muduo庫源碼學習(base)FileUtil

class ReadSmallFile : boost::noncopyable
{
 public:
  ReadSmallFile(StringArg filename);
  ~ReadSmallFile();

  // return errno
  template<typename String>
  int readToString(int maxSize,//最大的長度
                   String* content,//dest
                   int64_t* fileSize,//文件大小
                   int64_t* modifyTime,//修改大小
                   int64_t* createTime); //創建時間

  /// Read at maxium kBufferSize into buf_
  // return errno
  int readToBuffer(int* size);

  const char* buffer() const { return buf_; }

  static const int kBufferSize = 64*1024;

 private:
  int fd_;
  int err_;
  char buf_[kBufferSize];
};

// read the file content, returns errno if error happens.
template<typename String>
int readFile(StringArg filename,//讀取文件，是對ReadSmallFile的一個包裝。
             int maxSize,
             String* content,
             int64_t* fileSize = NULL,
             int64_t* modifyTime = NULL,
             int64_t* createTime = NULL)
{
  ReadSmallFile file(filename);
  return file.readToString(maxSize, content, fileSize, modifyTime, createTime);
}

// not thread safe
class AppendFile : boost::noncopyable
{
 public:
  explicit AppendFile(StringArg filename);

  ~AppendFile();

  void append(const char* logline, const size_t len);

  void flush();

  size_t writtenBytes() const { return writtenBytes_; }

 private:

  size_t write(const char* logline, size_t len);

  FILE* fp_;
  char buffer_[64*1024];
  size_t writtenBytes_;
};

FileUtil::AppendFile::AppendFile(StringArg filename)
  : fp_(::fopen(filename.c_str(), "ae")),  // 'e' for O_CLOEXEC//這裏用了fopen
    writtenBytes_(0)
{
  assert(fp_);
  ::setbuffer(fp_, buffer_, sizeof buffer_);//設置緩衝區
  // posix_fadvise POSIX_FADV_DONTNEED ?
}

FileUtil::AppendFile::~AppendFile()
{
  ::fclose(fp_);
}

void FileUtil::AppendFile::append(const char* logline, const size_t len)
{
  size_t n = write(logline, len);//調用fwrite
  size_t remain = len - n;
  while (remain > 0)
  {
    size_t x = write(logline + n, remain);
    if (x == 0)
    {
      int err = ferror(fp_);
      if (err)
      {
        fprintf(stderr, "AppendFile::append() failed %s\n", strerror_tl(err));
      }
      break;
    }
    n += x;
    remain = len - n; // remain -= x
  }

  writtenBytes_ += len;
}

void FileUtil::AppendFile::flush()//立刻刷新
{
  ::fflush(fp_);
}

size_t FileUtil::AppendFile::write(const char* logline, size_t len)
{
  // #undef fwrite_unlocked
  return ::fwrite_unlocked(logline, 1, len, fp_);
}

FileUtil::ReadSmallFile::ReadSmallFile(StringArg filename)
  : fd_(::open(filename.c_str(), O_RDONLY | O_CLOEXEC)),//構造函數打開文件
    err_(0)
{
  buf_[0] = '\0';
  if (fd_ < 0)
  {
    err_ = errno;
  }
}

FileUtil::ReadSmallFile::~ReadSmallFile()
{
  if (fd_ >= 0)//文件描述符有效則關閉
  {
    ::close(fd_); // FIXME: check EINTR
  }
}

// return errno
template<typename String>
int FileUtil::ReadSmallFile::readToString(int maxSize,//將文件讀到content
                                          String* content,
                                          int64_t* fileSize,
                                          int64_t* modifyTime,
                                          int64_t* createTime)
{
  BOOST_STATIC_ASSERT(sizeof(off_t) == 8);
  assert(content != NULL);
  int err = err_;
  if (fd_ >= 0)
  {
    content->clear();

    if (fileSize)//那爲什麼content就不檢查呢？
    {
      struct stat statbuf;//獲取文件信息
      if (::fstat(fd_, &statbuf) == 0)
      {
        if (S_ISREG(statbuf.st_mode))//是文件
        {
          *fileSize = statbuf.st_size;//文件大小
          content->reserve(static_cast<int>(std::min(implicit_cast<int64_t>(maxSize), *fileSize)));//預留大小。取文件和可讀的max的min
        }
        else if (S_ISDIR(statbuf.st_mode))//是目錄
        {
          err = EISDIR;
        }
        if (modifyTime)//保存修改時間
        {
          *modifyTime = statbuf.st_mtime;
        }
        if (createTime)//創建時間
        {
          *createTime = statbuf.st_ctime;
        }
      }
      else
      {
        err = errno;
      }
    }

    while (content->size() < implicit_cast<size_t>(maxSize))//也就是上面的比較結果，可讀的最大值超過文件大小，那就嘗試讀到最大
    {
		//思想就是儘量多地讀
      size_t toRead = std::min(implicit_cast<size_t>(maxSize) - content->size(), sizeof(buf_));
      ssize_t n = ::read(fd_, buf_, toRead);
      if (n > 0)//讀出n字節後，追加到content
      {
        content->append(buf_, n);
      }
      else//n==0表示讀到文件末尾
      {
        if (n < 0)//出錯
        {
          err = errno;
        }
        break;
      }
    }
  }
  return err;
}

int FileUtil::ReadSmallFile::readToBuffer(int* size)
{
  int err = err_;
  if (fd_ >= 0)
  {
    ssize_t n = ::pread(fd_, buf_, sizeof(buf_)-1, 0);//讀文件到buf,size是實際讀出的大小。@4是文件偏移遊標。
    if (n >= 0)
    {
      if (size)
      {
        *size = static_cast<int>(n);
      }
      buf_[n] = '\0';
    }
    else
    {
      err = errno;
    }
  }
  return err;
}

template int FileUtil::readFile(StringArg filename,
                                int maxSize,
                                string* content,
                                int64_t*, int64_t*, int64_t*);

template int FileUtil::ReadSmallFile::readToString(
    int maxSize,
    string* content,
    int64_t*, int64_t*, int64_t*);

#ifndef MUDUO_STD_STRING
template int FileUtil::readFile(StringArg filename,
                                int maxSize,
                                std::string* content,
                                int64_t*, int64_t*, int64_t*);

template int FileUtil::ReadSmallFile::readToString(
    int maxSize,
    std::string* content,
    int64_t*, int64_t*, int64_t*);
#endif