Step 2: Import WASI Functions Into WebAssembly

Step 2: Import WASI Functions Into WebAssembly

In parallel with the instructions in step 1, you should also add the following statements to your WebAssembly Text program.

2.1) Declare Function Signature Types

Although it is not a requirement, it is more idiomatic to declare WebAssembly function signature types. This also improves code reusability.

Both of the following sets of declarations must occur right at the start of the WAT coding.

(module
  ;; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  ;; Function types for WASI calls
  (type $type_wasi_args      (func (param i32 i32)                             (result i32)))
  (type $type_wasi_path_open (func (param i32 i32 i32 i32 i32 i64 i64 i32 i32) (result i32)))
  (type $type_wasi_fd_seek   (func (param i32 i64 i32 i32)                     (result i32)))
  (type $type_wasi_fd_io     (func (param i32 i32 i32 i32)                     (result i32)))
  (type $type_wasi_fd_close  (func (param i32)                                 (result i32)))

Once the function signature types have been declared, you can then declare which WASI functions need to be imported into your WAT program.

  ;; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  ;; Import OS system calls via WASI
  (import "wasi" "args_sizes_get" (func $wasi_args_sizes_get (type $type_wasi_args)))
  (import "wasi" "args_get"       (func $wasi_args_get       (type $type_wasi_args)))
  (import "wasi" "path_open"      (func $wasi_path_open      (type $type_wasi_path_open)))
  (import "wasi" "fd_seek"        (func $wasi_fd_seek        (type $type_wasi_fd_seek)))
  (import "wasi" "fd_read"        (func $wasi_fd_read        (type $type_wasi_fd_io)))
  (import "wasi" "fd_write"       (func $wasi_fd_write       (type $type_wasi_fd_io)))
  (import "wasi" "fd_close"       (func $wasi_fd_close       (type $type_wasi_fd_close)))

The WAT (import) statement creates a proxy function within WebAssembly (E.G. $wasi_fd_seek) that is mapped to an external WASI function (E.G. wasi.fd_seek).