1. 套接字选项概述

有很多方法来获取和设置套接字的选项, 以影响套接字行为:

• getsockopt和setsocketopt;
• fcntl;
• ioctl;

2. getsockopt和setsockopt

2个函数仅用于套接字, 分别用于获取和设置套接字选项

#include <sys/types.h>          /* See NOTES */
#include <sys/socket.h>

int getsockopt(int sockfd, int level, int optname, void *optval, socklen_t *optlen);
int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen);

• 参数
  sockfd 一个打开的套接字描述符
  level 级别, 指定系统中解释选项的代码或为通用套接字代码, 或为某个特定于协议的代码(e.g. IPv4/6, TCP, STCP)
  optval 指向某个变量(optval)的指针, setsockopt从optval中取得选项待设置的新值, getsockopt则把已获得的选项当前值存放到*optval中
  optlen 对setsockopt是值参数, 对getsockopt是值-结果参数, 用于指明optval的大小

套接字选项粗分为2个基本类型: 启用或进制某个特性的二元选项, 称为标志选项; 取得并返回我们可以设置或检查的特定值的选项, 称为值选项.

套接字层和IP层的套接字选项汇总表:

说明:

1. 数据类型形如linger{}, 表示struct linger;
2. 标有"标志"的列指出一个选项是否为标志选项. 当给这些标志选项调用getsockopt函数时, *optval是一个整数. *optval中返回的值为0, 表示相应的选项被禁止, 不为0表示相应的选项被启用;
3. setsockopt函数需要一个不为0 的optval值来启用选项, 为0 的optval来禁止选项;
4. 如果"标志"列不含"·", 那么相应选项用于在用户进程与系统之间传递所指定数据类型的值;

3. 检查选项是否受支持并获取默认值

思路: 利用getsockopt 获取每个level对应的选项名, 如果没有出错, 则证明支持该选项; 如果出错, 则说明不支持. 如果读取出来的值大小, 跟选项汇总表不一致, 则表明可能有哪里出问题, 打印出来.
一个sockopt的判断伪代码

sockfd = socket();

if (getsockopt(fd, opt_level, opt_name, &val, &len) < 0) {
    // getsockopt error
    ...
}
else {
// no getsockopt error
// print opt_val_str
}

如果IPV6_DONTFRAG, SCTP_AUTOCLOSE等宏定义无法识别, 请确认安装好libsctp-dev, lksctp-tools库后, 然后#include <netinet/sctp.h>
安装命令, 详见 Unix下基于SCTP socket的通信：一对一场景 | 简书

$ sudo apt-get install libsctp-dev lksctp-tools

源代码:

#include <sys/types.h> 
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <netinet/sctp.h>

/* UNP官方代码 */
#include "ourhdr.h" 

union val {
    int i_val;
    long l_val;
    struct linger linger_val;
    struct timeval timeval_val;
} val;

int Socket(int domain, int type, int protocol);
static char *sock_str_flag(union val *, int);
static char *sock_str_int(union val *, int);
static char *sock_str_linger(union val *, int);
static char *sock_str_timeval(union val *, int);

