c - Understanding set/getsockopt SO_SNDBUF size doubles

Question

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
", sendbuff);

 // Set buffer size
 sendbuff = 98304;

 printf("sets the send buffer to %d
", 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
", sendbuff);

 return 0;
}

The output on my machine is:

send buffer size = 129024

sets the send buffer to 98304

new send buffer size = 196608

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

Answer 1

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.
[...]