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.\" mono manual page.
.\" Copyright 2003 Ximian, Inc.
.\" Copyright 2004-2009 Novell, Inc.
.\" Author:
.\" Miguel de Icaza (
.TH Mono "Mono 2.5"
mono \- Mono's ECMA-CLI native code generator (Just-in-Time and Ahead-of-Time)
.B mono [options] file [arguments...]
.B mono-sgen [options] file [arguments...]
\fImono\fP 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
.I file
and optionally passes
.I arguments
to it. The
.I file
is an ECMA assembly. They typically have a .exe or .dll extension.
The runtime provides a number of configuration options for running
applications, for developing and debugging, and for testing and
debugging the runtime itself.
The \fImono\fP command uses the Boehm conservative garbage collector
while the \fImono-sgen\fP command uses a moving and generational
garbage collector.
On Unix-based systems, Mono provides a mechanism to emulate the
Windows-style file access, this includes providing a case insensitive
view of the file system, directory separator mapping (from \\ to /) and
stripping the drive letters.
This functionality is enabled by setting the
environment variable to one of
.B all, drive
.B case.
See the description for
in the environment variables section for more details.
The following options are available:
\fB--aot\fR, \fB--aot[=options]\fR
This option is used to precompile the CIL code in the specified
assembly to native code. The generated code is stored in a file with
the extension .so. This file will be automatically picked up by the
runtime when the assembly is executed.
Ahead-of-Time compilation is most useful if you use it in combination
with the -O=all,-shared flag which enables all of the optimizations in
the code generator to be performed. Some of those optimizations are
not practical for Just-in-Time compilation since they might be very
time consuming.
Unlike the .NET Framework, Ahead-of-Time compilation will not generate
domain independent code: it generates the same code that the
Just-in-Time compiler would produce. Since most applications use a
single domain, this is fine. If you want to optimize the generated
code for use in multi-domain applications, consider using the
-O=shared flag.
This pre-compiles the methods, but the original assembly is still
required to execute as this one contains the metadata and exception
information which is not available on the generated file. When
precompiling code, you might want to compile with all optimizations
(-O=all). Pre-compiled code is position independent code.
Pre compilation is just a mechanism to reduce startup time, increase
code sharing across multiple mono processes and avoid just-in-time
compilation program startup costs. The original assembly must still
be present, as the metadata is contained there.
AOT code typically can not be moved from one computer to another
(CPU-specific optimizations that are detected at runtime) so you
should not try to move the pre-generated assemblies or package the
pre-generated assemblies for deployment.
A few options are available as a parameter to the
.B --aot
command line option. The options are separated by commas, and more
than one can be specified:
.ne 8
.I bind-to-runtime-version
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.
.I 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.
.I outfile=[filename]
Instructs the AOT compiler to save the output to the specified file.
.I write-symbols
Instructs the AOT compiler to emit debug symbol information.
.I save-temps,keep-temps
Instructs the AOT compiler to keep temporary files.
.I threads=[number]
This is an experimental option for the AOT compiler to use multiple threads
when compiling the methods.
.I nodebug
Instructs the AOT compiler to not output any debugging information.
.I ntrampolines=[number]
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.
.I nrgctx-trampolines=[number]
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.
.I nimt-trampolines=[number]
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.
.I print-skipped-methods
If the AOT compiler cannot compile a method for any reason, enabling this flag
will output the skipped methods to the console.
.I autoreg
The AOT compiler will emit a (ELF only) library initializer to automatically
register the aot compiled module with the runtime. This is only useful in static
.I asmonly
Instructs the AOT compiler to output assembly code instead of an object file.
.I soft-debug
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
.I 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
\fBdisable\fR which disables the attach functionality.
\fB--config filename\fR
Load the specified configuration file instead of the default one(s).
The default files are /etc/mono/config and ~/.mono/config or the file
specified in the MONO_CONFIG environment variable, if set. See the
mono-config(5) man page for details on the format of this file.
