a47cb700ad
We get undefined behavior, at least in code that re-slices, unless capacity is set. This ensures the buffer under the string has a capacity set to the same length as its size. This also shows more explicitly the problems in TinyGo (ex some type mismatch you have to ignore until fixed). Signed-off-by: Adrian Cole <adrian@tetrate.io>
77 lines
2.2 KiB
Go
77 lines
2.2 KiB
Go
package main
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import (
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"fmt"
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"reflect"
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"unsafe"
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)
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// main is required for TinyGo to compile to Wasm.
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func main() {}
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// greet prints a greeting to the console.
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func greet(name string) {
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log(fmt.Sprint("wasm >> ", greeting(name)))
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}
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// log a message to the console using _log.
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func log(message string) {
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ptr, size := stringToPtr(message)
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_log(ptr, size)
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}
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// _log is a WebAssembly import which prints a string (linear memory offset,
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// byteCount) to the console.
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//
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// Note: In TinyGo "//export" on a func is actually an import!
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//go:wasm-module env
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//export log
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func _log(ptr uint32, size uint32)
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// greeting gets a greeting for the name.
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func greeting(name string) string {
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return fmt.Sprint("Hello, ", name, "!")
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}
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// _greet is a WebAssembly export that accepts a string pointer (linear memory
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// offset) and calls greet.
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//export greet
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func _greet(ptr, size uint32) {
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name := ptrToString(ptr, size)
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greet(name)
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}
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// _greet is a WebAssembly export that accepts a string pointer (linear memory
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// offset) and returns a pointer/size pair packed into a uint64.
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//
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// Note: This uses a uint64 instead of two result values for compatibility with
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// WebAssembly 1.0.
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//export greeting
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func _greeting(ptr, size uint32) (ptrSize uint64) {
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name := ptrToString(ptr, size)
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g := greeting(name)
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ptr, size = stringToPtr(g)
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return (uint64(ptr) << uint64(32)) | uint64(size)
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}
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// ptrToString returns a string from WebAssembly compatible numeric types
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// representing its pointer and length.
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func ptrToString(ptr uint32, size uint32) string {
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// Get a slice view of the underlying bytes in the stream. We use SliceHeader, not StringHeader
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// as it allows us to fix the capacity to what was allocated.
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return *(*string)(unsafe.Pointer(&reflect.SliceHeader{
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Data: uintptr(ptr),
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Len: uintptr(size), // Tinygo requires these as uintptrs even if they are int fields.
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Cap: uintptr(size), // ^^ See https://github.com/tinygo-org/tinygo/issues/1284
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}))
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}
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// stringToPtr returns a pointer and size pair for the given string in a way
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// compatible with WebAssembly numeric types.
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func stringToPtr(s string) (uint32, uint32) {
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buf := []byte(s)
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ptr := &buf[0]
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unsafePtr := uintptr(unsafe.Pointer(ptr))
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return uint32(unsafePtr), uint32(len(buf))
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}
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