作为进程间通信及网络通信的一种重要技术，在实际的开发中，socket编程是经常被用到的。关于socket编程的一般步骤，这里不再赘述，相关资料和文章很多，google/baidu即可。
本文主要是探讨如何更好地进行socket读写处理，以及如何检测连接断开。
首先，有以下几点需要注意：
- 对于全双工的socket，同时读写是没问题的。比如，一个socket程序有两个线程，一个线程对socket进行读操作(recv/read)，一个线程对socket进行写操作(send/write)，这里是不需要进行互斥操作或做临界区保护的。
- 在Unix系统下， 对一个对端已经关闭的socket调用两次write，第二次将会生成SIGPIPE信号，该信号的默认处理动作是终止进程。为了防止在这种情况下导致进程退出，我们需要屏蔽该信号的默认处理动作。有两种方法，a) 在程序开头调用signal(SIGPIPE, SIG_IGN)，忽略SIGPIPE信号的默认动作；b) 采用send函数的MSG_NOSIGNAL标志位，忽略SIGPIPE信号。当然，虽然我们忽略了SIGPIPE，但errno还是会被设置为EPIPE的。因此，我们可以根据这点，按照我们的情况来进行相应的处理了。
- 对文件描述符进行select/poll操作，a) 如果select/poll返回值大于0，此时进行recv/read，recv/read返回0，表明连接关闭; b) recv/read返回-1，并且errno为ECONNRESET、EBADF、EPIPE、ENOTSOCK之一时，也表明连接关闭。
- 对文件描述符进行send/write操作，如果send/write返回-1，并且errno为ECONNRESET、EBADF、EPIPE、ENOTSOCK之一时，也表明连接关闭。
下面是一个demo程序，server端代码(server.c)为：
#define _GNU_SOURCE
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <poll.h>
#include <pthread.h>
#include <errno.h> #define UNIX_PATH_MAX 108
#define SOCK_PATH "/tmp/test.sock" int sock_write(int fd, char* buf, int len)
{
int err_num, res;
char err_str[];
int disconnect = ; while(!disconnect){
/* Use send with a MSG_NOSIGNAL to ignore a SIGPIPE signal which
* would cause the process exit. */
res = send(fd, buf, len, MSG_NOSIGNAL | MSG_DONTWAIT);
if( - == res ){
err_num = errno;
printf("send error:%s!\n", strerror_r(err_num, err_str, sizeof(err_str))); switch(err_num){
case ECONNRESET:
case EBADF:
case EPIPE:
case ENOTSOCK:
disconnect = ;
break;
//case EWOULDBLOCK:
case EAGAIN:
usleep();
break;
case EINTR:
break;
default:
break;
}
}else if( res > ){
if( res < len ){
/* Incomplete information. */
buf += res;
len -= res;
}else if( res == len ){
/* Message has sended successfully. */
break;
}
}
}
return disconnect;
} void* write_handle(void* fd)
{
int len, connection_fd;
char buffer[];
char* end_flag = "\r\n"; connection_fd = *(int*)fd;
len = snprintf(buffer, , "buffer data ends with a end flag%s", end_flag);
buffer[len] = ; while( == sock_write(connection_fd, buffer, len)){
sleep();
}
} int main(void)
{
struct sockaddr_un address;
int socket_fd, connection_fd;
socklen_t address_length; socket_fd = socket(PF_UNIX, SOCK_STREAM, );
if(socket_fd < ) {
printf("%s:socket() failed\n", SOCK_PATH);
return -;
}
unlink(SOCK_PATH);
memset(&address, , sizeof(struct sockaddr_un));
address.sun_family = AF_UNIX;
snprintf(address.sun_path, UNIX_PATH_MAX, SOCK_PATH);
if(bind(socket_fd, (struct sockaddr *) &address, sizeof(struct sockaddr_un)) != )
{
printf("%s:bind() failed\n", SOCK_PATH);
return -;
}
if(listen(socket_fd, ) != )
{
printf("%s:listen() failed\n", SOCK_PATH);
return -;
}
while((connection_fd = accept(socket_fd, (struct sockaddr *) &address,&address_length)) > -)
{
pthread_t w_thread;
struct pollfd pfd;
char buffer[];
int nbytes;
int cnt;
int res;
int err_num;
int disconnect=;
#ifdef DEBUG
char str[];
#endif printf("\n\n%s:accept a connection!\n", SOCK_PATH); pthread_create(&w_thread, NULL, write_handle, &connection_fd); pfd.fd = connection_fd;
pfd.events = POLLIN | POLLHUP | POLLRDNORM;
pfd.revents = ; while(){
res = poll(&pfd, , );
if(res >= ){
// if result > 0, this means that there is either data available on the
// socket, or the socket has been closed
cnt = recv(connection_fd, buffer, sizeof(buffer), MSG_PEEK | MSG_DONTWAIT);
if( == cnt ){
if(res > ){
// if recv returns zero, that means the connection has been closed.
