= RunTimeOptions

Executables produced by MLton take command line arguments that control
the runtime system.  These arguments are optional, and occur before
the executable's usual arguments.  To use these options, the first
argument to the executable must be `@MLton`.  The optional arguments
then follow, must be terminated by `--`, and are followed by any
arguments to the program.  The optional arguments are _not_ made
available to the SML program via `CommandLine.arguments`.  For
example, a valid call to `hello-world` is:

----
hello-world @MLton gc-summary fixed-heap 10k -- a b c
----

In the above example,
`CommandLine.arguments () = ["a", "b", "c"]`.

It is allowed to have a sequence of `@MLton` arguments, as in:

----
hello-world @MLton gc-summary -- @MLton fixed-heap 10k -- a b c
----

Run-time options can also control MLton, as in

----
mlton @MLton fixed-heap 0.5g -- foo.sml
----


== Options

* ``fixed-heap __x__{k|K|m|M|g|G}``
+
Use a fixed size heap of size _x_, where _x_ is a real number and the
trailing letter indicates its units.
+
[cols="^25%,<75%"]
|===
| `k` or `K` | 1024
| `m` or `M` | 1,048,576
| `g` or `G` | 1,073,741,824
|===
+
A value of `0` means to use almost all the RAM present on the machine.
+
The heap size used by `fixed-heap` includes all memory allocated by
SML code, including memory for the stack (or stacks, if there are
multiple threads).  It does not, however, include any memory used for
code itself or memory used by C globals, the C stack, or malloc.

* ``gc-messages``
+
Print a message at the start and end of every garbage collection.

* ``gc-summary``
+
Print a summary of garbage collection statistics upon program
termination to standard error.

* ``gc-summary-file __file__``
+
Print a summary of garbage collection statistics upon program
termination to the file specified by _file_.

* ``load-world __world__``
+
Restart the computation with the file specified by _world_, which must
have been created by a call to `MLton.World.save` by the same
executable.  See <<MLtonWorld#>>.

* ``max-heap __x__{k|K|m|M|g|G}``
+
Run the computation with an automatically resized heap that is never
larger than _x_, where _x_ is a real number and the trailing letter
indicates the units as with `fixed-heap`.  The heap size for
`max-heap` is accounted for as with `fixed-heap`.

* ``may-page-heap {false|true}``
+
Enable paging the heap to disk when unable to grow the heap to a
desired size.

* ``no-load-world``
+
Disable `load-world`.  This can be used as an argument to the compiler
via `-runtime no-load-world` to create executables that will not load
a world.  This may be useful to ensure that set-uid executables do not
load some strange world.

* ``ram-slop __x__``
+
Multiply _x_ by the amount of RAM on the machine to obtain what the
runtime views as the amount of RAM it can use.  Typically _x_ is less
than 1, and is used to account for space used by other programs
running on the same machine.

* ``stop``
+
Causes the runtime to stop processing `@MLton` arguments once the next
`--` is reached.  This can be used as an argument to the compiler via
`-runtime stop` to create executables that don't process any `@MLton`
arguments.
