies located on the global assembly cache.

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


The following options are used to help when developing a JITed application.
--debug, --debug=OPTIONS
 Turns on the debugging mode in the runtime. If an assembly was compiled with debugging information, it will produce line number information for stack traces.

The optional OPTIONS argument is a comma separated list of debugging options. These options are turned off by default since they generate much larger and slower code at runtime.
The following options are supported:
 Produces a detailed error when throwing a InvalidCastException. This option needs to be enabled as this generates 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 mdb.

 Turns on profiling. For more information about profiling applications and code coverage see the sections "PROFILING" and "CODE COVERAGE" below.
 Shows method names as they are invoked. By default all methods are traced.
.Sp The trace can be customized to include or exclude methods, classes or assemblies. A trace expression is a comma separated list of targets, each target can be prefixed with a minus sign to turn off a particular target. The words ‘program’, ‘all’ and ‘disabled’ have special meaning. ‘program’ refers to the main program being executed, and ‘all’ means all the method calls.
.Sp 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.
.Sp 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

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

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

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

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

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

        mono --trace=N:System.Xml

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


The maintainer options are only used by those developing the runtime itself, and not typically of interest to runtime users or developers.
--break method
 Inserts a breakpoint before the method whose name is ‘method’ (namespace.class:methodname). Use ‘Main’ as method name to insert a breakpoint on the application’s main method.
 Inserts a breakpoint on exceptions. This allows you to debug your application with a native debugger when an exception is thrown.
--compile name
 This compiles a method (, this is used for testing the compiler performance or to examine the output of the code generator.
 Compiles all the methods in an assembly. This is used to test the compiler performance or to examine the output of the code generator
 This generates a postscript file with a graph with the details about the specified method ( This requires ‘dot’ and ghostview to be installed (it expects Ghostview to be called "gv").
.Sp 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
.Sp 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.
.Sp semdel will delete the global semaphore.
.Sp hps will list the currently used handles.
-v, --verbose
 Increases the verbosity level, each time it is listed, increases the verbosity level to include more information (including, for example, a disassembly of the native code produced, code selector info etc.).


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

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

The code is loaded into the root domain, and it starts execution on the special runtime attach thread. The attached program should create its own threads and return after 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 profilers, see the section "CUSTOM PROFILERS" for more details.

If a profiler is not specified, the default profiler is used.
.Sp The profiler_args is a profiler-specific string of options for the profiler itself.
.Sp 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.
.Sp For example:

        mono --profile program.exe

.Sp 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.
.Sp Note that the statistical profiler has a very low overhead and should be the preferred profiler to use (for better output use the full path to the mono binary when running and make sure you have installed the addr2line utility that comes from the binutils package).


The logging profiler will eventually replace the default profiler as it is more complete and encompasses the functionality of all the other profilers for Mono. It supports the following execution modes:
Statistical: the program instruction pointer is periodically sampled (it works also with unmanaged functions). If call chains are requested, for each sample the profiler gets a partial stack trace (up to a desired depth) so that caller-callee information is available.
Instrumenting: each method enter and exit is logged with a timestamp; further processing of the data can show the methods that took the longer to execute, with complete accounting for callers and callees. However, this way of profiling is rather intrusive and slows down the application significantly.
Allocation: each allocation is logged.
Allocation summary: shows, for each collection, a summary of the heap contents broken down by class (for each class the number of allocated and released objects is given, together with their aggregated size in bytes).
Heap snapshot mode: dumps the whole heap contents at every collection (or at user specified collections). It is also possible to request a collection and snapshot dump with a signal.
Moreover, other events can be logged and analyzed, like jit time for each method, load and unload for assemblies, modules and and individual classes, and appdomain and thread creation and destruction.
Instead of reporting the collected information at the end of the execution of the program, this profiler logs all the events periodically into a file during program execution. To minimize the performance impact with multi-threaded applications, the logging uses per-thread buffers that are routinely saved to disk.
The output file contains compressed events, to process the data you should use tools like the "Mono.Profiler" tool provided on the Mono SVN repository.
This profiler is activated passing the --profile=logging option to the mono runtime, and is controlled attaching further options, like --profile=logging:statistical for doing statistical profiling (multiple options are separated by commas).
As a quick primer, here are a few examples of the most common usage modes:
To write the resulting data to "mydata.mprof" (defaults to statistical profiling):

