EPIPE和ECONNRESET

RST包：

假设Server A上面有Process X，它有一个socket M，和另外的Server B上面的Process Y的 Socket N以TCP协议连接上了，那么，据我所知，有2种情况会出现RST包：

（1）X没有close socket就退出了，然后Y继续向M send数据，A的内核就会发送RST 到 socket N；
（2）X设置了SO_LINGER，其中l_onoff 非0， l_linger 为0，这样当A close socket M的时候，也会发送RST到socket N。

当socket N收到了RST，select的结果为socket可读，则：

（a）如果这个时候调用recv，返回-1，errno为ECONNRESET，如果再次调用recv，返回-1，errno为EPIPE，同事产生EPIPE信号；
（b）如果这个时候调用send，返回-1，errno为EPIPE，同时会产生SIGPIPE信号。

#include    "unp.h"#define MAXBUF 40960void processSignal(int signo)
{printf("Signal is %d\n", signo);signal(signo, processSignal);
}ssize_t                        /* Write "n" bytes to a descriptor. */
writen(int fd, const void *vptr, size_t n)
{size_t         nleft;ssize_t         nwritten;const char    *ptr;ptr = vptr;nleft = n;while (nleft > 0) {printf("Begin Writen %d\n", nleft);if ( (nwritten = write(fd, ptr, nleft)) <= 0) {if (nwritten < 0 && errno == EINTR) {printf("intterupt\n");nwritten = 0;        /* and call write() again */}elsereturn(-1);            /* error */}nleft -= nwritten;ptr += nwritten;printf("Already write %d, left %d, errno=%d\n", nwritten, nleft, errno);}return(n);
}
/* end writen */void
Writen(int fd, void *ptr, size_t nbytes)
{if (writen(fd, ptr, nbytes) != nbytes)err_sys("writen error");
}void
str_cli(FILE *fp, int sockfd)
{char sendline[MAXBUF], recvline[MAXBUF];while (1) {memset(sendline, 'a', sizeof(sendline));printf("Begin send %d data\n", MAXBUF);Writen(sockfd, sendline, sizeof(sendline));sleep(5);}
}int
main(int argc, char **argv)
{int                     sockfd;struct sockaddr_in     servaddr;signal(SIGPIPE, SIG_IGN);//signal(SIGPIPE, processSignal);if (argc != 2)err_quit("usage: tcpcli [port]");sockfd = Socket(AF_INET, SOCK_STREAM, 0);bzero(&servaddr, sizeof(servaddr));servaddr.sin_family = AF_INET;servaddr.sin_port = htons(atoi(argv[1]));Inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr);Connect(sockfd, (SA *) &servaddr, sizeof(servaddr));str_cli(stdin, sockfd);        /* do it all */exit(0);
}

NC模拟服务端

测试1：忽略SIGPIPE信号，writen之前，对方关闭接受进程

本机服务端：   nc -l -p 30000
本机客户端:
./client 30000
   Begin send 40960 data
   Begin Writen 40960
   Already write 40960, left 0, errno=0
   Begin send 40960 data
   Begin Writen 40960
   Already write 40960, left 0, errno=0
   执行到上步停止服务端,client会继续显示:Begin send 40960 data
   Begin Writen 40960
   writen error: Broken pipe(32)
结论:可见write之前，对方socket中断，发送端write会返回-1,errno号为EPIPE(32)

测试2：catch SIGPIPE信号，writen之前，对方关闭接受进程

修改客户端代码，catch sigpipe信号
//signal(SIGPIPE, SIG_IGN);
signal(SIGPIPE, processSignal);
本机服务端：
nc -l -p 30000
本机客户端:
./client 30000Begin send 40960 data
Begin Writen 40960
Already write 40960, left 0, errno=0
Begin send 40960 data
Begin Writen 40960
Already write 40960, left 0, errno=0
执行到上步停止服务端,client会继续显示:
Begin send 40960 data
Begin Writen 40960
Signal is 13
writen error: Broken pipe(32)
结论:可见write之前，对方socket中断，发送端write时，会先调用SIGPIPE响应函数，然后write返回-1,errno号为EPIPE(32)

测试3 writen过程中，对方关闭接受进程

为了方便操作，加大1次write的数据量，修改MAXBUF为4096000

本机服务端：
nc -l -p 30000
本机客户端:
./client 30000Begin send 4096000 data
Begin Writen 4096000
执行到上步停止服务端,client会继续显示:
Already write 589821, left 3506179, errno=0
Begin Writen 3506179
writen error: Connection reset by peer(104)

结论:
可见socket write中，对方socket中断，发送端write会先返回已经发送的字节数,
再次write时返回-1,errno号为ECONNRESET(104)

具体解释：

The client's call to readline may happen before the server's RST is received by the client, or it may happen after. If the readline happens before the RST is received, as we've shown in our example, the result is an unexpected EOF in the client. But if the RST arrives first, the result is an ECONNRESET ("Connection reset by peer") error return from readline.

What happens if the client ignores the error return from readline and writes more data to the server? This can happen, for example, if the client needs to perform two writes to the server before reading anything back, with the first write eliciting the RST.

The rule that applies is: When a process writes to a socket that has received an RST, the SIGPIPE signal is sent to the process. The default action of this signal is to terminate the process, so the process must catch the signal to avoid being involuntarily terminated.
If the process either catches the signal and returns from the signal handler, or ignores the signal, the write operation returns EPIPE.
以上解释了测试1，2的现象,write一个已经接受到RST的socket，系统内核会发送SIGPIPE给发送进程，如果进程catch/ignore这个信号，write都返回EPIPE错误.

因此,UNP建议应用根据需要处理SIGPIPE信号，至少不要用系统缺省的处理方式处理这个信号，系统缺省的处理方式是退出进程，这样你的应用就很难查处处理进程为什么退出。