This instructs the Mono runtime to
start a debugging agent inside the Mono runtime and connect it to a
client user interface will control the Mono process.
This option is typically used by IDEs, like the MonoDevelop IDE.
configuration is specified using one of more of the following options:
.ne 8
.I transport=transport_name
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'.
.I address=host:port
Use this option to specify the IP address where your debugger client is
listening to.
.I loglevel=LEVEL
Specifies the diagnostics log level for
.I logfile=filename
Used to specify the file where the log will be stored, it defaults to
standard output.
.I server=[y/n]
Defaults to no, with the default option Mono will actively connect to the
host/port configured with the \fBaddress\fR option. If you set it to 'y', it
instructs the Mono runtime to start debugging in server mode, where Mono
actively waits for the debugger front end to connect to the Mono process.
Mono will print out to stdout the IP address and port where it is listening.
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.
\fB--gc=boehm\fR, \fB--gc=sgen\fR
Selects the Garbage Collector engine for Mono to use, Boehm or SGen.
Currently this merely ensures that you are running either the
\fImono\fR or \fImono-sgen\fR commands. This flag can be set in the
\fBMONO_ENV_OPTIONS\fR environment variable to force all of your child
processes to use one particular kind of GC with the Mono runtime.
\fB--help\fR, \fB-h\fR
Displays usage instructions.
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:
When using a Mono that has been compiled with LLVM support, it forces
Mono to fallback to its JIT engine and not use the LLVM backend.
\fB--optimize=MODE\fR, \fB-O=MODE\fR
MODE is a comma separated list of optimizations. They also allow
optimizations to be turned off by prefixing the optimization name with
a minus sign.
In general, Mono has been tuned to use the default set of flags,
before using these flags for a deployment setting, you might want to
actually measure the benefits of using them.
The following 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 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
\fB--security\fR, \fB--security=mode\fR
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.
.ne 8
Using security without parameters is equivalent as calling it with the
"cas" parameter.
The following modes are supported:
.I cas
This allows mono to support declarative security attributes,
e.g. execution of Code Access Security (CAS) or non-CAS demands.
.I core-clr
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.
.I validil
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.
.I verifiable
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
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
This is different from \fB--security\fR's verifiable
or validil in that these options only check user code and skip
mscorlib and assemblies located on the global assembly cache.
\fB-V\fR, \fB--version\fR
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.
\fB--debug\fR, \fB--debug=OPTIONS\fR
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.
.ne 8
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:
.I casts
Produces a detailed error when throwing a InvalidCastException. This
option needs to be enabled as this generates more verbose code at
execution time.
.I mdb-optimizations
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.
.I gdb
Generate and register debugging information with gdb. This is only supported on some
platforms, and only when using gdb 7.0 or later.
Turns on profiling. For more information about profiling applications
and code coverage see the sections "PROFILING" and "CODE COVERAGE"
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
The trace can be customized to include or exclude methods, classes or
assemblies. A trace expression is a comma separated list of targets,
each target can be prefixed with a minus sign to turn off a particular
target. The words `program', `all' and `disabled' have special
meaning. `program' refers to the main program being executed, and
`all' means all the method calls.
The `disabled' option is used to start up with tracing disabled. It
can be enabled at a later point in time in the program by sending the
SIGUSR2 signal to the runtime.
Assemblies are specified by their name, for example, to trace all
calls in the System assembly, use:
mono --trace=System app.exe
Classes are specified with the T: prefix. For example, to trace all
calls to the System.String class, use:
mono --trace=T:System.String app.exe
And individual methods are referenced with the M: prefix, and the
standard method 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.
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.
Generate a JIT method map in a /tmp/ file. This file is then
used, for example, by the perf tool included in recent Linux kernels.
Each line in the file has:
Currently this option is only supported on Linux.
The maintainer options are only used by those developing the runtime
itself, and not typically of interest to runtime users or developers.
\fB--break method\fR
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.