        mono --profile=logging:output=mydata.mprof program.exe

To perform statistical profiling, inspecting call chains up to depth 8:

        mono --profile=logging:statistical=8 program.exe

To profile allocations (by default the call stack will be analized for each allocation, producing detailed caller method attribution infornation):

        mono --profile=logging:allocations program.exe

To profile garbage collection activity at a high level (collection time and objects freed at each collection for each class are reported, but heap snapshots are not saved to disk):

        mono --profile=logging:allocations-summary program.exe

To perform heap profiling taking heap snapshots:

        mono --profile=logging:heap=all program.exe         mono --profile=logging:heap=<signal> program.exe

If "all" is specified, a heap snapshot is taken at each collection. If, instead, a signal name or number is given (one of SIGUSR1, SIGUSR2 and SIGPROF, remembering that SIGPROF is not compatible with the statistical profiler), a garbage collection is forced and a snapshot is taken every time the signal is received. Heap profiling also enables full allocation profiling (with call stacks), and each allocation can be related to its corresponding object in the snapshots, enabling investigations like "find all objects of a given class allocated by a given method and still live at a given collection, and then find all objects referencing them".

Then you would need to invoke the decoder mprof-decoder(1) on the output file to see the profiling results, or to examine heap snapshots and allocations in detail mprof-heap-viewer(1).

