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来源：http://blog.21ic.com/user1/2613/archives/2008/48341.html
这个是单个，先弄懂，然后再弄懂多个，在该链接下面有少量的介绍
Asynchronous Input 异步输入模式

＃i nclude <termios.h>    
＃i nclude <stdio.h>      
＃i nclude <unistd.h>      
＃i nclude <fcntl.h>      
＃i nclude <sys/signal.h>    
＃i nclude <sys/types.h>      
      
#define BAUDRATE B38400      
#define MODEMDEVICE "/dev/ttyS1"      

#define _POSIX_SOURCE 1 /* POSIX compliant source */      
#define FALSE 0      
#define TRUE 1      

volatile int STOP=FALSE;
void signal_handler_IO (int status);   /* definition of signal handler */  
                                     // 定义信号处理程序
int wait_flag=TRUE;                   /* TRUE while no signal received */    
                                    // TRUE 代表没有受到信号，正在等待中  
main()   {        
int fd,c, res;  
struct termios oldtio,newtio;  
struct sigaction saio;        
/* definition of signal action */      
// 定义信号处理的结构
char buf[255];        

/* open the device to be non-blocking (read will return immediatly) */    
// 是用非阻塞模式打开设备 read 函数立刻返回，不会阻塞    
    fd = open(MODEMDEVICE, O_RDWR &#124; O_NOCTTY &#124; O_NONBLOCK);    
    if (fd <0) {perror(MODEMDEVICE); exit(-1); }        

/* install the signal handler before making the device asynchronous */    
    // 在进行设备异步传输前，安装信号处理程序    
    saio.sa_handler = signal_handler_IO;    
saio.sa_mask = 0;  
saio.sa_flags = 0;    
saio.sa_restorer = NULL;    
    sigaction(SIGIO,&saio,NULL);  

/* allow the process to receive SIGIO */
// 允许进程接收 SIGIO 信号      
fcntl(fd, F_SETOWN, getpid());    
    /* Make the file descriptor asynchronous (the manual page says only  
O_APPEND and O_NONBLOCK, will work with F_SETFL...) */  
// 设置串口的文件描述符为异步，man上说，只有 O_APPEND 和 O_NONBLOCK 才能使用F_SETFL
fcntl(fd, F_SETFL, FASYNC);        
tcgetattr(fd,&oldtio); /* save current port settings */  

    /* set new port settings for canonical input processing */  
// 设置新的串口为标准输入模式      
newtio.c_cflag = BAUDRATE &#124; CRTSCTS &#124; CS8 &#124; CLOCAL &#124; CREAD;      
newtio.c_iflag = IGNPAR &#124; ICRNL;    
newtio.c_oflag = 0;      
newtio.c_lflag = ICANON;  
newtio.c_cc[VMIN]=1;    
    newtio.c_cc[VTIME]=0;    
    tcflush(fd, TCIFLUSH);    
tcsetattr(fd,TCSANOW,&newtio);    

/* loop while waiting for input. normally we would do something      
useful here 循环等待输入，通常我们会在这里做些其它的事情 */
while (STOP==FALSE) {      
  printf(".\n");usleep(100000);        
  /* after receiving SIGIO, wait_flag = FALSE, input is availableand can be read */
  // 在收到 SIGIO 信号后，wait_flag = FALSE, 表示有输入进来，可以读取了
  if (wait_flag==FALSE) {        
   res = read(fd,buf,255);    
   buf[res]=0;        
   printf(":%s:%d\n", buf, res);  
   if (res==1) STOP=TRUE; /* stop loop if only a CR was input */    
   wait_flag = TRUE;      /* wait for new input 等待新的输入*/  
       }      
  }      
/* restore old port settings */    
tcsetattr(fd,TCSANOW,&oldtio);  
}            

/***************************************************************************    
* signal handler. sets wait_flag to FALSE, to indicate above loop that    *
* characters have been received.                                          *
***************************************************************************/  

// 信号处理函数，设置 wait_flag 为 FALSE, 以告知上面的循环函数串口收到字符了  
void signal_handler_IO (int status)   {  
printf("received SIGIO signal.\n");    
wait_flag = FALSE;  
}

作者：jackxiang@向东博客 专注WEB应用 构架之美 --- 构架之美，在于尽态极妍 | 应用之美，在于药到病除
地址：http://jackxiang.com/post/1805/
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