\fB--compile name\fR
This compiles a method (, this is used for
testing the compiler performance or to examine the output of the code
Compiles all the methods in an assembly. This is used to test the
compiler performance or to examine the output of the code generator
\fB--graph=TYPE METHOD\fR
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
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
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.
\fB-v\fR, \fB--verbose\fR
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
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
.I profiler
is not specified, the default profiler is used.
.I profiler_args
is a profiler-specific string of options for the profiler itself.
The default profiler accepts the following options 'alloc' to profile
memory consumption by the application; 'time' to profile the time
spent on each routine; 'jit' to collect time spent JIT-compiling methods
and 'stat' to perform sample statistical profiling.
If no options are provided the default is 'alloc,time,jit'.
By default the
profile data is printed to stdout: to change this, use the 'file=filename'
option to output the data to filename.
For example:
mono --profile program.exe
That will run the program with the default profiler and will do time
and allocation 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 statistical profiler has a very low overhead and should
be the preferred profiler to use (for better output use the full path
to the mono binary when running and make sure you have installed the
addr2line utility that comes from the binutils package).
This is the most advanced profiler.
The Mono \f[I]log\f[] 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)
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 `'. 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:
\fB--profile=cov[:assembly-name[/namespace]] test-suite.exe\fR
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
.I assembly-name
does not include the extension.
You can further restrict the code coverage output by specifying a
mono --profile=cov:demo/My.Utilities demo.exe
Which will only perform code coverage in the given assembly and
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
.B 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
.B 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 \fB--debug=casts\fR 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.
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
Use "info" to track the dynamic loading of assemblies.
Use the
environment variable to limit the extent of the messages you get:
If set, the log mask is changed to the set value. Possible values are
"asm" (assembly loader), "type", "dll" (native library loader), "gc"
(garbage collector), "cfg" (config file loader), "aot" (precompiler),
"security" (e.g. Moonlight CoreCLR support) and "all".
The default value is "all". Changing the mask value allows you to display only
messages for a certain component. You can use multiple masks by comma
separating them. For example to see config file messages and assembly loader
messages set you mask to "asm,cfg".
The following is a common use to track down problems with P/Invoke:
$ MONO_LOG_LEVEL="debug" MONO_LOG_MASK="dll" mono glue.exe
Mono's XML serialization engine by default will use a reflection-based
approach to serialize which might be slow for continuous processing
(web service applications). The serialization engine will determine
when a class must use a hand-tuned serializer based on a few
parameters and if needed it will produce a customized C# serializer
for your types at runtime. This customized serializer then gets
dynamically loaded into your application.
You can control this with the MONO_XMLSERIALIZER_THS environment
The possible values are
.B `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
.B `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 \fBLLVM_COUNT=10\fR would only
compile 10 methods with LLVM and then switch to the Mono JIT engine.
\fBLLVM_COUNT=0\fR would disable the LLVM engine altogether.
If set, this variable will instruct Mono to ahead-of-time compile new
assemblies on demand and store the result into a cache in
Mono 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 $prefix/etc/mono/2.0/ file and, optionally, in the file found in the top-level directory of your ASP.NET application.
Both files are read by System.Web on application startup, if they are found at the
above locations. If you don't want the mapping to be performed you can set this
variable in your environment before starting the application and no action will
be taken.
If set, this variable overrides the default system configuration directory
($PREFIX/etc). It's used to locate machine.config file.
Sets the style of COM interop. If the value of this variable is "MS"
Mono will use string marhsalling routines from the liboleaut32 for the
BSTR type library, any other values will use the mono-builtin BSTR
string marshalling.
If set, this variable overrides the default runtime configuration file
($PREFIX/etc/mono/config). The --config command line options overrides the
environment variable.
Override the automatic cpu detection mechanism. Currently used only on arm.
The format of the value is as follows:
"armvV [thumb]"
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:
.I "local[:path]"
Persists event logs and entries to the local file system.
The directory in which to persist the event logs, event sources and entries
can be specified as part of the value.
