From http://stackoverflow.com/questions/2031109/understanding-set-getsockopt-so-sndbuf

readme

 the function the follow  utility：
 1. inquire socket buffer before set socket buffer
 2. set socket buffer
 3. get the info after set buffer
 
 conclusion:  a. the get value of socket buffer is double of the set value.
 b. if set value > the max of os max  value ,  the value we get is double *maxsocketbuffer of os
 for example  
   if max of os socket buffer is 200000 bytes and we set value is 3000000 , 
   then we get the value is 400000 not 600000.

selectScoketBuffer.c

#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;
}