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eWASM Contract Interface (ECI) Specification (Revision 0)

The eWASM Contract Interface (ECI) specifies the structure of a contract module.

Wire format

Every contract must be stored in the WebAssembly Binary Encoding format (in short, WASM bytecode).

Imports

A contract can only import symbols specified in the Ethereum Environment Interface.

Exports

A contract must have exactly two exported symbols:

  • memory: the shared memory space available for the EEI to write into.
  • main: a function with no parameters and no result value.

Entry point

The method exported as main will be executed by the VM.

On successful execution, the code should return via a normal code path.

If it needs to abort due to a failure, an unreachable instruction should be executed.

Debug-mode

Debug-mode is a special VM option, where an additional set of debugging interfaces are available to contracts. On a live VM, any bytecode trying to import these symbols should be rejected.

The imports are available under the debug namespace:

  • print(i64): print a number
  • printMem(i32 offset, i32 length): print a string as pointed by offset
  • printMemHex(i32 offset, i32 length): print a hex representation of the memory pointed to by offset