CODEWITHSUNDEEP
webassembly
MatthewScarpino
MatthewScarpino

Reputation: 5936

Setting data in a data expression

In the WebAssembly text format, every data expression I've seen provides data as a string, as in "hello". But the spec says that the last argument of data can be a concat((𝑏*)*), which apparently represents a concatenation of data elements.

Does anyone have an example of this? I can't find anything helpful. Thanks.

Upvotes: 0

Views: 123

Answers (2)

Andreas Rossberg
Andreas Rossberg

Reputation: 36118

A data segment can be written with multiple strings, which are simply concatenated:

(data (offset (i32.const 0))
  "... part 1 ..."
  "... part 2 ..."
  "... part 3 ..."
)

The only reason for this feature is to enable splitting the string over multiple lines (cf. string literals in C).

Upvotes: 3

Alexandr Skachkov
Alexandr Skachkov

Reputation: 560

As I understand spec in data section last argument is size of current block. As data uses linear memory possible you can guess size of the string and return concat of strings

(module
  (import "imp" "log" (func $log (param i32 i32)))
  (memory (import "imp" "mem") 1)
  (func $writeHi
    i32.const 0
    i32.const 13
    call $log
  )
  (data (i32.const 0) "hello")
  (data (i32.const 5) " ")
  (data (i32.const 6) "world")
  (export "writeLog" (func $writeHi))
)

with this js 

function consoleLogString(memory, offset, length) {
        var bytes = new Uint8Array(memory.buffer, offset, length);
        var string = new TextDecoder('utf8').decode(bytes);
        console.log(string);
    }
    const memory = new WebAssembly.Memory({ initial: 2 });
    const consoleLogStringWrapper = memory => (offset, length) => consoleLogString(memory, offset, length);
    var importObj = {
        imp : {
            mem: memory,
            log: consoleLogStringWrapper(memory)
        }
    };

    const wasmInstance = new WebAssembly.Instance(wasmModule, importObj);
    const { writeHi } = wasmInstance.exports;
    writeHi();

will return hello world, you can check on wat2wasm

Upvotes: 0

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