If the path is not explicitly set, it defaults to "/var/lib/mono/eventlog"
on unix and "%APPDATA%\mono\eventlog" on Windows.
.I "win32"
Uses the native win32 API to write events and registers event logs and
event sources in the registry. This is only available on Windows.
On Unix, the directory permission for individual event log and event source
directories is set to 777 (with +t bit) allowing everyone to read and write
event log entries while only allowing entries to be deleted by the user(s)
that created them.
.I "null"
Silently discards any events.
The default is "null" on Unix (and versions of Windows before NT), and
"win32" on Windows NT (and higher).
If set, contains a colon-separated list of text encodings to try when
turning externally-generated text (e.g. command-line arguments or
filenames) into Unicode. The encoding names come from the list
provided by iconv, and the special case "default_locale" which refers
to the current locale's default encoding.
When reading externally-generated text strings UTF-8 is tried first,
and then this list is tried in order with the first successful
conversion ending the search. When writing external text (e.g. new
filenames or arguments to new processes) the first item in this list
is used, or UTF-8 if the environment variable is not set.
The problem with using MONO_EXTERNAL_ENCODINGS to process your
files is that it results in a problem: although its possible to get
the right file name it is not necessarily possible to open the file.
In general if you have problems with encodings in your filenames you
should use the "convmv" program.
When using Mono with the SGen garbage collector this variable controls
several parameters of the collector. The variable's value is a comma
separated list of words.
.ne 8
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
Sets the size of the major heap (not including the large object space)
for the fixed-heap Mark&Sweep collector (i.e. `marksweep-fixed' and
`marksweep-fixed-par'). The size is in bytes, with optional suffixes
`k', `m' and `g' to specify kilo-, mega- and gigabytes, respectively.
The default is 512 megabytes.
Specifies which write barrier to use. Options are `cardtable' and
`remset'. The card table barrier is faster but less precise, and only
supported for the Mark&Sweep major collector on 32 bit platforms. The
default is `cardtable' if it is supported, 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
Specifies how application threads should be scanned. Options are
`precise` and `conservative`. Precise marking allow the collector
to know what values on stack are references and what are not.
Conservative marking threats all values as potentially references
and leave them untouched. Precise marking reduces floating garbage
and can speed up nursery collection and allocation rate, it has
the downside of requiring a significant extra memory per compiled
method. The right option, unfortunately, requires experimentation.
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.
.ne 8
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 references that are not on the remembered
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 gacutil /gacdir command. Example:
.B /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
directive instead, like this:
MonoIOMAP <appalias> all
See mod_mono(8) for more details.
Additionally. Mono includes a profiler module which allows one to track what
adjustements to file paths IOMAP code needs to do. The tracking code reports
the managed location (full stack trace) from which the IOMAP-ed call was made and,
on process exit, the locations where all the IOMAP-ed strings were created in
managed code. The latter report is only approximate as it is not always possible
to estimate the actual location where the string was created. The code uses simple
heuristics - it analyzes stack trace leading back to the string allocation location
and ignores all the managed code which lives in assemblies installed in GAC as well as in the
class libraries shipped with Mono (since they are assumed to be free of case-sensitivity
issues). It then reports the first location in the user's code - in most cases this will be
the place where the string is allocated or very close to the location. The reporting code
is implemented as a custom profiler module (see the "PROFILING" section) and can be loaded
in the following way:
mono --profile=iomap yourapplication.exe
Note, however, that Mono currently supports only one profiler module
at a time.
When Mono is using the LLVM code generation backend you can use this
environment variable to pass code generation options to the LLVM
If set to "disabled", System.IO.FileSystemWatcher will use a file watcher
implementation which silently ignores all the watching requests.
If set to any other value, System.IO.FileSystemWatcher will use the default
managed implementation (slow). If unset, mono will try to use inotify, FAM,
Gamin, kevent under Unix systems and native API calls on Windows, falling
back to the managed implementation on error.
Mono supports a plugin model for its implementation of System.Messaging making
it possible to support a variety of messaging implementations (e.g. AMQP, ActiveMQ).