The operating modes described above are the default ones, and are sufficient to use the profiler.
To further customize the profiler behavior there are more options, described below. These options can be individually enabled and disabled prefixing them with an (optional) ’+’ character or a ’-’ character. For instance, the "allocations" option by default records also the full call stack at each allocation. If only the caller is wanted, one should use "allocations,-save-allocation-stack", or to disable call tracking completely (making the profiler less intrusive) "allocations,-save-allocation-caller,-save-allocation-stack". In practice the "allocation" option by default behaves like "allocations,save-allocation-caller,save-allocation-stack", but the user can tune this to his needs.
These are all the available options, organized by category:
Execution profiling modes
statistical, stat or s
 Performs statistical profiling. This is a lightweight profiling mechanism and it has a much lower overhead than the enter-leave profiling as it works by sampling where the program is spending its time by using a timer. If specified with s=<number>, also inspect call chains up to level <number>.
enter-leave, calls or c
 Measure the time spent inside each method call, this is done by logging the time when a method enters and when the method leaves. This can be a time consuming operation.
jit, j Collect information about time spent by the JIT engine compiling methods.
Allocation profiling modes
allocations, alloc or a
 Collect information about each allocation (object class and size). By default this also implies "+save-allocation-caller" and "+save-allocation-stack".
save-allocation-caller, sac
 Save the direct caller of each allocation. The profiler filters out wrapper methods, and also recognizes if the allocation has been performed by the runtime while jitting a method.
save-allocation-stack, sas
 Save the full managed execution stack at each allocation. While the "sac" option saves the direct caller, this one records the whole stack trace. Note that in the call stack the wrapper methods are not filtered out. Anyway the "sac" and "sas" options can be combined, and the decoder will attribute the allocation to the correct method even if the wrapper is at the top of the stack trace.
allocations-summary or as
 At each collection dump a summary of the heap contents (for each class, the number and collective size of all live and freed heap objects). This very lightweight compared to full heap snapshots.
unreachable, free or f
 Performs a lightweight profile of the garbage collector. On each collection performed by the GC, the list of unreachable objects is recorded, and for each object the class and size is provided. This information can be used to compute the heap size broken down by class (combined with "a" can give the same information of "as", but the log file contains info about each individual object, while in "as" the processing is done directly at runtime and the log file contains only the summarized data broken down by class).
gc or g Measure the time spent in each collection, and also trace heap resizes.
heap-shot, heap or h
 Performs full heap profiling. In this case on each collection a full heap snapshot is recorded to disk. Inside the snapshots, each object reference is still represented so that it’s possible to investigate who is responsible for keeping objects alive.
The actual production of heap snapshots could produce large log files, so it can be controlled in three ways:
gc-dumps=N, gc-d=N, gcd=N
 states the number of snapshots that must be dumped (since the application starts). Zero means no dumps at all, -1 means dump at all collections.
gc-signal=<signal>, gc-s or gcs
 (where <signal> is one of "SIGUSR1", "SIGUSR2", or "SIGPROF") specifies a signal that will immediately trigger a collection and a dump.
gc-commands=FILE, gc-c=FILE or gcc=FILE
 specify a "command file". The file must contain an integer value in ASCII form, and the profiler will stat it at every collection. If it has been modified it will interpret its contents as a gcd=N option value, and dump the required number of snapshots from that moment onwards. If the file is present at application startup it takes precedence over an eventual gcd=N option.
Profiler activity control
output=FILE, out=FILE or o=FILE
 Use this option to provide the output file name for the profile log. If this option is not specified, it will default to "<program-name>.mprof".
output-suffix=SUFFIX, suffix=SUFFIX or os=SUFFIX: makes
 the output file name equals to "<program-name>-SUFFIX.mprof".
start-enabled or se: start with the profiler active
 (which is the default).
start-disabled or sd: start with the profiler inactive.
toggle-signal=<SIGNAL> or ts=<SIGNAL> (where <SIGNAL>
 is one of SIGUSR1, SIGUSR2 or SIGPROF): Choose a signal that will be used to toggle the profiler activity on and off. This way you can decide to profile only portions of the applicatopn lifetime (for instance, you can decide to avoid profiling an initial setup phase using sd, and enable the profiler later delivering the signal to the application).
force-accurate-timer (or fac): the profiler by default uses
 rtdsc to acquire timestamps for frequent events, but this can be imprecise; using this option you force the use of "gettimeofday" at every event, which is more accurate but much slower.
Internal buffer sizes
per-thread-buffer-size=N, tbs=N
 Use to specify the number of events that a thread buffer can hold. When the thread buffer is full, a log block is written to disk.
.Sp This defaults to tbs=10000.
statistical-thread-buffer-size=N, sbs=N
 The number of statistical samples that are held in memory before they are dumped to disk (the system does double-buffering and the statistical samples are written by a helper thread, so the statistical profiler never stops and is able to profile the profiler itself).
.Sp This defaults to sbs=10000.
write-buffer-size, wbs
 Specifies the size in bytes of the internal write buffers.
.Sp This defaults to wbs=1024.
In its current state, this profiler can also perform heap analysis (like heap-shot), and the decoder is already able to read the data, however the user interface for this feature is experimental (the mprof-heap-viewer tool in the mono-tools module).

Another known issue is that if the timer is not strictly monotonic (like rtdsc), differences between times can underflow (they are handled as unsigned integers) and weird numbers can show up in the logs.

Finally, it can happen that when exceptions are thrown the profiler temporarily loses track of the execution stack and misattributes the caller for a few allocations (and method execution time).

More explanations are provided here: "".


There are a number of external profilers that have been developed for Mono, we will update this section to contain the profilers.

The heap Shot profiler can track all live objects, and references to these objects, and includes a GUI tool, this is our recommended profiler. To install you must download the profiler from Mono’s SVN:         svn co svn://         cd heap-shot         ./autogen         make         make install

See the included documentation for details on using it.

