WebAssembly execution for programmable escrows. When an escrow is finished, its contract runs to decide whether the release conditions are met. Specification: XLS-0102: WASM VM.

The engine itself is Rust (crates/xrpl-wasm-vm, over wasmi), reached through a cxx bridge.

What is in this directory

WasmVM.h — the entry points xrpld calls: runEscrowWasm (execute a contract, returning a result and its gas cost, or a WasmTER) and preflightEscrowWasm (screen a module with no host and no execution). Both own their TER maps.
HostFunc.h — the HostFunctions interface: one virtual per host function, each defaulting to Unimplemented, returning std::expected<T, HostFunctionError>.
HostFuncImpl.h — WasmHostFunctionsImpl, the implementation over an ApplyContext&. Bodies are split across HostFuncImpl*.cpp by category.
HostContext.h — the bridge's C++ half: an ABI-shaped, noexcept view of HostFunctions that the engine calls back into. Nothing may unwind into Rust, so every method routes through guarded().
WasmCommon.h — the shared vocabulary: HostFunctionError (the codes a contract sees), Bytes, FieldLocator, WasmTER, adjustWasmEndianess, which is where the boundary's byte order is decided, and guarded(), the one catch every crossing of the bridge's C++ half goes through.

Host functions

Grouped by what they reach: ledger information; transaction and ledger-object field access; keylet construction; cryptography; float arithmetic; NFT queries; tracing.

The wire names and per-call gas costs are declared in crates/xrpl-host-functions — one host_functions! block that generates the ABI trait and the spec table. That declaration is the single source of truth; HostFunc.h is the C++ side of it.

Entry point

A module must export escrow_finish (escrowFunctionName) taking no parameters and returning int32_t: positive means the conditions are met, zero or negative rejects the finish. Everything the contract needs it asks for through a host call.