了解set/getsockopt SO_SNDBUF大小加倍 [英] Understanding set/getsockopt SO_SNDBUF size doubles

问题描述

我有以下程序来检查UDP套接字的发送缓冲区大小.但是，我的返回值让我有些困惑.我使用以下简单的应用程序:

Hi I have the following program to check the send buffer size for a UDP socket. However, I the return value is a bit confusing to me. I use the following simple app:

#include <sys/socket.h>
#include <stdio.h>

int main(int argc, char **argv)
{
 int sockfd, sendbuff;
 socklen_t optlen;

 sockfd = socket(AF_INET, SOCK_DGRAM, 0);
 if(sockfd == -1)
     printf("Error");

 int res = 0;

 // Get buffer size
 optlen = sizeof(sendbuff);
 res = getsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &sendbuff, &optlen);

 if(res == -1)
     printf("Error getsockopt one");
 else
     printf("send buffer size = %d\n", sendbuff);

 // Set buffer size
 sendbuff = 98304;

 printf("sets the send buffer to %d\n", sendbuff);
 res = setsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &sendbuff, sizeof(sendbuff));

 if(res == -1)
     printf("Error setsockopt");


 // Get buffer size
 optlen = sizeof(sendbuff);
 res = getsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &sendbuff, &optlen);

 if(res == -1)
     printf("Error getsockopt two");
 else
     printf("send buffer size = %d\n", sendbuff);

 return 0;
}

我的机器上的输出是:

发送缓冲区大小= 129024

send buffer size = 129024

将发送缓冲区设置为98304

sets the send buffer to 98304

新发送缓冲区的大小= 196608

new send buffer size = 196608

任何人都可以在这里弄清楚我在做什么错或如何解释输出吗?

Can anybody clarify what I'm doing wrong here or how to interpret the output?

推荐答案

您没有做错任何事情.在设置时，Linux将值加倍(在内核内)，并在查询时返回加倍的值. man 7 socket说:

You're not doing anything wrong. Linux doubles the value (within the kernel) when you set it, and returns the doubled value when you query it. man 7 socket says:

[...]

    SO_SNDBUF
              Sets or gets the maximum socket send buffer in bytes.  The  ker-
              nel doubles this value (to allow space for bookkeeping overhead)
              when it is set using setsockopt(), and  this  doubled  value  is
              returned  by  getsockopt().   The  default  value  is set by the
              wmem_default sysctl and the maximum allowed value is set by  the
              wmem_max sysctl.  The minimum (doubled) value for this option is
              2048.
[...]

NOTES
       Linux assumes that half of the send/receive buffer is used for internal
       kernel structures; thus the sysctls are twice what can be  observed  on
       the wire.
[...]

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