可以为posix recv设置超时会导致丢失udp数据包吗? [英] Can setting a timeout for posix recv cause lost udp packets?
问题描述
我找到了此答案有关如何设置posix套接字超时的信息.答案的linux部分:
I found this answer on how to set a timeout for posix socket. The linux part of that answer:
// LINUX
struct timeval tv;
tv.tv_sec = timeout_in_seconds;
tv.tv_usec = 0;
setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO, (const char*)&tv, sizeof tv);
以及posix文档中的引言:
and the quote from the posix documentation:
SO_RCVTIMEO
设置超时值，该值指定输入功能完成之前等待的最长时间.它接受一个时间间隔 具有秒数和微秒数的结构，用于指定 限制等待输入操作完成的时间.如果一个 接收操作已阻塞了这么长时间而没有接收 附加数据，则应返回部分计数或将errno设置为 如果未接收到数据，则为[EAGAIN]或[EWOULDBLOCK].此的默认值 option为零，表示接收操作不应 暂停.此选项采用timeval结构.请注意，并非全部 实施允许设置此选项.
Sets the timeout value that specifies the maximum amount of time an input function waits until it completes. It accepts a timeval structure with the number of seconds and microseconds specifying the limit on how long to wait for an input operation to complete. If a receive operation has blocked for this much time without receiving additional data, it shall return with a partial count or errno set to [EAGAIN] or [EWOULDBLOCK] if no data is received. The default for this option is zero, which indicates that a receive operation shall not time out. This option takes a timeval structure. Note that not all implementations allow this option to be set.
我不明白的是:这会导致udp软件包丢失吗? 如果在收到udp软件包时达到超时怎么办?
What I dont understand is: Can this cause loosing udp packages? What if the timeout is reached while a udp package is received?
PS:我知道UDP本质上是不可靠的，所以我的问题实际上主要是关于在处理udp消息时发生超时的情况.
PS: I am aware that UDP is inherently unreliable, so my question is really mainly about the case where the timeout occurs while an udp message is processed.
推荐答案
否；它不会使您更有可能丢弃数据包.
No; it doesn't make you more likely to drop packets.
了解较低级别的网络传输是如何发生的；您有一个网卡.当此卡接收数据时，无论您的程序在做什么，它都会将数据存储到其自己的存储区中.当您致电recv时；您要求操作系统将数据从网卡内存移至程序内存.这意味着，如果您的线程正在执行其他操作时有数据包进入；它不仅被删除，而且在您的线程下次获取数据时进行处理.
Looking at how network transport happens at a lower level; you have a network card. As this card receives data, irrespective of what your program is doing, it stores the data into it's own memory area. When you call recv; you're asking the OS to move data from the network cards memory to your programs memory. This means that if a packet comes in while your thread is doing something else; it isn't just dropped, but processed the next time your thread comes to get data.
如果您的线程没有足够频繁地调用recv；则网卡的内存将已满.发生这种情况时，将无法存储新的数据包；如果使用的是TCP，则路由器将被告知无法处理.如果是UDP，则将其丢弃.正是这一部分使UDP本质上不可靠，因为它可能在数据包传输期间的任何时候发生.
If your thread doesn't call recv often enough; then the memory for the network card will become full. When this happens no new packets can be stored; and if it's using TCP then the router will be told that it's not able to process it; if it's UDP then it will simply be dropped. It is this part that makes UDP inherently unreliable as it can happen at any point during the transport of the packet.
超时会影响线程等待数据在网卡存储区中显示的时间；除非您再也不会致电recv；不会影响丢包.
The timeout impacts how long the thread will wait for data to appear in the networkcard memory area; and unless you never call recv again; does not impact dropped packets or not.
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