通过C中的TCP通信，您如何指示在不知道有关数据的任何先验信息的情况下停止对请求的read()调用? [英] Over TCP communication in C, how can you indicate to stop calling read() for a request without knowing any prior information about the data?

问题描述

我目前正在用C对服务器进行编程，该服务器使用TCP通过套接字进行通信.客户端应该发送{文件名\ n} + {文件内容\ n}，服务器将存储该信息，然后在成功时发送响应.但是，我们不能确定客户端是否会实际上以结构化协议发送适当的信息.

I am currently programming a server in C that communicates over sockets using TCP. The client is supposed to send {filename\n} + {file contents\n} and the server will store that information and then send a response on success. However, we can't be sure the client will actually send the proper information in a structured protocol.

通常在更简单的情况下，我们知道之前要发送指定数量的字节，并且可以等待直到达到指定的数量.在这种情况下，我们不需要，当前代码如下所示:

Often in simpler situations, we know a specified amount of bytes to be sent before and can wait until that specified number has been reached. In this case, we don't and currently the code looks like this:

//buffer is a resize-able array, 
//assume it's correct + don't worry about memory leakage for now
resizeablearray_t *buffer;

char data[255];
char retry = 1; 
while (retry) {
   int bytes = read(socketfd, data, 255);
   if (bytes <= 0) {
      fprintf(stderr, "Error %s\n", strerror(errno));
      return;
   }
   push(buffer, data, bytes);
}

因此，我们遇到了一个巨大的问题:我们如何指示c中的read()函数我们已经读取了先前调用中的所有信息，我们不应该再次调用它?

Therefore, we're given a huge problem: how do we indicate to the read() function in c that we've read in all the information on the prior call and we shouldn't call it again?

read()函数将阻塞，直到需要通过服务器读取的字节为止.但是，在我们的模型中，如果由于计数不足而继续尝试读取，并且缓冲区中没有剩余，我们将永远等待.

The read() function blocks until there are bytes to be read over the server. However, in our model, if we continue to attempt to read due to short counts and there is nothing left in the buffer, we will wait FOREVER.

关于如何在另一个read()调用之前指示任何信息的想法，而无需任何有关传入消息及其结构的先验信息，我们可以从while循环中断开并停止尝试从套接字读取消息?

Any ideas on how to indicate before another read() call, without any prior information on the incoming message and its structure, that we can break from the while loop and stop attempting to read from the socket?

推荐答案

在现代文件传输协议中，有一个指示文件的大小，或更直接地，指示事务的有效负载中有多少个字节.这使另一端可以知道将要传送多少字节.如果没有提供足够的字节，则读取器需要继续读取.交付完所有字节后，就不会出现您所询问的额外"调用.

In modern file transfer protocols, there is an indication of the size of the file, or more directly, how many bytes in the payload of the transaction. This allows the other side to know how many bytes will be delivered. If not enough bytes are delivered, the reader needs to keep reading. When all the bytes are delivered, there is no "extra" call to read in the sense you are asking about.

即使发送方正在动态创建数据，因此也不知道在事务开始时总共要发送多少个字节，它也至少可以报告当前发送中要发送多少个字节.完成后，它可以发送指示已完成发送的最终报告.接收者会阅读直到看到最终报告.

Even if the sender is creating data dynamically, and thus doesn't know how many bytes are going to be sent in total at the beginning of the transaction, it can at least report how many bytes are being delivered in the current send. When it is done, it can send a final report that indicates it is done sending. The receiver reads until it sees this final report.

如果使用recv而不是read调用，则可以传递标志以指示您不希望永远阻止等待，而是希望立即获得是否还有剩余的状态读.但是，这是确定传输是否完成的不可靠方法.发件人可能很慢，文件的其余部分很可能会在一段时间后出现.

If you use the recv call instead of read, you will be able to pass in flags to indicate that you do not wish to block waiting forever, and instead want an immediate status of whether there is anything left to read. However, this is an unreliable method to determine if the transfer is complete. The sender could just be slow, and the rest of the file may very well come some time later.

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