[Muduo网络库源码分析] (12) net/Buffer_cc_h缓冲区(非阻塞 IO 必备)

缓冲区(非阻塞 IO 必备)

实现：实现Buffer类

---

功能：缓冲区，为非阻塞IO暂时存储数据，实现高效IO

---

知识点：

• string的函数实现
• resize()

Resizes the string content to n characters.

If n is smaller than the current length of the string, the content is reduced to its first n characters, the rest being dropped.

If n is greater than the current length of the string, the content is expanded by appending as many instances of the c character as needed to reach a size of n characters.

The second version, actually calls: resize(n,char()), so when a string is resized to a greater size without passing a second argument, the new character positions are filled with the default value of a char, which is the null character.

• swap()

Swaps the contents of the string with those of string object str, such that after the call to this member function, the contents of this string are those which were in str before the call, and the contents of str are those which were in this string.

• capacity()

Returns the size of the allocated storage space in the string object.

• copy函数

template<class InputIterator, class OutputIterator>
  OutputIterator copy ( InputIterator first, InputIterator last, OutputIterator result )
{
  while (first!=last) *result++ = *first++;
  return result;
}

• memchr函数

Searches within the first num bytes of the block of memory pointed by ptr for the first occurrence of value (interpreted as an unsigned char), and returns a pointer to it.

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用途：

作为非阻塞 IO的缓冲区。

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模块拆分与封装：

已把其线程对象模块单独分离出来，并利用Makefile统一编译，模块代码地址如下：https://github.com/chudongfang/Learn_From_Muduo/tree/master/Buffer

---

代码及分析：

Buffer.h

// Copyright 2010, Shuo Chen.  All rights reserved.
// http://code.google.com/p/muduo/
//
// Use of this source code is governed by a BSD-style license
// that can be found in the License file.
// Author: Shuo Chen (chenshuo at chenshuo dot com)
//
// This is a public header file, it must only include public header files.
#ifndef MUDUO_NET_BUFFER_H
#define MUDUO_NET_BUFFER_H
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
//#include   // ssize_t
namespace muduo
{
namespace net
{
/// A buffer class modeled after org.jboss.netty.buffer.ChannelBuffer
///
/// @code
/// +-------------------+------------------+------------------+
/// | prependable bytes |  readable bytes  |  writable bytes  |
/// |                   |     (CONTENT)    |                  |
/// +-------------------+------------------+------------------+
/// |                   |                  |                  |
/// 0      <=      readerIndex   <=   writerIndex    <=     size
/// @endcode
class Buffer : public muduo::copyable
{
 public:
  // kCheapPrepend 和 kInitialSize，定义了 prependable 的初始大小和 writable 的初始大小
  static const size_t kCheapPrepend = 8;
  static const size_t kInitialSize = 1024;
  //初始化指针位置
  explicit Buffer(size_t initialSize = kInitialSize)
    : buffer_(kCheapPrepend + initialSize),
      readerIndex_(kCheapPrepend),
      writerIndex_(kCheapPrepend)
  {
    assert(readableBytes() == 0);
    assert(writableBytes() == initialSize);
    assert(prependableBytes() == kCheapPrepend);
  }
  // implicit copy-ctor, move-ctor, dtor and assignment are fine
  // NOTE: implicit move-ctor is added in g++ 4.6
  // swap Buffer
  void swap(Buffer& rhs)
  {
    buffer_.swap(rhs.buffer_);
    std::swap(readerIndex_, rhs.readerIndex_);
    std::swap(writerIndex_, rhs.writerIndex_);
  }
  //返回可读的字节数
  size_t readableBytes() const
  { return writerIndex_ - readerIndex_; }
  //返回可写的字节数
  size_t writableBytes() const
  { return buffer_.size() - writerIndex_; }
  //返回已读的数目
  size_t prependableBytes() const
  { return readerIndex_; }

