sendmsg() doesn't function as is expected - sockets

Question

I wrote a program with a simple pair of client and server,the first of which generate a bunch of strings and sends them via API provided by socket,namely sendmsg(),and the second of which receive the strings with facilitation of epoll.
According to what's captured by the tcpdump and the result of client execution,the client is able to connect to the server and send a few messages correctly,but only a few,the following strings failed to be recieved completely,and at the end of server print of the received codes there is infinite dupilcation of the same string segment.
Please contact me if any more detail is in need.
BTW,is there any recommendation of references which provide detailed description about the usage of sendmsg()?
This is main codes of client:
void MultithreadedLogAnalyzer::SendToServer(string Addr,uint16_t Port) {
int connfd = socket(AF_INET,SOCK_STREAM,0);
sockaddr_in servaddr;
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(Port);
inet_pton(AF_INET,Addr.c_str(),&servaddr.sin_addr);
/*connect failure process to be added*/
int st = connect(connfd,(struct sockaddr *)&servaddr,sizeof(sockaddr));
if(-1 == st){
perror("connection failed");
}
Handle(connfd,servaddr);
close(connfd);
}
void MultithreadedLogAnalyzer::Handle(int connfd,const sockaddr_in &servaddr){
int n= 0,counter =0;
for(vector<string>::iterator si = mFilterResult.begin();si != mFilterResult.end(); ++si)
{
msghdr msg;
iovec iov;
memset(&msg,0,sizeof(msg));
memset(&iov,0,sizeof(iov));
msg.msg_control = NULL;
msg.msg_controllen = 0;
msg.msg_flags = 0;
char data[1024];
memset(data,0, 1024);
memcpy(data,si->c_str(),si->size());
iov.iov_base = data;
iov.iov_len = (*si).size() ;//check first to locate error
cout << "size:" << (*si).size() << endl;
msg.msg_name = NULL;
msg.msg_namelen = 0;
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
//msg.msg_accrights = NULL;
//msg.msg_accrightslen = 0;
n = sendmsg(connfd,&msg,0);
cout << "n: " << n << endl;
++counter;
if (-1 == n)
perror("sendmsg error\n");
}
cout << "number of strings send: " << counter << endl;
The codes of server follows as below:
#include <sys/socket.h>
#include <stdlib.h>
#include <iostream>
#include <string.h>
#include <fcntl.h>
#include <stdio.h>
#include <strings.h>
#include <netinet/in.h>
#include <sys/epoll.h>
#define POLLSIZE 100
#define STRINGSIZE 300
using namespace std;
void handle(int fd){
int c = 0;
do{
msghdr msg;
memset(&msg,0,sizeof(msghdr));
msg.msg_name = NULL;
msg.msg_namelen = 0;
char data[1024];
iovec iov;
iov.iov_base = data;
iov.iov_len = 1024;
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
msg.msg_control = NULL;
msg.msg_controllen = 0;
msg.msg_flags = 0;
c = recvmsg(fd,&msg,0);
if(msg.msg_iovlen==0
){
return;
}
cout << "msg length:" << msg.msg_iovlen<< endl;
cout << string((char *)(msg.msg_iov[0].iov_base)) << endl;
}while(c != -1);
}
int main() {
int listenfd;
listenfd = socket(AF_INET, SOCK_STREAM, 0);
sockaddr_in servaddr,cliaddr;
socklen_t socklen = 0;
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
int serverPort = 2000;
servaddr.sin_port = htons(serverPort);
bind(listenfd,(struct sockaddr *)&servaddr,sizeof(servaddr));
int listennumnber = 20;
listen(listenfd,listennumnber);
//the rest to be commented
int epfd = epoll_create(POLLSIZE);
if (epfd < 0)
perror("epoll_create");
struct epoll_event ev;
ev.events = EPOLLIN|EPOLLET;
ev.data.fd = listenfd;
if(epoll_ctl(epfd,EPOLL_CTL_ADD,listenfd,&ev) < 0)
perror("epoll_ctl");
int curfds = 1;
struct epoll_event *events = (struct epoll_event*)malloc(sizeof(struct epoll_event));
for (;;){
int n;
int nfds = epoll_wait(epfd,events,curfds,-1);
if (-1 == nfds)
continue;
for(n = 0;n < nfds; ++n){
if(events[n].data.fd == listenfd){
int connfd = accept(listenfd,(struct sockaddr *)&cliaddr ,&socklen);
cout << "connect established through connfd: " << connfd << endl;
if (connfd < 0)
continue;
// if(setnonblocking(connfd) < 0)
// perror("set setnonblocking error");
ev.events = EPOLLIN | EPOLLET;
ev.data.fd = connfd;
if(epoll_ctl(epfd,EPOLL_CTL_ADD,connfd,&ev) < 0)
return -1;
curfds++;
continue;
}
handle(events[n].data.fd);
}
}
return 0;
}
Problem solved by adding send() and recv() functions in appropriate places of the message transferring loop.

Answer 1

cout << "msg length:" << msg.msg_iovlen<< endl;
cout << string((char *)(msg.msg_iov[0].iov_base)) << endl;
This message should only print msg.msg_iov[0].iov_len bytes.
}while(c != -1);
This is in the wrong place. It should be a test immediately after c = recvmsg(...); it should be if (c > 0), and if c == -1 you should either call perror() or use errno or strerror() in a message, immediately, without calling any other system calls.
Basically you're continuing to receive beyond end of stream; you're printing junk after end of stream or an error; and you're ignoring the actual message length when printing the message.