SO_SNDBUF 的影响
我无法理解以下代码段如何以及为何工作：
/* Now lets try to set the send buffer size to 5000 bytes */
size = 5000;
err = setsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &size, sizeof(int));
if (err != 0) {
printf("Unable to set send buffer size, continuing with default size\n");
}
如果我们检查发送缓冲区的值，它确实被正确设置为 5000*2 = 10000。 但是，如果我们尝试发送超过发送缓冲区大小的内容，它会发送全部内容。例如：
n = send(sockfd, buf, 30000, 0);
/* Lets check how much us actually sent */
printf("No. of bytes sent is %d\n", n);
打印出 30000。
这到底是如何工作的？发送缓冲区大小限制为 10000 没有任何影响吗？如果确实如此，到底发生了什么？某种碎片化？
更新：如果套接字处于非阻塞模式会发生什么？我尝试了以下操作：
- 将缓冲区大小更改为 10000 (5000*2) 会导致发送 16384 字节
- 将缓冲区大小更改为 20000 (10000*2) 会导致发送 30000 字节
再次，为什么？
I am unable to make sense of how and why the following code segments work :
/* Now lets try to set the send buffer size to 5000 bytes */
size = 5000;
err = setsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &size, sizeof(int));
if (err != 0) {
printf("Unable to set send buffer size, continuing with default size\n");
}
If we check the value of the send buffer, it is indeed correctly set to 5000*2 = 10000.
However, if we try to send more than the send buffer size, it does send all of it. For example:
n = send(sockfd, buf, 30000, 0);
/* Lets check how much us actually sent */
printf("No. of bytes sent is %d\n", n);
This prints out 30000.
How exactly did this work? Didn't the fact that the send buffer size was limited to 10000 have any effect? If it did, what exactly happened? Some kind of fragmentation?
UPDATE: What happens if the socket is in non-blocking mode? I tried the following:
- Changing buffer size to 10000 (5000*2) causes 16384 bytes to be sent
- Changing buffer size to 20000 (10000*2) causes 30000 bytes to be sent
Once again, why?
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SO_SNDBUF选项的效果对于TCP和UDP是不同的。由于看起来您正在谈论 TCP，因此您观察到的效果是通过套接字处于阻塞模式来解释的，因此
send(2)会阻塞，直到内核可以接受所有数据和/或网络堆栈异步地将数据出队并将其推送到网卡，从而释放缓冲区中的空间。另外，TCP 是一种流协议，它不保留任何“消息”结构。一个
send(2)可以对应另一侧的多个recv(2)，反之亦然。将其视为字节流。The effect of setting
SO_SNDBUFoption is different for TCP and UDP.Since it looks like you are talking about TCP, the effect you are observing is explained by the socket being in blocking mode, so
send(2)blocks until kernel can accept all of your data, and/or the network stack asynchronously de-queueing data and pushing it to the network card, thus freeing space in the buffer.Also, TCP is a stream protocol, it does not preserve any "message" structure. One
send(2)can correspond to multiplerecv(2)s on the other side, and the other way around. Treat it as byte-stream.SO_SNDBUF配置套接字实现内部使用的缓冲区。如果您的套接字是非阻塞的，您只能发送配置的大小，如果您的套接字是阻塞的，则您的调用没有限制。SO_SNDBUFconfigures the buffer that the socket implementation uses internally. If your socket is non-blocking you can only send up to the configured size, if your socket is blocking there is no limitation for your call.