  //返回读指针
  const char* peek() const
  { return begin() + readerIndex_; }

  //在写入的数据中从头寻找crlf串
  const char* findCRLF() const
  {
    // FIXME: replace with memmem()?
    const char* crlf = std::search(peek(), beginWrite(), kCRLF, kCRLF+2);
    return crlf == beginWrite() ? NULL : crlf;
  }
  //在写入的数据中从start寻找crlf串
  const char* findCRLF(const char* start) const
  {
    assert(peek() <= start);
    assert(start <= beginWrite());
    // FIXME: replace with memmem()?
    const char* crlf = std::search(start, beginWrite(), kCRLF, kCRLF+2);
    return crlf == beginWrite() ? NULL : crlf;
  }
  //memchr
  //
  //Searches within the first num bytes of the block of memory 
  //pointed by ptr for the first occurrence of value (interpreted as an unsigned char),
  //and returns a pointer to it.

  //在写入的数据中从头寻找第一个出现的'\n'
  const char* findEOL() const
  {
    const void* eol = memchr(peek(), '\n', readableBytes());
    return static_cast<const char*>(eol);
  }

  //在写入的数据中从start寻找第一个出现的'\n'
  const char* findEOL(const char* start) const
  {
    assert(peek() <= start);
    assert(start <= beginWrite());
    const void* eol = memchr(start, '\n', beginWrite() - start);
    return static_cast<const char*>(eol);
  }
  // retrieve returns void, to prevent
  // string str(retrieve(readableBytes()), readableBytes());
  // the evaluation of two functions are unspecified
  //读指针后移len位，如果大于可读，则重新初始化buffer指针位置`
  void retrieve(size_t len)
  {
    assert(len <= readableBytes());
    if (len < readableBytes())
    {
      readerIndex_ += len;
    }
    else
    {
      retrieveAll();
    }
  }
  //移动读指针到end
  void retrieveUntil(const char* end)
  {
    assert(peek() <= end);
    assert(end <= beginWrite());
    retrieve(end - peek());
  }
  //读指针后移一个64位的数
  void retrieveInt64()
  {
    retrieve(sizeof(int64_t));
  }
  //读指针后移一个32位的数
  void retrieveInt32()
  {
    retrieve(sizeof(int32_t));
  }
  //读指针后移一个16位的数
  void retrieveInt16()
  {
    retrieve(sizeof(int16_t));
  }
  //读指针后移一个8位的数
  void retrieveInt8()
  {
    retrieve(sizeof(int8_t));
  }
  //初始化指针位置
  void retrieveAll()
  {
    readerIndex_ = kCheapPrepend;
    writerIndex_ = kCheapPrepend;
  }
  //读完所有内容到string中，并返回
  string retrieveAllAsString()
  {
    return retrieveAsString(readableBytes());
  }
  //读取长度为len的字符串，并返回
  string retrieveAsString(size_t len)
  {
    assert(len <= readableBytes());
    string result(peek(), len);
    retrieve(len);
    return result;
  }
  StringPiece toStringPiece() const
  {
    return StringPiece(peek(), static_cast<int>(readableBytes()));
  }
  void append(const StringPiece& str)
  {
    append(str.data(), str.size());
  }

  // the functions copy
  //
  //template
  //OutputIterator copy ( InputIterator first, InputIterator last, OutputIterator result )
  //{
  //    while (first!=last) *result++ = *first++;
  //      return result;
  //}

  //首先判断其是否有足够空间，然后copy ，最后更新指针
  void append(const char* /*restrict*/ data, size_t len)
  {
    ensureWritableBytes(len);
    std::copy(data, data+len, beginWrite());
    hasWritten(len);
  }

  void append(const void* /*restrict*/ data, size_t len)
  {
    append(static_cast<const char*>(data), len);
  }

  //确认可写的字节数满足len个字节数的要求，如果len大于其可写字节数，则分配内存
  void ensureWritableBytes(size_t len)
  {
    if (writableBytes() < len)
    {
      makeSpace(len);
    }
    assert(writableBytes() >= len);
  }
  //返回写指针
  char* beginWrite()
  { return begin() + writerIndex_; }
  //返回写指针
  const char* beginWrite() const
  { return begin() + writerIndex_; }

