| ; 0, TIME serves as a timeout value. The read will be satisfied if a single character is read, or TIME is exceeded (t = TIME *0.1 s). If TIME is exceeded, no character will be returned.
If MIN > 0 and TIME > 0, TIME serves as an inter-character timer. The read will be satisfied if MIN characters are received, or the time between two characters exceeds TIME. The timer is restarted every time a character is received and only becomes active after the first character has been received.
If MIN = 0 and TIME = 0, read will be satisfied immediately. The number of characters currently available, or the number of characters requested will be returned. According to Antonino (see contributions), you could issue a fcntl(fd, F_SETFL, FNDELAY); before reading to get the same result.
By modifying newtio.c_cc[VTIME] and newtio.c_cc[VMIN] all modes described above can be tested.
在非标准输入模式中，输入的数据并不组合成行，也不会进行 erase, kill, delete 等输入处理。我们只是用两个参数来控制这种模式的输入行为： c_cc[VTIME] 设定字符输入间隔时间的计时器，而 c_cc[VMIN] 设置满足读取函数的最少字节数。
MIN > 0, TIME = 0 ： 读取函数在读到了 MIN 值的字符数后返回。
MIN = 0, TIME > 0 ： TIME 决定了超时值，读取函数在读到一个字节的字符，或者等待读取时间超过 TIME （t = TIME * 0.1s）以后返回，也就是说，即使没有从串口中读到数据，读取函数也会在 TIME 时间后返回。
MIN > 0, TIME > 0 ： 读取函数会在收到了 MIN 字节的数据后，或者超过 TIME 时间没收到数据后返回。此计时器会在每次收到字符的时候重新计时，也只会在收到第一个字节后才启动。
MIN = 0, TIME = 0 ： 读取函数会立即返回。实际读取到的字符数，或者要读到的字符数，会作为返回值返回。根据 Antonino（参考 conditions）, 可以使用 fcntl(fd, F_SETFL, FNDELAY), 在读取前获得同样的结果。
改变了 nettio.c_cc[VTIME] 和 newtio.c_cc[VMIN], 就可以测试以上的设置了。
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <stdio.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;
main() {
int fd,c, res;
struct termios oldtio,newtio;
char buf[255];
fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY );
if (fd <0) {perror(MODEMDEVICE); exit(-1); }
tcgetattr(fd,&oldtio); /* save current port settings */
bzero(&newtio, sizeof(newtio));
newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;
newtio.c_iflag = IGNPAR;
newtio.c_oflag = 0;
/* set input mode (non-canonical, no echo,...) */
// 设置输入模式为非标准输入
newtio.c_lflag = 0;
newtio.c_cc[VTIME] = 0; /* inter-character timer unused */
// 不是用字符间隔计时器
newtio.c_cc[VMIN] = 5; /* blocking read until 5 chars received */
//收到5个字符数以后，read 函数才返回
tcflush(fd, TCIFLUSH);
tcsetattr(fd,TCSANOW,&newtio);
while (STOP==FALSE) { /* loop for input */
res = read(fd,buf,255); /* returns after 5 chars have been input */
buf[res]=0; /* so we can printf... */
printf(":%s:%d\n", buf, res);
if (buf[0]==''''z'''') STOP=TRUE;
}
tcsetattr(fd,TCSANOW,&oldtio);
}
3.3. Asynchronous Input 异步输入模式
#include <termios.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/signal.h>
#include <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 | O_NOCTTY | 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
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