通过套接字从 Python 发送对象到 C/Fortran

Question

以下代码是更大软件包的sn-ps。我想了解代码如何管理发送 numpy 数组、单元格等对象。我想知道的是代码似乎没有序列化对象，但它确实有效。为什么会起作用？

Python 客户端代码（sn-p）：

class DriverSocket(socket.socket):

   def __init__(self, _socket_interface):
       super(DriverSocket,self).__init__(_sock=_socket_interface)


   def sendpos(self, pos, cell):
      """Sends the position and cell data to the driver.

      Args:
         pos: An array containing the atom positions.
         cell: A cell object giving the system box.

      Raises:
         InvalidStatus: Raised if the status is not Ready.
      """

      if (self.status & Status.Ready):
         try:
            self.sendall(Message("posdata"))
            self.sendall(cell.h)
            self.sendall(cell.ih)
            self.sendall(np.int32(len(pos)/3))
            self.sendall(pos)
         except:
            self.poll()
            return
      else:
         raise InvalidStatus("Status in sendpos was " + self.status)

在 C (sn-p) 中接收代码：

void open_socket_(int *psockfd, int* inet, int* port, char* host)
/* Opens a socket.

Note that fortran passes an extra argument for the string length, but this is
ignored here for C compatibility.

Args:
   psockfd: The id of the socket that will be created.
   inet: An integer that determines whether the socket will be an inet or unix
      domain socket. Gives unix if 0, inet otherwise.
   port: The port number for the socket to be created. Low numbers are often
      reserved for important channels, so use of numbers of 4 or more digits is
      recommended.
   host: The name of the host server.
*/

{
   int sockfd, portno, n;
   struct hostent *server;

   struct sockaddr * psock; int ssock;

   if (*inet>0)
   {  // creates an internet socket
      struct sockaddr_in serv_addr;      psock=(struct sockaddr *)&serv_addr;     ssock=sizeof(serv_addr);
      sockfd = socket(AF_INET, SOCK_STREAM, 0);
      if (sockfd < 0)  error("Error opening socket");

      server = gethostbyname(host);
      if (server == NULL)
      {
         fprintf(stderr, "Error opening socket: no such host %s \n", host);
         exit(-1);
      }

      bzero((char *) &serv_addr, sizeof(serv_addr));
      serv_addr.sin_family = AF_INET;
      bcopy((char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr, server->h_length);
      serv_addr.sin_port = htons(*port);
      if (connect(sockfd, psock, ssock) < 0) error("Error opening socket: wrong host address, or broken connection");
   }
   else ...



void readbuffer_(int *psockfd, char *data, int* plen)
/* Reads from a socket.

Args:
   psockfd: The id of the socket that will be read from.
   data: The storage array for data read from the socket.
   plen: The length of the data in bytes.
*/

{
   int n, nr;
   int sockfd=*psockfd;
   int len=*plen;

   n = nr = read(sockfd,data,len);

   while (nr>0 && n<len )
   {  nr=read(sockfd,&data[n],len-n); n+=nr; }

   if (n == 0) error("Error reading from socket: server has quit or connection broke");
}

然后是使用 C-socket 代码的 Fortran 代码

CALL open_socket(socket, inet, port, host)
...
CALL readbuffer(socket, msgbuffer, nat*3*8)

而这个接收代码确实得到了一个二维数组等等。反之亦然。

Answer 1

如果您尝试通过send/sendall 发送任意对象，您将看到一个异常，指出需要类似字节。所以想法很简单，你发送的 numpy 结构提供了类似字节的接口，转换为字节只留下原始二进制数据。

最简单的实现是从bytes继承：

class A: pass
sock.sendall(A())  # exception

class B(bytes): pass
sock.sendall(B())  # no exception

但是，numpy 是一个用 Python、Cython 和 C 编写的复杂框架。它们可能使用C API 来提供类似的功能。

还值得注意的是，duck-typing 在这里不起作用：

class C:
    def __bytes__(self):
        return bytes()

sock.sendall(C())  # exception