C/C ++-通过套接字发送time_t的最安全方法 [英] c/c++ - safest way to send time_t over socket

问题描述

我已经建立了一个C ++服务器/客户端环境，并试图将time_t值从服务器发送到客户端(在任何服务器中都是有用的).但是我很头疼:time_t似乎不在任何大小规格范围内.我想知道通过网络发送time_t的最安全(更可移植)的方法是什么.

I've set up a C++ server/client environment, and am trying to send a time_t value from the server to the client (an useful thing in any server). But I'm coming accross a headache: time_t seems to not be under any size specifications. I'm wondering what is the safest (more portable) way to send time_t over the network.

这是一个小例子:

time_t T = time(NULL);
unsigned char * P = (unsigned char *)&T;
// ... Convert it to network byte order, etc.

// Here, 's' would be the socket, and 'S'
// the size of the data that is going to
// be sent
send(s, P, S, 0);

我有两个问题:

• time_t大小不固定
• time_t可以是有符号或无符号整数数据类型的typedef

如何处理这两个问题，以便可以在(几乎)任何体系结构之间安全地发送它?

How can I handle that two issues so that I can send it safely between (almost) any architecture?

谢谢.

看到相同的三个答案后，我发现最好在这里做出答复.抱歉，我之前没有对此进行澄清.我更喜欢以纯"字节发送它，尽管以字符串形式发送它不是问题.据我了解，我需要再次牢记主机系统中time_t数据类型的签名和大小，不是吗? (谢谢，抱歉)

After seeing the same three answers, I find it better to response here. I'm sorry I did not clarify that before. I prefer sending it in 'pure' bytes, although sending it as a string is not a problem. As far as I understand, I would need to keep in mind (again) the signedness and the size of time_t data type in the host system, isn't it? (thanks and sorry)

推荐答案

首先，我认为您需要确定是否要处理C标准使time_t的含义过于含糊的问题(不一定)以秒为单位表示，甚至没有任何有意义的数值属性(例如订单/比较).这与每个现有和历史实施的行为相反，其中time_t以秒为单位. C和POSIX都允许time_t为浮点类型；据我所知，没有实现可以利用它，而在POSIX上这将是相当有害的，因为time_t的 value 必须是它在struct timespec中使用的方式的整数，依此类推

First, I think you need to decide if you want to deal with the issue that the C standard leaves the meaning of time_t overly vague (it's not necessarily represented in seconds and doesn't even have any meaningful numerical properties like order/comparison). This is contrary to the behavior of every existing and historical implementation, where time_t is in seconds. Both C and POSIX also allow time_t to be a floating point type; as far as I know, no implementations make use of this, and on POSIX it would be rather harmful since the value of time_t has to be an integer the way it's used in struct timespec, etc.

如果您认为自纪元以来time_t始终是整数秒数，即 values 对于系统之间的交换有意义，则感到满意，那么只需格式化它们即可.最安全的做法是简单地将其强制转换为一个整数类型，该类型足够大以存储任何有意义的值，并且在所有系统上的大小都相同:int64_t.然后使用不影响字节序差异的正常方式使用int64_t进行序列化.

If you decide you're happy assuming time_t is always an integral number of seconds since the epoch, i.e. the values are meaningful for interchange between systems, then it's just a matter of formatting them. The safest thing to do would be to simply cast to a integer type that's large enough to store any meaningful value and that's the same size on all systems: that would be int64_t. Then use whatever normal means you use for serializing int64_t in a way that's immune to endian differences.

另一方面，如果您希望绝对"便携，则应为"epoch"(标准的或您自己选择的epoch)计算自己的time_t值，然后使用difftime转换为代表距纪元后的秒数"的双精度，并用snprintf(buf, sizeof buf, "%.0f", diff)格式化double.请注意，在便携式C中为时期计算time_t值实际上是非常困难的，因为大多数标准函数都在本地时间工作，而您则需要通用时间.您可以使用gmtime，mktime和localtime函数来解决这些问题，但这并不容易...

If on the other hand you want to be "absolutely" portable, you should compute your own time_t value for "the epoch" (either the standard one or your own choice of epoch), then use difftime to convert to a double representing "seconds since the epoch", and format the double with snprintf(buf, sizeof buf, "%.0f", diff). Note that computing a time_t value for the epoch in portable C is actually quite difficult because most of the standard functions work in local time, whereas you need universal time. There are tricks you can do with the gmtime, mktime, and localtime functions to figure it out, but it's nontrivial...

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