To specify which messaging implementation is to be used the evironement variable
needs to be set to the full class name for the provider. E.g. to use the RabbitMQ based
AMQP implementation the variable should be set to:
If set causes the mono process to be bound to a single processor. This may be
useful when debugging or working around race conditions.
Disable inlining of thread local accesses. Try setting this if you get a segfault
early on in the execution of mono.
Provides a search path to the runtime where to look for library
files. This is a tool convenient for debugging applications, but
should not be used by deployed applications as it breaks the assembly
loader in subtle ways.
Directories are separated by the platform path separator (colons on unix). Example:
.B /home/username/lib:/usr/local/mono/lib
Alternative solutions to MONO_PATH include: installing libraries into
the Global Assembly Cache (see gacutil(1)) or having the dependent
libraries side-by-side with the main executable.
For a complete description of recommended practices for application
deployment, see
Experimental RTC support in the statistical profiler: if the user has
the permission, more accurate statistics are gathered. The MONO_RTC
value must be restricted to what the Linux rtc allows: power of two
from 64 to 8192 Hz. To enable higher frequencies like 4096 Hz, run as root:
echo 4096 > /proc/sys/dev/rtc/max-user-freq
For example:
MONO_RTC=4096 mono --profiler=default:stat program.exe
If set its the directory where the ".wapi" handle state is stored.
This is the directory where the Windows I/O Emulation layer stores its
shared state data (files, events, mutexes, pipes). By default Mono
will store the ".wapi" directory in the users's home directory.
Uses the string value of this variable as a replacement for the host name when
creating file names in the ".wapi" directory. This helps if the host name of
your machine is likely to be changed when a mono application is running or if
you have a .wapi directory shared among several different computers.
Mono typically uses the hostname to create the files that are used to
share state across multiple Mono processes. This is done to support
home directories that might be shared over the network.
If set, extra checks are made during IO operations. Currently, this
includes only advisory locks around file writes.
The name of the theme to be used by Windows.Forms. Available themes today
include "clearlooks", "nice" and "win32".
The default is "win32".
The time, in seconds, that the SSL/TLS session cache will keep it's entry to
avoid a new negotiation between the client and a server. Negotiation are very
CPU intensive so an application-specific custom value may prove useful for
small embedded 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).
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
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:
.ne 8
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
This option can be used to get more detailed information from
InvalidCast exceptions, it will provide information about the types
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
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.
Equivalent to setting the \fBMONO_XDEBUG\fR 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
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
Captures the interrupt signal (Control-C) and displays a stack trace
when pressed. Useful to find out where the program is executing at a
given point. This only displays the stack trace of a single thread.
Instructs the runtime to initialize the stack with
some known values (0x2a on x86-64) at the start of a method to assist
in debuggin the JIT engine.
This option will leak delegate trampolines that are no longer
referenced as to present the user with more information about a
delegate misuse. Basically a delegate instance might be created,
passed to unmanaged code, and no references kept in managed code,
which will garbage collect the code. With this option it is possible
to track down the source of the problems.
This option will cause mono to abort with a descriptive message when
during stack unwinding after an exception it reaches a native stack
frame. This happens when a managed delegate is passed to native code,
and the managed delegate throws an exception. Mono will normally try
to unwind the stack to the first (managed) exception handler, and it
will skip any native stack frames in the process. This leads to
undefined behaviour (since mono doesn't know how to process native
frames), leaks, and possibly crashes too.
This option will disable the GDB backtrace emitted by the runtime
after a SIGSEGV or SIGABRT in unmanaged code.
This option will suspend the program when a native SIGSEGV is received.
This is useful for debugging crashes which do not happen under gdb,
since a live process contains more information than a core file.
The logging level, possible values are `error', `critical', `warning',
`message', `info' and `debug'. See the DEBUGGING section for more
Controls the domain of the Mono runtime that logging will apply to.
