zmq::message_t 发送后可以重复使用吗? [英] Can zmq::message_t's be reused after being sent?

问题描述

我正在使用 ZeroMQ 来实现一个玩具通信协议；这是我第一次使用这个框架/库.

I'm using ZeroMQ to implement a toy communications protocol; and this is my first time using this framework/library.

现在，在我的协议中，某一方发送了多条连续的消息，所有消息的大小都相同.所以 - 我想，我会避免重新分配它们，而只是尝试用不同的内容重新填充消息数据缓冲区，例如:

Now, in my protocol, multiple consecutive messages get sent by a certain party, all of which have the same size. So - I thought, I'd avoid reallocating them, and just try to refill the message data buffer with different content, e.g.:

zmq::message_t msg { fixed_common_size };
while (some_condition()) {
    my_filling_routine(msg.data(), fixed_common_size);
    the_socket.send(msg);
}

但是在这个循环的第二次迭代中，我得到了一个分段错误；msg.data() 虽然不是 nullptr.我突然想到，也许 ZeroMQ 以某种方式蚕食了内存，因此我需要编写如下内容:

but on the second iteration of this loop, I get a segmentation fault; msg.data() is not nullptr though. It occurred to me, that maybe ZeroMQ cannibalizes the memory somehow, and thus I need to write something like:

zmq::message_t msg { fixed_common_size };
char buffer[fixed_common_size];
while (some_condition()) {
    my_filling_routine(buffer, fixed_common_size);
    msg.rebuild(buffer, fixed_common_size);
    the_socket.send(msg);
}

但我确信这会导致取消分配和重新分配.

But I'm sure this causes de-allocation and re-allocation.

那么，rebuild() 真的是必要的，还是只是我代码中的一些错误?

So, is it really the case that a rebuild() is necessary, or could it just be some bug in my code?

注意:我使用的是 Unix 套接字，以防万一答案取决于它.

Note: I'm using Unix sockets, in case the answer depends on that somehow.

推荐答案

不，zmq::message_t 发送后不能重复使用.

首先，欢迎来到 ZeroMQ，这是一个很酷的地方，分布式系统从业者.

ZeroMQ API 非常明确地警告这些问题.成功调用 zmq_msg_send()，引用消息有效负载，并不意味着消息本身已实际发送.

The ZeroMQ API warns about these issues quite explicitly. A successful call to zmq_msg_send(), referencing a message-payload, does not mean that the message itself has been actually sent.

你最好试着想象调用只是将责任从你的应用程序代码转移"到ZeroMQ引擎(在工厂内部，在中实例化了一个IO线程池)Context()-instance ...).

It's best you try to imagine that the call just "moves" the responsiblity from your application code to the ZeroMQ engine (inside the factory with a pool-of-IOthreads instantiated inside the Context()-instance ... ).

传递给 zmq_msg_send() 的 zmq_msg_t 结构在打电话.

The zmq_msg_t structure passed to zmq_msg_send() is nullified during the call.

和下一个更好的永远不要触碰它，如上所述:o)

and next better never touch it anytime later as reported above :o)

分配器最大的生态"旁路是重用整个消息，如:

The maximum "ecological"-bypass of allocator is to re-use the whole-message, as :

如果您想将相同的消息发送到多个套接字，您必须使用(例如使用 zmq_msg_copy() )进行复制.

If you want to send the same message to multiple sockets you have to copy it using ( e.g. using zmq_msg_copy() ).

<小时>

复制警告...

再加一点:可以复制，但以后敢尝试修改...

---

A bonus warning on copying ...

plus one more raised point: may copy, but dare to try to modify it after this ...

避免在使用 zmq_msg_copy() 复制消息后修改消息内容，这样做会导致未定义的行为.如果您需要的是实际的硬拷贝，请使用 zmq_msg_init_size() 分配新消息并使用 memcpy() 复制消息内容.

永远不要直接访问 zmq_msg_t 成员，而是始终使用 zmq_msg 函数系列.

Avoid modifying message content after a message has been copied with zmq_msg_copy(), doing so can result in undefined behaviour. If what you need is an actual hard copy, allocate a new message using zmq_msg_init_size() and copy the message content using memcpy().

Never access zmq_msg_t members directly, instead always use the zmq_msg-family of functions.

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