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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 */
#ifndef nsStringBuffer_h__
#define nsStringBuffer_h__
#include <atomic>
#include "mozilla/MemoryReporting.h"
#include "nsStringFwd.h"
template <class T>
struct already_AddRefed;
* This structure precedes the string buffers "we" allocate. It may be the
* case that nsTAString::mData does not point to one of these special
* buffers. The mDataFlags member variable distinguishes the buffer type.
* When this header is in use, it enables reference counting, and capacity
* tracking. NOTE: A string buffer can be modified only if its reference
* count is 1.
class nsStringBuffer {
friend class CheckStaticAtomSizes;
std::atomic<uint32_t> mRefCount;
uint32_t mStorageSize;
* Allocates a new string buffer, with given size in bytes and a
* reference count of one. When the string buffer is no longer needed,
* it should be released via Release.
* It is up to the caller to set the bytes corresponding to the string
* buffer by calling the Data method to fetch the raw data pointer. Care
* must be taken to properly null terminate the character array. The
* storage size can be greater than the length of the actual string
* (i.e., it is not required that the null terminator appear in the last
* storage unit of the string buffer's data).
* This guarantees that StorageSize() returns aStorageSize if the returned
* buffer is non-null. Some callers like nsAttrValue rely on it.
* @return new string buffer or null if out of memory.
static already_AddRefed<nsStringBuffer> Alloc(size_t aStorageSize);
* Returns a string buffer initialized with the given string on it, or null on
* OOM.
* Note that this will allocate extra space for the trailing null byte, which
* this method will add.
static already_AddRefed<nsStringBuffer> Create(const char16_t* aData,
size_t aLength);
static already_AddRefed<nsStringBuffer> Create(const char* aData,
size_t aLength);
* Resizes the given string buffer to the specified storage size. This
* method must not be called on a readonly string buffer. Use this API
* carefully!!
* This method behaves like the ANSI-C realloc function. (i.e., If the
* allocation fails, null will be returned and the given string buffer
* will remain unmodified.)
* @see IsReadonly
static nsStringBuffer* Realloc(nsStringBuffer* aBuf, size_t aStorageSize);
* Increment the reference count on this string buffer.
void NS_FASTCALL AddRef();
* Decrement the reference count on this string buffer. The string
* buffer will be destroyed when its reference count reaches zero.
void NS_FASTCALL Release();
* This method returns the string buffer corresponding to the given data
* pointer. The data pointer must have been returned previously by a
* call to the nsStringBuffer::Data method.
static nsStringBuffer* FromData(void* aData) {
return reinterpret_cast<nsStringBuffer*>(aData) - 1;
* This method returns the data pointer for this string buffer.
void* Data() const {
return const_cast<char*>(reinterpret_cast<const char*>(this + 1));
* This function returns the storage size of a string buffer in bytes.
* This value is the same value that was originally passed to Alloc (or
* Realloc).
uint32_t StorageSize() const { return mStorageSize; }
* If this method returns false, then the caller can be sure that their
* reference to the string buffer is the only reference to the string
* buffer, and therefore it has exclusive access to the string buffer and
* associated data. However, if this function returns true, then other
* consumers may rely on the data in this buffer being immutable and
* other threads may access this buffer simultaneously.
bool IsReadonly() const {
// This doesn't lead to the destruction of the buffer, so we don't
// need to perform acquire memory synchronization for the normal
// reason that a reference count needs acquire synchronization
// (ensuring that all writes to the object made on other threads are
// visible to the thread destroying the object).
// We then need to consider the possibility that there were prior
// writes to the buffer on a different thread: one that has either
// since released its reference count, or one that also has access
// to this buffer through the same reference. There are two ways
// for that to happen: either the buffer pointer or a data structure
// (e.g., string object) pointing to the buffer was transferred from
// one thread to another, or the data structure pointing to the
// buffer was already visible on both threads. In the first case
// (transfer), the transfer of data from one thread to another would
// have handled the memory synchronization. In the latter case
// (data structure visible on both threads), the caller needed some
// sort of higher level memory synchronization to protect against
// the string object being mutated at the same time on multiple
// threads.
// See bug 1603504. TSan might complain about a race when using
// memory_order_relaxed, so use memory_order_acquire for making TSan
// happy.
#if defined(MOZ_TSAN)
return mRefCount.load(std::memory_order_acquire) > 1;
return mRefCount.load(std::memory_order_relaxed) > 1;
* The FromString methods return a string buffer for the given string
* object or null if the string object does not have a string buffer.
* The reference count of the string buffer is NOT incremented by these
* methods. If the caller wishes to hold onto the returned value, then
* the returned string buffer must have its reference count incremented
* via a call to the AddRef method.
static nsStringBuffer* FromString(const nsAString& aStr);
static nsStringBuffer* FromString(const nsACString& aStr);
* The ToString methods assign this string buffer to a given string
* object. If the string object does not support sharable string
* buffers, then its value will be set to a copy of the given string
* buffer. Otherwise, these methods increment the reference count of the
* given string buffer. It is important to specify the length (in
* storage units) of the string contained in the string buffer since the
* length of the string may be less than its storage size. The string
* must have a null terminator at the offset specified by |len|.
* NOTE: storage size is measured in bytes even for wide strings;
* however, string length is always measured in storage units
* (2-byte units for wide strings).
void ToString(uint32_t aLen, nsAString& aStr, bool aMoveOwnership = false);
void ToString(uint32_t aLen, nsACString& aStr, bool aMoveOwnership = false);
* This measures the size only if the StringBuffer is unshared.
size_t SizeOfIncludingThisIfUnshared(
mozilla::MallocSizeOf aMallocSizeOf) const;
* This measures the size regardless of whether the StringBuffer is
* unshared.
* WARNING: Only use this if you really know what you are doing, because
* it can easily lead to double-counting strings. If you do use them,
* please explain clearly in a comment why it's safe and won't lead to
* double-counting.
size_t SizeOfIncludingThisEvenIfShared(
mozilla::MallocSizeOf aMallocSizeOf) const;
#endif /* !defined(nsStringBuffer_h__ */