#ifdef DEBUG
printf("connection disconnect 1!\n");
#endif
disconnect = ;
break;
}
}else if( - == cnt ){
err_num = errno;
#ifdef DEBUG
printf("recv error:%s!\n", strerror_r(errno, str, sizeof(str)));
#endif
switch(err_num){
case ECONNRESET:
case EBADF:
case EPIPE:
case ENOTSOCK:
disconnect = ;
break;
default:
break;
}
if( disconnect ){
#ifdef DEBUG
printf("connection disconnect 2!\n");
#endif
break;
}
}else if( cnt > ){
/* discard everything received from client.*/
while((nbytes = recv(connection_fd, buffer, sizeof(buffer)-, MSG_DONTWAIT)) > ){
buffer[nbytes] = ;
#ifdef DEBUG
printf("buffer:%s\n", buffer);
#endif
}
#ifdef DEBUG
if( == nbytes ){
printf("All received!\n");
}
#endif
}
}
#ifdef DEBUG
else if(res == -){
/* This case shouldn't happen, we sleep 5 seconds here in case and retry it. */
printf("Error: poll return -1!!!\n");
sleep();
}
#endif
}
close(connection_fd);
pthread_cancel(w_thread);
} close(socket_fd);
unlink(SOCK_PATH);
return ;
}
client端代码(client.c)为:
#define _GNU_SOURCE
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include <string.h>
#include <pthread.h>
#include <poll.h>
#include <errno.h> #define UNIX_PATH_MAX 108
#define SOCK_PATH "/tmp/test.sock" int main(void)
{
struct sockaddr_un address;
int socket_fd, res;
char buffer[];
pthread_t w_thread;
struct pollfd pfd;
int err_num;
int disconnect; while(){
socket_fd = socket(PF_UNIX, SOCK_STREAM, );
if(socket_fd < )
{
printf("%s:socket() failed\n", SOCK_PATH);
sleep();
continue;
}
memset(&address, , sizeof(struct sockaddr_un));
address.sun_family = AF_UNIX;
snprintf(address.sun_path, UNIX_PATH_MAX, SOCK_PATH);
if(connect(socket_fd, (struct sockaddr *) &address, sizeof(struct sockaddr_un)) != )
{
printf("%s:connect() failed\n", SOCK_PATH);
close(socket_fd);
socket_fd = -;
sleep();
continue;
}
#ifdef DEBUG
printf("connect success!\n");
#endif
#if 1
pfd.fd = socket_fd;
pfd.events = POLLIN | POLLHUP | POLLRDNORM;
pfd.revents = ;
disconnect = ;
while(){
// use the poll system call to be notified about socket status changes
res = poll(&pfd, , );
if(res >= ){
// if result > 0, this means that there is either data available on the
// socket, or the socket has been closed
char buffer[];
int cnt, nbytes;
cnt = recv(socket_fd, buffer, sizeof(buffer)-, MSG_PEEK | MSG_DONTWAIT);
if( - == cnt){
err_num = errno;
switch(err_num){
case ECONNRESET:
case EBADF:
case EPIPE:
case ENOTSOCK:
disconnect = ;
break;
default:
break;
}
if(disconnect){
break;
}
}
else if( == cnt){
if(res > ){
// if recv returns zero, that means the connection has been closed:
disconnect = ;
break;
}
}
else if( cnt > ){
while((nbytes = recv(socket_fd, buffer, sizeof(buffer)-, MSG_DONTWAIT)) > ){
buffer[nbytes] = ;
printf("buffer:%s\n", buffer);
}
}
}
}
#endif
close(socket_fd);
socket_fd = -;
#ifdef DEBUG
printf("server disconnect!\n");
#endif
/* here sleep 5 seconds and re-connect. */
sleep();
} return ;
}