  //把写指针向后移动len 
  void hasWritten(size_t len)
  {
    assert(len <= writableBytes());
    writerIndex_ += len;
  }
  //把写指针向前移动len
  void unwrite(size_t len)
  {
    assert(len <= readableBytes());
    writerIndex_ -= len;
  }
  ///
  /// Append int64_t using network endian
  ///

  //写入一个64位的数
  void appendInt64(int64_t x)
  {
    int64_t be64 = sockets::hostToNetwork64(x);
    append(&be64, sizeof be64);
  }
  ///
  /// Append int32_t using network endian
  ///

  //写入一个32位的数
  void appendInt32(int32_t x)
  {
    int32_t be32 = sockets::hostToNetwork32(x);
    append(&be32, sizeof be32);
  }
  //写入一个16位的数
  void appendInt16(int16_t x)
  {
    int16_t be16 = sockets::hostToNetwork16(x);
    append(&be16, sizeof be16);
  }
  //写入一个16位的数
  void appendInt8(int8_t x)
  {
    append(&x, sizeof x);
  }
  ///
  /// Read int64_t from network endian
  ///
  /// Require: buf->readableBytes() >= sizeof(int32_t)
  //读取一个64位的数
  int64_t readInt64()
  {
    int64_t result = peekInt64();
    retrieveInt64();
    return result;
  }
  ///
  /// Read int32_t from network endian
  ///
  /// Require: buf->readableBytes() >= sizeof(int32_t)
  //读取一个32位的数
  int32_t readInt32()
  {
    int32_t result = peekInt32();
    retrieveInt32();
    return result;
  }
  //读取一个16位的数
  int16_t readInt16()
  {
    int16_t result = peekInt16();
    retrieveInt16();
    return result;
  }
  //读取一个8位的数
  int8_t readInt8()
  {
    int8_t result = peekInt8();
    retrieveInt8();
    return result;
  }
  ///
  /// Peek int64_t from network endian
  ///
  /// Require: buf->readableBytes() >= sizeof(int64_t)
  //读取并返回一个Int64
  int64_t peekInt64() const
  {
    assert(readableBytes() >= sizeof(int64_t));
    int64_t be64 = 0;
    ::memcpy(&be64, peek(), sizeof be64);
    return sockets::networkToHost64(be64);
  }
  ///
  /// Peek int32_t from network endian
  ///
  /// Require: buf->readableBytes() >= sizeof(int32_t)
  //读取并返回一个Int32
  int32_t peekInt32() const
  {
    assert(readableBytes() >= sizeof(int32_t));
    int32_t be32 = 0;
    ::memcpy(&be32, peek(), sizeof be32);
    return sockets::networkToHost32(be32);
  }
  //读取并返回一个Int16
  int16_t peekInt16() const
  {
    assert(readableBytes() >= sizeof(int16_t));
    int16_t be16 = 0;
    ::memcpy(&be16, peek(), sizeof be16);
    return sockets::networkToHost16(be16);
  }
  //读取并返回一个Int8
  int8_t peekInt8() const
  {
    assert(readableBytes() >= sizeof(int8_t));
    int8_t x = *peek();
    return x;
  }
  ///
  /// Prepend int64_t using network endian
  ///
  //将int64放入prepend
  void prependInt64(int64_t x)
  {
    int64_t be64 = sockets::hostToNetwork64(x);
    prepend(&be64, sizeof be64);
  }
  ///
  /// Prepend int32_t using network endian
  ///

  //将int32放入prepend
  void prependInt32(int32_t x)
  {
    int32_t be32 = sockets::hostToNetwork32(x);
    prepend(&be32, sizeof be32);
  }
  //将int16放入prepend
  void prependInt16(int16_t x)
  {
    int16_t be16 = sockets::hostToNetwork16(x);
    prepend(&be16, sizeof be16);
  }