The Live Type profiler shows at every GC iteration all of the live objects of a given type. To install you must download the profiler from Mono’s SVN:         svn co svn://         cd heap-prof         ./autogen         make         make install

To use the profiler, execute:         mono --profile=desc-heap program.exe

The output of this profiler looks like this:         Checkpoint at 102 for heap-resize          System.MonoType : 708          System.Threading.Thread : 352          System.String : 3230          System.String[] : 104          Gnome.ModuleInfo : 112          System.Object[] : 160          System.Collections.Hashtable : 96          System.Int32[] : 212          System.Collections.Hashtable+Slot[] : 296          System.Globalization.CultureInfo : 108          System.Globalization.NumberFormatInfo : 144

The first line describes the iteration number for the GC, in this case checkpoint 102.

Then on each line the type is displayed as well as the number of bytes that are being consumed by live instances of this object.

The AOT profiler is used to feed back information to the AOT compiler about how to order code based on the access patterns for pages. To use it, use:         mono --profile=aot program.exe The output of this profile can be fed back into Mono’s AOT compiler to order the functions on the disk to produce precompiled images that have methods in sequential pages.


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 ‘’. 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 ‘’ where ‘NAME’ is the name of your profiler.

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


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

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

        mono --profile=cov:demo demo.exe

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

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

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

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

Typical output looks like this:

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

The offsets displayed are IL offsets.

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


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

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

        kill -QUIT pid

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

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

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

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

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

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

Use the MONO_LOG_MASK environment variable to limit the extent of the messages you get: If set, the log mask is changed to the set value. Possible values are "asm" (assembly loader), "type", "dll" (native library loader), "gc" (garbage collector), "cfg" (config file loader), "aot" (precompiler) 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 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
 If set, this variable will instruct Mono to ahead-of-time compile new assemblies on demand and store the result into a cache in ~/.mono/aot-cache.
 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.
 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
 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.
.Sp 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).
 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.
 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 disable the GDB backtrace emitted by the runtime after a SIGSEGV or SIGABRT in unmanaged code.
 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.
 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.
 Sets the type of event log provider to use (for System.Diagnostics.EventLog).
.Sp Possible values are:
.Sp Persists event logs and entries to the local file system.
.Sp The directory in which to persist the event logs, event sources and entries can be specified as part of the value.
.Sp If the path is not explicitly set, it defaults to "/var/lib/mono/eventlog" on unix and "%APPDATA%\mono\ventlog" on Windows.
.Sp Uses the native win32 API to write events and registers event logs and event sources in the registry. This is only available on Windows.
.Sp On Unix, the directory permission for individual event log and event source directories is set to 777 (with +t bit) allowing everyone to read and write event log entries while only allowing entries to be deleted by the user(s) that created them.
.Sp Silently discards any events.
The default is "null" on Unix (and versions of Windows before NT), and "win32" on Windows NT (and higher).
 If set, contains a colon-separated list of text encodings to try when turning externally-generated text (e.g. command-line arguments or filenames) into Unicode. The encoding names come from the list provided by iconv, and the special case "default_locale" which refers to the current locale’s default encoding.
When reading externally-generated text strings UTF-8 is tried first, and then this list is tried in order with the first successful conversion ending the search. When writing external text (e.g. new filenames or arguments to new processes) the first item in this list is used, or UTF-8 if the environment variable is not set.
The problem with using MONO_EXTERNAL_ENCODINGS to process your files is that it results in a problem: although its possible to get the right file name it is not necessarily possible to open the file. In general if you have problems with encodings in your filenames you should use the "convmv" program.
 Provides a prefix the runtime uses to look for Global Assembly Caches. Directories are separated by the platform path separator (colons on unix). MONO_GAC_PREFIX should point to the top directory of a prefixed install. Or to the directory provided in the gacutil /gacdir command. Example: /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.)
.Sp For example, this would work from the shell:

