但是,它似乎是不正确的,它有时可能会返回零,也许是因为文件尚未准备好被读取？在从磁盘读取文件之前,我应该调用select()来查看它是否准备好了吗？

请注意,nBytes是：1,445,888

一些示例代码：

fd_set readFdSet;
timeval timeOutTv;

timeOutTv.tv_sec = 0;
timeOutTv.tv_usec = 0;

// Let's see if we'll block on the read.
FD_ZERO(&readFdSet);
FD_SET(fd,&readFdSet);

int selectReturn = ::select(fd + 1,&readFdSet,NULL,&timeOutTv);

if (selectReturn == 0) {
  // There is still more to read.
  return false; // But return early.
} else if (selectReturn < 0) {
  clog << "Error: select failure: " << strerror(errno) << endl;
  abort();
} else {
  assert(FD_ISSET(fd,&readFdSet));

  try {
    const int bufferSizeAvailable = _bufferSize - _availableIn;

    if (_availableIn) {
      assert(_availableIn <= _bufferSize);

      memmove(_buffer,_buffer + bufferSizeAvailable,_availableIn);
    }

    ssize_t got = ::read(fd,_buffer + _availableIn,bufferSizeAvailable);

    clog << " available: " << bufferSizeAvailable << " availableIn: "
         << _availableIn << " bufferSize: " << _bufferSize << " got "
         << got << endl;

    return got == 0;
  } catch (Err &err) {
    err.append("During load from file.");
    throw;
  }
}

输出读取(失败时没有数据读取)：

available: 1445888 availableIn: 0 bufferSize: 1445888 got: 0

这是使用VMware Server 1.0.10作为虚拟机在centos4 32位上运行的.正在读取的文件系统是虚拟机的本地系统.主机是Windows Server 2008 32位.

uname -a说：

Linux q-centos4x32 2.6.9-89.0.25.ELsmp #1 SMP Thu May 6 12:28:03 EDT 2010 i686 i686 i386 GNU/Linux

我注意到下面给出的链接http://opengroup.org/onlinepubs/007908775/xsh/read.html说明：

The value returned may be less than nbyte if the number of bytes left in the file is less than nbyte,if the read() request was interrupted by a signal...

If a read() is interrupted by a signal before it reads any data,it will return -1 with errno set to [EINTR].

If a read() is interrupted by a signal after it has successfully read some data,it will return the number of bytes read.

所以,也许我得到一个信号中断读取,因此返回的值是零因为一个错误或它认为零字节被读取？