  //将int8放入prepend
  void prependInt8(int8_t x)
  {
    prepend(&x, sizeof x);
  }
  //将长度为len的data放入到prepend中
  void prepend(const void* /*restrict*/ data, size_t len)
  {
    assert(len <= prependableBytes());
    readerIndex_ -= len;
    const char* d = static_cast<const char*>(data);
    std::copy(d, d+len, begin()+readerIndex_);
  }
  //Requests the container to reduce its capacity to fit its size.
  void shrink(size_t reserve)
  {
    // FIXME: use vector::shrink_to_fit() in C++ 11 if possible.
    Buffer other;
    other.ensureWritableBytes(readableBytes()+reserve);
    other.append(toStringPiece());
    swap(other);
  }
  //返回当前buffer分配的内存可以容纳的字符数
  size_t internalCapacity() const
  {
    return buffer_.capacity();
  }
  /// Read data directly into buffer.
  ///
  /// It may implement with readv(2)
  /// @return result of read(2), @c errno is saved
  ssize_t readFd(int fd, int* savedErrno);
 private:
  //返回缓冲区开始时的指针
  char* begin()
  { return &*buffer_.begin(); }
  const char* begin() const
  { return &*buffer_.begin(); }

  //当内存不够用时，重新分配内存
  void makeSpace(size_t len)
  {
    if (writableBytes() + prependableBytes() < len + kCheapPrepend)
    {
      // FIXME: move readable data
      //分配足够多的内存
      buffer_.resize(writerIndex_+len);
    }
    else
    {
      // move readable data to the front, make space inside buffer
      assert(kCheapPrepend < readerIndex_);
      //先把已有的数据移到前面去，腾出 writable 空间
      size_t readable = readableBytes();
      std::copy(begin()+readerIndex_,
                begin()+writerIndex_,
                begin()+kCheapPrepend);
      readerIndex_ = kCheapPrepend;
      writerIndex_ = readerIndex_ + readable;
      assert(readable == readableBytes());
    }
  }
 private:
  std::vector<char> buffer_;
  size_t readerIndex_;
  size_t writerIndex_;
  static const char kCRLF[];
};
}
}
#endif  // MUDUO_NET_BUFFER_H

---

Buffer.cc

// Copyright 2010, Shuo Chen.  All rights reserved.
// http://code.google.com/p/muduo/
//
// Use of this source code is governed by a BSD-style license
// that can be found in the License file.

// Author: Shuo Chen (chenshuo at chenshuo dot com)
//

#include 

#include 

#include 
#include 

using namespace muduo;
using namespace muduo::net;

const char Buffer::kCRLF[] = "\r\n";

const size_t Buffer::kCheapPrepend;
const size_t Buffer::kInitialSize;


//从fd中读取内容
//返回读到的字节数
ssize_t Buffer::readFd(int fd, int* savedErrno)
{
  // saved an ioctl()/FIONREAD call to tell how much to read
  char extrabuf[65536];
  struct iovec vec[2];
  const size_t writable = writableBytes();
  vec[0].iov_base = begin()+writerIndex_;
  vec[0].iov_len = writable;
  vec[1].iov_base = extrabuf;
  vec[1].iov_len = sizeof extrabuf;
  // when there is enough space in this buffer, don't read into extrabuf.
  // when extrabuf is used, we read 128k-1 bytes at most.
  const int iovcnt = (writable < sizeof extrabuf) ? 2 : 1;
  const ssize_t n = sockets::readv(fd, vec, iovcnt);
  if (n < 0)
  {
    *savedErrno = errno;
  }
  else if (implicit_cast(n) <= writable)
  {
    writerIndex_ += n;
  }
  //如果buffer缓冲区未装下，把extrabuf中的内容添加进buffer
  else
  {
    writerIndex_ = buffer_.size();
    append(extrabuf, n - writable);
  }
  // if (n == writable + sizeof extrabuf)
  // {
  //   goto line_30;
  // }
  return n;
}