If set, the log mask is changed to the set value. Possible values are
"asm" (assembly loader), "type", "dll" (native library loader), "gc"
(garbage collector), "cfg" (config file loader), "aot" (precompiler),
"security" (e.g. Moonlight CoreCLR support) and "all".
The default value is "all". Changing the mask value allows you to display only
messages for a certain component. You can use multiple masks by comma
separating them. For example to see config file messages and assembly loader
messages set you mask to "asm,cfg".
Used for runtime tracing of method calls. The format of the comma separated
trace options is:
[-]M:method name
[-]T:class name
disabled Trace output off upon start.
You can toggle trace output on/off sending a SIGUSR2 signal to the program.
If set, enables the System.Diagnostics.DefaultTraceListener, which will
print the output of the System.Diagnostics Trace and Debug classes.
It can be set to a filename, and to Console.Out or Console.Error to display
output to standard output or standard error, respectively. If it's set to
Console.Out or Console.Error you can append an optional prefix that will
be used when writing messages like this: Console.Error:MyProgramName.
See the System.Diagnostics.DefaultTraceListener documentation for more
This eases WCF diagnostics functionality by simply outputs all log messages from WCF engine to "stdout", "stderr" or any file passed to this environment variable. The log format is the same as usual diagnostic output.
This throws an exception when a X11 error is encountered; by default a
message is displayed but execution continues
Set this value to 1 to prevent the serializer from removing the
temporary files that are created for fast serialization; This might
be useful when debugging.
This is used in the System.Windows.Forms implementation when running
with the X11 backend. This is used to debug problems in Windows.Forms
as it forces all of the commands send to X11 server to be done
synchronously. The default mode of operation is asynchronous which
makes it hard to isolate the root of certain 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
Permissible values are:
.I "all"
All generated code can be shared.
.I "collections"
Only the classes in System.Collections.Generic will have its code
shared (this is the default value).
.I "corlib"
Only code in corlib will have its code shared.
.I "none"
No generic code sharing will be performed.
Generic code sharing by default only applies to collections. The
Mono JIT by default turns this on.
When the the MONO_XDEBUG env var is set, debugging info for JITted
code is emitted into a shared library, loadable into gdb. This enables,
for example, to see managed frame names on gdb backtraces.
Enables the maximum JIT verbosity for the specified method. This is
very helpfull to diagnose a miscompilation problems of a specific
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'.
.B ves-init-begin, ves-init-end
Begin and end of runtime initialization.
.B method-compile-begin, method-compile-end
Begin and end of method compilation.
The probe arguments are class name, method name and signature,
and in case of method-compile-end success or failure of compilation.
.B gc-begin, gc-end
Begin and end of Garbage Collection.
To verify the availability of the probes, run:
dtrace -P mono'$target' -l -c mono
Mono's Ping implementation for detecting network reachability can
create the ICMP packets itself without requiring the system ping
command to do the work. If you want to enable this on Linux for
non-root users, you need to give the Mono binary special permissions.
As root, run this command:
# setcap cap_net_raw=+ep /usr/bin/mono
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.
.B ~/.mono/aot-cache
The directory for the ahead-of-time compiler demand creation
assemblies are located.
.B /etc/mono/config, ~/.mono/config
Mono runtime configuration file. See the mono-config(5) manual page
for more information.
.B ~/.config/.mono/certs, /usr/share/.mono/certs
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.
.B ~/.mono/assemblies/ASSEMBLY/ASSEMBLY.config
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.
.B ~/.config/.mono/keypairs, /usr/share/.mono/keypairs
Contains Mono cryptographic keypairs for users / machine. They can be
accessed by using a CspParameters object with DSACryptoServiceProvider
and RSACryptoServiceProvider classes.
.B ~/.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.
.B <assembly>.config
Configuration information for individual assemblies is loaded by the
runtime from side-by-side files with the .config files, see the for more information.
.B Web.config, web.config
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
Mailing lists are listed at the
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:
For ASP.NET-related documentation, see the xsp(1) manual page
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