這兩個函數是最通用的I/O函數。實際上我們甚至可以說用recvmsg來代替read、readv、recv和recvfrom。sendmsg也類似。
需要的頭文件
#include<sys/socket.h>
函數原型
ssize_t recvmsg(int sockfd,struct msghdr *msg,int flags);
ssize_t sendmsg(int sockfd,struct msghdr *msg,int flags);
返回值
出錯 -1
參數說明
sockfd 套接字描述符
flags對套接字接口設置
這兩個函數把大部分參數封裝到一個msghdr結構中:
struct msghdr {
void *msg_name;
socklen_t msg_namelen;
struct iovec *msg_iov;
int msg_iovlen;
void *msg_control;
socklen_t msg_controllen;
int msg_flags;
};
msg_name指向一個套接字結構地址
msg_namelen套接字結構地長度
msg_iov輸入輸出緩衝區地址
msg_iovlen緩衝區個數
msg_control輔助數據地址
msg_controllen輔助數據長度
msg_flags接收消息的標識
參數使用
msg_name和msg_namelen用於保存當前使用的協議的地址,比如使用了tcp協議、udp協議、UNIX domain協議等.
struct iovc{
void* io_base;//緩衝區空間的基地址
size_t iov_len;//緩衝區空間的長度
}
msg_control字段的也是指向一段內存,msg_controllen是指該內存的總大小長度,通常該內存被用來存儲輔助數據,輔助數據可用於一些特殊的處理。msg_control通常指向一個控制消息頭部,其結構體如下所示:
struct cmsghdr {
socklen_t cmsg_len; //包含該頭部的數據長度
int cmsg_level; //具體的協議標識
int cmsg_type; //協議中的類型
}
舉例說明
客戶端
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string.h>
int main(int argc, char *argv[])
{
int sockfd;
struct sockaddr_in servaddr;
sockfd=socket(AF_INET,SOCK_STREAM,0);
if(sockfd<0)
{
printf("Socket created failed.\n");
return -1;
}
servaddr.sin_family=AF_INET;
servaddr.sin_port=htons(9966);
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
printf("connecting...\n");
if(connect(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr))<0)
{
printf("Connect server failed.\n");
return -1;
}
struct msghdr msg;
struct iovec iov[2];
msg.msg_name = &servaddr;
msg.msg_namelen = sizeof(servaddr);
msg.msg_iov = iov;
msg.msg_iovlen = 2;
iov[0].iov_base = "hello\0";
iov[0].iov_len = strlen("hello\0");
iov[1].iov_base = "anyone\0";
iov[1].iov_len = strlen("anyone\0");
msg.msg_control = 0;
msg.msg_controllen = 0;
msg.msg_flags = 0;
int ret = sendmsg(sockfd, &msg, 0);
printf("%d\n",ret);
printf("%s\n",(msg.msg_iov+1)->iov_base);
return 0;
}
服務端
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string.h>
int main()
{
int listenfd, connfd;
struct sockaddr_in servaddr, cliaddr;
socklen_t len;
struct msghdr msg;
struct iovec iov;
char buf[1024];
struct sockaddr_in *cli_msg;
cli_msg = (struct sockaddr_in*)malloc(sizeof(struct sockaddr_in));
msg.msg_name = &cli_msg;
msg.msg_namelen = sizeof(cli_msg);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
iov.iov_base = buf;
iov.iov_len = 1024;
msg.msg_control = 0;
msg.msg_controllen = 0;
msg.msg_flags = 0;
listenfd=socket(AF_INET, SOCK_STREAM,0);
if(listenfd<0)
{
printf("Socket created failed.\n");
return -1;
}
servaddr.sin_family=AF_INET;
servaddr.sin_port=htons(9966);
servaddr.sin_addr.s_addr=htonl(INADDR_ANY);
if(bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr))<0)
{
printf("bind failed.\n");
return -1;
}
printf("listening....\n");
listen(listenfd, 5);
while(1)
{
len = sizeof(cliaddr);
connfd=accept(listenfd,(struct sockaddr *)&cliaddr,&len);
int n = recvmsg(connfd, &msg, 0);
cli_msg = (struct sockaddr_in*)msg.msg_name;
char ip[16];
inet_ntop(AF_INET, &(cli_msg->sin_addr), ip, sizeof(ip));
printf("recv %s is %d \n",ip, n);
printf("%s\n", msg.msg_iov->iov_base);
close(connfd);
}
}