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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
* Infrastructure for sharing DOMString data with JSStrings.
* Importing an nsAString into JS:
* If possible (GetSharedBufferHandle works) use the external string support in
* JS to create a JSString that points to the readable's buffer. We keep a
* reference to the buffer handle until the JSString is finalized.
* Exporting a JSString as an nsAReadable:
* Wrap the JSString with a root-holding XPCJSReadableStringWrapper, which roots
* the string and exposes its buffer via the nsAString interface, as
* well as providing refcounting support.
#include "nscore.h"
#include "nsString.h"
#include "nsStringBuffer.h"
#include "jsapi.h"
#include "xpcpublic.h"
using namespace JS;
const XPCStringConvert::LiteralExternalString
const XPCStringConvert::DOMStringExternalString
const XPCStringConvert::DynamicAtomExternalString
void XPCStringConvert::LiteralExternalString::finalize(char16_t* aChars) const {
// Nothing to do.
size_t XPCStringConvert::LiteralExternalString::sizeOfBuffer(
const char16_t* aChars, mozilla::MallocSizeOf aMallocSizeOf) const {
// This string's buffer is not heap-allocated, so its malloc size is 0.
return 0;
void XPCStringConvert::DOMStringExternalString::finalize(
char16_t* aChars) const {
nsStringBuffer* buf = nsStringBuffer::FromData(aChars);
size_t XPCStringConvert::DOMStringExternalString::sizeOfBuffer(
const char16_t* aChars, mozilla::MallocSizeOf aMallocSizeOf) const {
// We promised the JS engine we would not GC. Enforce that:
JS::AutoCheckCannotGC autoCannotGC;
const nsStringBuffer* buf =
// We want sizeof including this, because the entire string buffer is owned by
// the external string. But only report here if we're unshared; if we're
// shared then we don't know who really owns this data.
return buf->SizeOfIncludingThisIfUnshared(aMallocSizeOf);
void XPCStringConvert::DynamicAtomExternalString::finalize(
char16_t* aChars) const {
nsDynamicAtom* atom = nsDynamicAtom::FromChars(aChars);
// nsDynamicAtom::Release is always-inline and defined in a translation unit
// we can't get to here. So we need to go through nsAtom::Release to call
// it.
size_t XPCStringConvert::DynamicAtomExternalString::sizeOfBuffer(
const char16_t* aChars, mozilla::MallocSizeOf aMallocSizeOf) const {
// We return 0 here because NS_AddSizeOfAtoms reports all memory associated
// with atoms in the atom table.
return 0;
// convert a readable to a JSString, copying string data
// static
bool XPCStringConvert::ReadableToJSVal(JSContext* cx, const nsAString& readable,
nsStringBuffer** sharedBuffer,
MutableHandleValue vp) {
*sharedBuffer = nullptr;
uint32_t length = readable.Length();
if (readable.IsLiteral()) {
return StringLiteralToJSVal(cx, readable.BeginReading(), length, vp);
nsStringBuffer* buf = nsStringBuffer::FromString(readable);
if (buf) {
bool shared;
if (!StringBufferToJSVal(cx, buf, length, vp, &shared)) {
return false;
if (shared) {
*sharedBuffer = buf;
return true;
// blech, have to copy.
JSString* str = JS_NewUCStringCopyN(cx, readable.BeginReading(), length);
if (!str) {
return false;
return true;
namespace xpc {
bool NonVoidStringToJsval(JSContext* cx, nsAString& str,
MutableHandleValue rval) {
nsStringBuffer* sharedBuffer;
if (!XPCStringConvert::ReadableToJSVal(cx, str, &sharedBuffer, rval)) {
return false;
if (sharedBuffer) {
// The string was shared but ReadableToJSVal didn't addref it.
// Move the ownership from str to jsstr.
return true;
} // namespace xpc