OpenRTMFP/Cumulus Primer（10）IO 管理之流缓冲区

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OpenRTMFP/Cumulus Primer（10）IO 管理之流缓冲区

Author: 柳大·Poechant（钟超）
Email: zhongchao.ustc#gmail.com (#->@)
Blog: Blog.CSDN.net/Poechant
Date: April 24^th, 2012

本文主要分析 MemoryStream.h 文件中定义的类。

1 了解 std::streambuf

首先要了解 streambuf 内置了一个 get 指针和一个 put 指针。streambuf 的所有操作基本都是对这两个指针的操作。其一些成员函数的缩写中的 g 和 p 就分别表示 get pointer 和 put pointer。

1.1 单步移动内置指针

Increase get pointer: Advances the get pointer by n positions. The get pointer is the internal pointer that points to the next location in the controlled input sequence.
```
  void gbump ( int n );
```
Increase put pointer: Advances the put pointer by n positions. The put pointer is the internal pointer that points to the next location of the controlled output sequence.
```
  void pbump ( int n );
```

1.2 获取 get 指针和 put 指针的位置

Pointer to current position of input sequence: Returns a reference to the current element of the controlled input sequence (i.e., the “get pointer”).
```
  char * gptr ( ) const;
```
Pointer to current position of output sequence: Returns a reference to the current element of the output sequence (the put pointer).
```
  char * pptr ( ) const;
```

1.3 设置 get 和 put 指针可达区域的上下界

Set input sequence pointers: Sets values for the pointers that define both the boundaries of the accessible part of the controlled input sequence and the get pointer itself.
```
  void setg ( char* gbeg, char* gnext, char* gend );
```

gbeg: New value for the pointer to the beginning of the accessible part of the controlled input sequence.
gnext: New value for the get pointer, which points to the next element within the controlled input sequence where the next input operation shall be performed.
gend: New value for the end pointer, just past the end of the accessible part of the controlled input sequence.

Set output sequence pointers: Sets the values that define the boundaries of the accessible part of the controlled output sequence.
```
  void setp ( char* pbeg, char* pend );
```

pbeg: New value for the pointer to the beginning of the accessible part of the controlled output sequenceand put pointer.
pend: New value for the end pointer, just past the end of the accessible part of the controlled output sequence.

2 MemoryStreamBuf

类定义：

class MemoryStreamBuf: public std::streambuf {    friend class ScopedMemoryClip;public:    MemoryStreamBuf(char* pBuffer,Poco::UInt32 bufferSize);    MemoryStreamBuf(MemoryStreamBuf&);    ~MemoryStreamBuf();    void            next(Poco::UInt32 size); // Explaint below    Poco::UInt32    written(); // Explaint below    void            written(Poco::UInt32 size);    Poco::UInt32    size();  // Explaint below    void            resize(Poco::UInt32 newSize); // Explaint below    char*           begin(); // Explaint below    void            position(Poco::UInt32 pos=0); // Explaint below    char*           gCurrent(); // Explaint below    char*           pCurrent(); // Explaint belowprivate:    virtual int overflow(int_type c);    virtual int underflow();    virtual int sync();    Poco::UInt32    _written;    char*           _pBuffer;    Poco::UInt32    _bufferSize;    MemoryStreamBuf();    MemoryStreamBuf& operator = (const MemoryStreamBuf&);};

ScopedMemoryClip 是 MemoryStreamBuf 的友元，其内部有 MemoryStreamBuf 的成员，这里暂且不管。构造函数传入的参数是缓冲区的地址和缓冲区大小（字节数）。拷贝构造函数和析构函数不必赘述。

2.1 移动内置的 get 和 put 指针：

put 和 get 指针都移动：

void MemoryStreamBuf::next(UInt32 size) {    pbump(size);    gbump(size);}

2.2 获取 get 和 put 指针当前位置：

封装 streambuf 的 gptr 和 pptr：

inline char* MemoryStreamBuf::gCurrent() {    return gptr();}inline char* MemoryStreamBuf::pCurrent() {    return pptr();}

2.3 获取缓冲区的起始位置和大小：

依赖于内置成员变量 pBuffer 和 bufferSize：

inline char* MemoryStreamBuf::begin() {    return _pBuffer;}inline Poco::UInt32 MemoryStreamBuf::size() {    return _bufferSize;}

2.4 缓冲区的已写字节数

读取（其中也可能发生设置操作）：

UInt32 MemoryStreamBuf::written() {    int written = pCurrent() - begin(); // 已写字节数    if (written < 0)        written = 0;    if (written > _written) // 保存已写字节数        _written = (UInt32)written;    return _written;}

设置：

void MemoryStreamBuf::written(UInt32 size) {    _written=size;}

2.5 显式设定 put 和 get 指针位置

设定 put 和 get 指针为以缓冲区首地址为开始偏移量为 pos 的位置：

void MemoryStreamBuf::position(UInt32 pos) {    // 保存已写字节数    written(); // Save nb char written    // 移动 put 指针    setp(_pBuffer, _pBuffer + _bufferSize);    if (pos > _bufferSize)        pos = _bufferSize;    pbump((int) pos);    // 移动 get 指针    setg(_pBuffer, _pBuffer + pos, _pBuffer + _bufferSize);}

2.6 修改缓冲区大小

void MemoryStreamBuf::resize(UInt32 newSize) {    // 大小标识    _bufferSize = newSize;    // gptr 当前位置    int pos = gCurrent() - _pBuffer;    if (pos > _bufferSize) pos = _bufferSize;    // 设置 gptr 可达范围和当前位置    setg(_pBuffer, _pBuffer + pos, _pBuffer + _bufferSize);     // pptr 当前位置    pos = pCurrent() - _pBuffer;    if (pos > _bufferSize) pos = _bufferSize;    // 设置 pptr 可达范围和当前位置    setp(_pBuffer,_pBuffer + _bufferSize);        pbump(pos);}

2.7 构造函数、拷贝构造函数和析构函数

构造函数会设定 pptr 和 gptr，并初始化 pBuffer 和 bufferSize

MemoryStreamBuf::MemoryStreamBuf(char* pBuffer, UInt32 bufferSize):     _pBuffer(pBuffer),_bufferSize(bufferSize),_written(0) {    setg(_pBuffer, _pBuffer,_pBuffer + _bufferSize);    setp(_pBuffer, _pBuffer + _bufferSize);}

析构函数会拷贝对方的 pBuffer、bufferSizse、_written，并设定 gptr、pptr。注意设定 pptr 时，要分别调用 setp 和 pbump，因为 setp 仅将 pptr 设定为传入的首个参数值（与可达范围的首地址相同）。

MemoryStreamBuf::MemoryStreamBuf(MemoryStreamBuf& other):   _pBuffer(other._pBuffer),_bufferSize(other._bufferSize),_written(other._written) {    setg(_pBuffer, other.gCurrent(), _pBuffer + _bufferSize);    setp(_pBuffer, _pBuffer + _bufferSize);    pbump((int)(other.pCurrent()-_pBuffer));}

析构函数：

MemoryStreamBuf::~MemoryStreamBuf() {}

Reference

http://www.cplusplus.com/reference/iostream/streambuf/gbump/
http://www.cplusplus.com/reference/iostream/streambuf/pbump/

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