Trait wiggle::GuestMemory
source · pub unsafe trait GuestMemory: Send + Sync {
// Required methods
fn base(&self) -> &[UnsafeCell<u8>];
fn can_read(&self, r: Region) -> bool;
fn can_write(&self, r: Region) -> bool;
fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>;
fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>;
fn mut_unborrow(&self, h: BorrowHandle);
fn shared_unborrow(&self, h: BorrowHandle);
// Provided methods
fn ptr<'a, T>(&'a self, offset: T::Pointer) -> GuestPtr<'a, T>
where Self: Sized,
T: ?Sized + Pointee { ... }
fn is_shared_memory(&self) -> bool { ... }
}Expand description
A trait which abstracts how to get at the region of host memory that contains guest memory.
All GuestPtr types will contain a handle to this trait, signifying where
the pointer is actually pointing into. This type will need to be implemented
for the host’s memory storage object.
§Safety
Safety around this type is tricky, and the trait is unsafe since there are
a few contracts you need to uphold to implement this type correctly and have
everything else in this crate work out safely.
The most important method of this trait is the base method. This returns,
in host memory, a pointer and a length. The pointer should point to valid
memory for the guest to read/write for the length contiguous bytes
afterwards.
The region returned by base must not only be valid, however, but it must
be valid for “a period of time before the guest is reentered”. This isn’t
exactly well defined but the general idea is that GuestMemory is allowed
to change under our feet to accommodate instructions like memory.grow or
other guest modifications. Memory, however, cannot be changed if the guest
is not reentered or if no explicitly action is taken to modify the guest
memory.
This provides the guarantee that host pointers based on the return value of
base have a dynamic period for which they are valid. This time duration
must be “somehow nonzero in length” to allow users of GuestMemory and
GuestPtr to safely read and write interior data.
This type also provides methods for run-time borrow checking of references into the memory. The safety of this mechanism depends on there being exactly one associated tracking of borrows for a given WebAssembly memory. There must be no other reads or writes of WebAssembly the memory by either Rust or WebAssembly code while there are any outstanding borrows.
§Using References
The GuestPtr::as_slice or GuestPtr::as_str will return smart
pointers GuestSlice and GuestStr. These types, which implement
std::ops::Deref and std::ops::DerefMut, provide mutable references
into the memory region given by a GuestMemory.
These smart pointers are dynamically borrow-checked by the borrow checker
methods on this trait. While a GuestSlice or a GuestStr are live,
WebAssembly cannot be reentered because the store’s borrow is connected to
the relevant 'a lifetime on the guest pointer.
Required Methods§
sourcefn base(&self) -> &[UnsafeCell<u8>]
fn base(&self) -> &[UnsafeCell<u8>]
Returns the base allocation of this guest memory, located in host memory.
A pointer/length pair are returned to signify where the guest memory lives in the host, and how many contiguous bytes the memory is valid for after the returned pointer.
Note that there are safety guarantees about this method that
implementations must uphold, and for more details see the
GuestMemory documentation.
sourcefn can_read(&self, r: Region) -> bool
fn can_read(&self, r: Region) -> bool
Check if a region of memory can be read.
This will only return true if there are no active mutable borrows.
sourcefn can_write(&self, r: Region) -> bool
fn can_write(&self, r: Region) -> bool
Check if a region of memory can be written.
This will only return true if there are no active borrows.
sourcefn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>
fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>
Acquires a mutable borrow on a region of memory.
Only succeeds if there are no active shared or mutable borrows and this
is not a shared WebAssembly memory.
Acquires a shared borrow on a region of memory.
Only succeeds if there are no active mutable borrows and this is not a
shared WebAssembly memory.
sourcefn mut_unborrow(&self, h: BorrowHandle)
fn mut_unborrow(&self, h: BorrowHandle)
Undoes a borrow by mut_borrow.
Undoes a borrow by shared_borrow.
Provided Methods§
sourcefn ptr<'a, T>(&'a self, offset: T::Pointer) -> GuestPtr<'a, T>
fn ptr<'a, T>(&'a self, offset: T::Pointer) -> GuestPtr<'a, T>
Convenience method for creating a GuestPtr at a particular offset.
Note that T can be almost any type, and typically offset is a u32.
The exception is slices and strings, in which case offset is a (u32, u32) of (offset, length).
Check if the underlying memory is shared across multiple threads; e.g., with a WebAssembly shared memory.