/**
* 自定义结构sock_opts, 包含了获得或输出套接字选项的所有信息
*/
struct sock_opts {
    const char *opt_str; /// 字符名称
    int opt_level; /// 级别
    int opt_name;  /// 名称
    char *(*opt_val_str)(union val *, int); /// 函数指针, 用于输出
} sock_opts[] = {
    {"SO_BROADCAST",  SOL_SOCKET,  SO_BROADCAST,     sock_str_flag},
    {"SO_DEBUG",      SOL_SOCKET,  SO_DEBUG,         sock_str_flag},
    {"SO_DONTROUTE",  SOL_SOCKET,  SO_DONTROUTE,     sock_str_flag},
    {"SO_ERROR",      SOL_SOCKET,  SO_ERROR,         sock_str_int },
    {"SO_KEEPALIVE",  SOL_SOCKET,  SO_KEEPALIVE,     sock_str_flag},
    {"SO_LINGER",     SOL_SOCKET,  SO_LINGER,        sock_str_linger},
	{"SO_OOBINLINE",  SOL_SOCKET,  SO_OOBINLINE,     sock_str_flag},
	{"SO_RCVBUF",     SOL_SOCKET,  SO_RCVBUF,        sock_str_int },
	{"SO_SNDBUF",     SOL_SOCKET,  SO_SNDBUF,        sock_str_int },
	{"SO_RCVLOWAT",   SOL_SOCKET,  SO_RCVLOWAT,      sock_str_int },
	{"SO_SNDLOWAT",   SOL_SOCKET,  SO_SNDLOWAT,      sock_str_int },
	{"SO_RCVTIMEO",   SOL_SOCKET,  SO_RCVTIMEO,      sock_str_timeval },
	{"SO_SNDTIMEO",   SOL_SOCKET,  SO_SNDTIMEO,      sock_str_timeval },

	{"SO_REUSEADDR",  SOL_SOCKET,  SO_REUSEADDR,     sock_str_flag },
#ifdef SO_REUSEPORT
	{"SO_REUSEPORT",  SOL_SOCKET,  SO_REUSEPORT,     sock_str_flag},
#else
	{"SO_REUSEPORT",  0,  		   0,     			 NULL},
#endif

	{"SO_TYPE",  	  SOL_SOCKET,  SO_TYPE,       	 sock_str_int },
#ifdef SO_USELOOPBACK
	{"SO_USELOOPBACK",SOL_SOCKET,  SO_USELOOPBACK,   sock_str_flag},
#else
	{"SO_USELOOPBACK",  0,  		   0,     			 NULL},
#endif

	{"IP_TOS",  	  IPPROTO_IP,  IP_TOS,     	     sock_str_int },
	{"IP_TTL",  	  IPPROTO_IP,  IP_TTL,  	     sock_str_int },

	{"IPV6_DONTFRAG", 		IPPROTO_IPV6, IPV6_DONTFRAG,    	 sock_str_flag},
	{"IPV6_UNICAST_HOPS",  	IPPROTO_IPV6, IPV6_UNICAST_HOPS,     sock_str_int },
	{"IPV6_V6ONLY",  		IPPROTO_IPV6, IPV6_V6ONLY,     		 sock_str_flag},
	
	{"TCP_MAXSEG",   IPPROTO_TCP,  TCP_MAXSEG,        sock_str_int },
	{"TCP_NODELAY",  IPPROTO_TCP,  TCP_NODELAY,      sock_str_flag},

	{"SCTP_AUTOCLOSE",  IPPROTO_SCTP,  SCTP_AUTOCLOSE,     sock_str_int },
#ifdef SCTP_MAXBURST
	{"SCTP_MAXBURST",   IPPROTO_SCTP,  SCTP_MAXBURST,      sock_str_int },
#else
	{"SCTP_MAXBURST",  0,  		   0,     			 NULL},
#endif

	{"SCTP_MAXSEG",  	IPPROTO_SCTP,  SCTP_MAXSEG,    	   sock_str_int },
	{"SCTP_NODELAY",    IPPROTO_SCTP,  SCTP_NODELAY,       sock_str_flag},

	{NULL, 				0, 			   0, 				   NULL}
};

int main()
{
    int fd;
    socklen_t len;
    struct sock_opts *ptr;

    for (ptr = sock_opts; ptr->opt_str != NULL; ++ptr) {
        printf("%s: ", ptr->opt_str);
        if (ptr->opt_val_str == NULL) {
            printf("(undefined)\n");
        }
        else {
            switch(ptr->opt_level) {
            case SOL_SOCKET:
            case IPPROTO_IP:
            case IPPROTO_TCP:
                fd = Socket(AF_INET, SOCK_STREAM, 0);
                break;