        MONO_IOMAP=drive:case         export MONO_IOMAP

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

        MonoSetEnv MONO_IOMAP=all

 If set to "disabled", System.IO.FileSystemWatcher will use a file watcher implementation which silently ignores all the watching requests. If set to any other value, System.IO.FileSystemWatcher will use the default managed implementation (slow). If unset, mono will try to use inotify, FAM, Gamin, kevent under Unix systems and native API calls on Windows, falling back to the managed implementation on error.
 If set causes the mono process to be bound to a single processor. This may be useful when debugging or working around race conditions.
 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.
.Sp Directories are separated by the platform path separator (colons on unix). Example: /home/username/lib:/usr/local/mono/lib
.Sp 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.
.Sp For a complete description of recommended practices for application deployment, see the page.
 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

.Sp For example:

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

 Disable inlining of thread local accesses. Try setting this if you get a segfault early on in the execution of mono.
 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.
.Sp 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.
 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.
.Sp This is can also be enabled by default by passing the "--disable-shared-handles" option to configure.
 The name of the theme to be used by Windows.Forms. Available themes today include "clearlooks", "nice" and "win32".
.Sp The default is "win32".
 The time, in seconds, that the SSL/TLS session cache will keep it’s entry to avoid a new negotiation between the client and a server. Negotiation are very CPU intensive so an application-specific custom value may prove useful for small embedded systems.
.Sp 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.
 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.
 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/ file and, optionally, in the 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 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.
 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) and "all". The default value is "all". Changing the mask value allows you to display only messages for a certain component. You can use multiple masks by comma separating them. For example to see config file messages and assembly loader messages set you mask to "asm,cfg".
 Used for runtime tracing of method calls. The format of the comma separated trace options is:

        [-]M:method name         [-]N:namespace         [-]T:class name         [-]all         [-]program         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 throws an exception when a X11 error is encountered; by default a message is displayed but execution continues
 This is used in the System.Windows.Forms implementation when running with the X11 backend. This is used to debug problems in Windows.Forms as it forces all of the commands send to X11 server to be done synchronously. The default mode of operation is asynchronous which makes it hard to isolate the root of certain 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.
.Sp The variable controls which classes will have generic code sharing enabled.
.Sp 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).
 Only code in corlib will have its code shared.
none No generic code sharing will be performed.

.Sp Generic code sharing by default only applies to collections. The Mono JIT by default turns this on.
 When the the MONO_XDEBUG env var is set, debugging info for JITted code is emitted into a shared library, loadable into gdb. This enables, for example, to see managed frame names on gdb backtraces.
 Enables the maximum JIT verbosity for the specified method. This is very helpfull to diagnose a miscompilation problems of a specific method.


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


On some platforms, Mono can expose a set of DTrace probes (also known as user-land statically defined, USDT Probes).
They are defined in the file ‘mono.d’.
ves-init-begin, ves-init-end
.Sp Begin and end of runtime initialization.
method-compile-begin, method-compile-end
.Sp 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.
gc-begin, gc-end
.Sp Begin and end of Garbage Collection.
To verify the availability of the probes, run:
  dtrace -P mono’$target’ -l -c 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.
.Sp The directory for the ahead-of-time compiler demand creation assemblies are located.
/etc/mono/config, ~/.mono/config
.Sp Mono runtime configuration file. See the mono-config(5) manual page for more information.
~/.config/.mono/certs, /usr/share/.mono/certs
.Sp 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.
.Sp 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.
~/.config/.mono/keypairs, /usr/share/.mono/keypairs
.Sp Contains Mono cryptographic keypairs for users / machine. They can be accessed by using a CspParameters object with DSACryptoServiceProvider and RSACryptoServiceProvider classes.
~/.config/.isolatedstorage, ~/.local/share/.isolatedstorage, /usr/share/.isolatedstorage
.Sp 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.
.Sp Configuration information for individual assemblies is loaded by the runtime from side-by-side files with the .config files, see the for more information.
Web.config, web.config
.Sp 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 page.


Mailing lists are listed at the



certmgr(1), csharp(1), mcs(1), mdb(1), monocov(1), monodis(1), mono-config(5), mozroots(1), xsp(1).

For more information on AOT:

For ASP.NET-related documentation, see the xsp(1) manual page

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