streambuf定义了对流缓冲区操作的接口，主要分为输出缓冲流和输入缓冲流。streambuf有两种用法，一是直接使用各个接口，二是继承并实现新的I/O channels。

1. 输出缓冲流:

• pbase(): put base，输出缓冲流的首指针。
• pptr(): put pointer，当前可写位置
• epptr(): end put pointer，输出缓冲流的尾指针+1，即one past the last character。

函数：
setp(char new_pbase, char new_epptr)**
注：设置pbase,pptr,epptr三个指针的位置：

• pbase()=new_pbase;
• pptr()=bew_pbase;
• epptr()=new_epptr;

pbump(int offset)
注： 移动pptr offset个位置。当我们写入num个数据，pbump(num)。

overflow
当缓冲区满时的操作；
pptr() == epptr()

2. 输入缓冲流:

• eback(): 指向输入缓冲流的首指针
• gptr(): 当前输入缓冲流的位置
• egptr(): 输入缓冲流的末尾
• gbump(): 移动gptr
• setg(): 设置eback/gptr/egptr的位置

underflow()
当无缓冲区数据可读时。
cond: gptr()=egptr()

setg
setg (eback_base,new_gptr, new_egptr)
分别设置：

• eback
• gptr
• egptr

3. 基于streambuf自定义一个输入输出缓冲流

prebufferstream.hpp:

/*  Author: zhangwen1@unionpay.com */
/*  Date: 2020-11-24               */

/*
 * Provide Custom Streambufeer Example 
*/ 

#ifndef PREBUFFERSTREAM_HPP
#define PREBUFFERSTREAM_HPP

#include <streambuf>
#include <cstdio>
#include <exception>

class prebuffer: public std::streambuf{

public:
  typedef std::char_traits<char> traits_ty;
  typedef traits_ty::int_type int_type;
  typedef traits_ty::pos_type pos_type;
  typedef traits_ty::off_type off_type;

private:
  int m_mode;
  int m_filedes;

  static const int s_buf_size = 4092;
  static const int s_pback_size = 4;
  char buffer[s_buf_size];

  void init(int fd, int om, bool throw_exception);

  int flushoutput();

public:

  prebuffer(int fd, int om, bool throw_exception=false);
  prebuffer(const prebuffer&)=delete;
  prebuffer& operator=(const prebuffer&)=delete;
  prebuffer(prebuffer&&)=delete;
  prebuffer& operator=(prebuffer&&)=delete;

  ~prebuffer(){close();}

  virtual int_type overflow(int_type c) override;
  virtual int_type underflow() override;

  bool is_open() const { return m_filedes >= 0; }

  void close();
  virtual int sync() override;

  std::string get_str(){
    return std::string(buffer,this->pptr()-buffer);
  }

};


class FileError: public std::exception{
  const char* what(void) const throw(){
    return "Invaild fd";
  }
};

#endif

prebufferstream.cc:

#include "prebufferstream.hpp"

#include <iostream>
#include <unistd.h>
#include <cstring>

void prebuffer::init(int fd, int om, bool throw_exception){
  m_filedes = fd;
  if (m_filedes > 0){
    m_mode = om;
  }
  if (throw_exception && !is_open()){
      throw FileError();
  }
  setg(buffer+s_pback_size,
       buffer+s_pback_size,
       buffer+s_pback_size);
  setp(buffer,
       buffer+s_buf_size-1);
}

prebuffer::prebuffer(int fd, int om, bool throw_exception){
  init(fd,om,throw_exception);
}

void prebuffer::close(){
  if (is_open()){
    this->sync();
    m_mode = 0;
    m_filedes = -1;
  }
}

prebuffer::int_type prebuffer::underflow(){

  if (gptr() < egptr()){
    return static_cast<unsigned char>(*gptr());
  }
  int num_putback=0;
  if (s_pback_size > 0){
    num_putback = gptr()-eback();
    if (num_putback > s_pback_size){
      num_putback = s_pback_size;
    }
    std::memcpy(buffer+s_pback_size-num_putback,gptr()-num_putback,num_putback);
  }

  const int num = ::read(m_filedes,buffer+s_pback_size,s_buf_size-s_pback_size);

  if (num <= 0){return EOF;}
  setg(buffer+s_pback_size-num_putback,buffer+s_pback_size,buffer+s_pback_size+num);

  return static_cast<unsigned char>(*gptr());

}

prebuffer::int_type prebuffer::overflow(int_type c){
  std::cout<<"Overflow Happend"<<std::endl;
  if (!(m_mode & std::ios::out) || !is_open()) return EOF;
  if (c != EOF){
    *pptr() = c;
    pbump(1);
  }
  if (flushoutput() == EOF)
    {
      return -1; 
    }

  return c;
}

int prebuffer::flushoutput(){
  if (!(m_mode & std::ios::out) || !is_open()) return EOF;
  size_t num = pptr()-pbase();
  if (::write(m_filedes,buffer,num) == -1){
    return EOF;
  }
  pbump(-num);
  return num;
}

int prebuffer::sync(){
  if (flushoutput() == EOF){
    return -1;
  }
  return 0;
}

测试, main.cc：

#include "prebufferstream.hpp"
#include <fcntl.h> 
#include <ostream>
#include <istream>
#include <unistd.h>
#include <iostream>

int main(int argc,char* argv[]){

  std::string file_name1 = "test";
  std::string file_name2 = "intest";
  int fdout;
  int fdin;
  if ((fdout=open(file_name1.c_str(),O_RDWR | O_CREAT | O_APPEND)) < 0){
    std::cout<<"ERROR Open File"<<std::endl;
  }
  if ((fdin=open(file_name2.c_str(),O_RDONLY | O_CREAT | O_APPEND)) < 0){
    std::cout<<"ERROR Open File"<<std::endl;
  }


  prebuffer ot{fdout,std::ios::out};
  prebuffer it{fdin,std::ios::in};
  std::ostream dout(&ot);
  std::istream dint(&it);
  dout<<1<<2<<3<<4<<5<<"\n";

  std::cout<<ot.get_str()<<std::endl;

  ot.sync();
  int line;
  while (dint>>line){
    std::cout<<line<<" ";
  }

  close(fdout);
  close(fdin);

  return 0;
}