#ifdef IPV6
            case IPPROTO_IPV6:
                fd = Socket(AF_INET6, SOCK_STREAM, 0);
                break;
#endif
#ifdef IPPROTO_SCTP
            case IPPROTO_SCTP:
                fd = Socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP);
                break;
#endif
            default:
                // err_quit("Can‘t create fd for level %d\n", ptr->opt_level);
				fprintf(stderr, "Can‘t create fd for level %d\n", ptr->opt_level);
            }

            len = sizeof(val);
			
			// if (fd < 0) continue;

            if (getsockopt(fd, ptr->opt_level, ptr->opt_name, &val, &len) == -1) {
                err_ret("getsockopt error");
            }
            else {
                printf("default = %s\n", (*ptr->opt_val_str)(&val, len));
            }

            close(fd);
        }
    }
    
    return 0;
}

int Socket(int domain, int type, int protocol)
{
    int fd = socket(domain, type, protocol);

    if (fd < 0) {
        perror("socket error");
        return -1;
    }

    return fd;
}


static char strres[128];
static char *sock_str_flag(union val *ptr, int len) 
{
    if (len != sizeof(int)) 
        snprintf(strres, sizeof(strres), "size (%d) not sizeof(int)", len);
    else 
        snprintf(strres, sizeof(strres), "%s", (ptr->i_val == 0) ? "off": "on");

    return strres;
}

static char *sock_str_int(union val *ptr, int len)
{
    if (len != sizeof(int)) 
        snprintf(strres, sizeof(strres), "size (%d) not sizeof(int)", len);
    else 
        snprintf(strres, sizeof(strres), "%d\t", ptr->i_val);

    return strres;
}

static char *sock_str_linger(union val *ptr, int len)
{
    if (len != sizeof(struct linger)) 
        snprintf(strres, sizeof(strres), "size (%d) not sizeof(struct linger)", len);
    else 
        snprintf(strres, sizeof(strres), "l_onoff = %d, l_linger = %d\t", ptr->linger_val.l_onoff, ptr->linger_val.l_linger);

    return strres;
}

static char *sock_str_timeval(union val *ptr, int len)
{
    if (len != sizeof(struct timeval)) 
        snprintf(strres, sizeof(strres), "size (%d) not sizeof(struct timeval)", len);
    else 
        snprintf(strres, sizeof(strres), "%ld sec, %ldusec\t", ptr->timeval_val.tv_sec, ptr->timeval_val.tv_usec);

    return strres;
}

运行结果:

SO_BROADCAST: default = off
SO_DEBUG: default = off
SO_DONTROUTE: default = off
SO_ERROR: default = 0	
SO_KEEPALIVE: default = off
SO_LINGER: default = l_onoff = 0, l_linger = 0	
SO_OOBINLINE: default = off
SO_RCVBUF: default = 131072	
SO_SNDBUF: default = 16384	
SO_RCVLOWAT: default = 1	
SO_SNDLOWAT: default = 1	
SO_RCVTIMEO: default = 0 sec, 0usec	
SO_SNDTIMEO: default = 0 sec, 0usec	
SO_REUSEADDR: default = off
SO_REUSEPORT: default = off
SO_TYPE: default = 1	
SO_USELOOPBACK: (undefined)
IP_TOS: default = 0	
IP_TTL: default = 64	
Can‘t create fd for level 41
IPV6_DONTFRAG: getsockopt error: Bad file descriptor
Can‘t create fd for level 41
IPV6_UNICAST_HOPS: getsockopt error: Bad file descriptor
Can‘t create fd for level 41
IPV6_V6ONLY: getsockopt error: Bad file descriptor
TCP_MAXSEG: default = 536	
TCP_NODELAY: default = off
SCTP_AUTOCLOSE: default = 0	
SCTP_MAXBURST: (undefined)
SCTP_MAXSEG: default = size (8) not sizeof(int)
SCTP_NODELAY: default = off

Linux socket编程 套接字选项