一个例子看不明白

sgb0321

一个例子看不明白 [复制链接]

/*
打问号的地方不明白，希望大家给加下注释，谢谢了
这是一个蜂鸣器的程序（运行在arm9（mini2440）上的Linux下）
*/
#include
#include
#include
#include

#define PWM_IOCTL_SET_FREQ 1
#define PWM_IOCTL_STOP 2

#define ESC_KEY 0x1b

static int getch(void)
{
struct termios oldt,newt;
int ch;

if (!isatty(STDIN_FILENO)) {
fprintf(stderr, "this problem should be run at a terminal\n");
exit(1);
}
// save terminal setting
if(tcgetattr(STDIN_FILENO, &oldt) < 0) {
perror("save the terminal setting");
exit(1);
}

// set terminal as need
newt = oldt;
newt.c_lflag &= ~( ICANON | ECHO );//???为什么设置为除了这两个属性外的其他所有属性
if(tcsetattr(STDIN_FILENO,TCSANOW, &newt) < 0) {
perror("set terminal");
exit(1);
}

ch = getchar();

// restore termial setting
if(tcsetattr(STDIN_FILENO,TCSANOW,&oldt) < 0) {
perror("restore the termial setting");
exit(1);
}
return ch;
}

static int fd = -1;
static void close_buzzer(void);
static void open_buzzer(void)
{
fd = open("/dev/pwm", 0);
if (fd < 0) {
perror("open pwm_buzzer device");
exit(1);
}

// any function exit call will stop the buzzer
atexit(close_buzzer);
}

static void close_buzzer(void)
{
if (fd >= 0) {
ioctl(fd, PWM_IOCTL_STOP);
close(fd);
fd = -1;
}
}

static void set_buzzer_freq(int freq)
{
// this IOCTL command is the key to set frequency
int ret = ioctl(fd, PWM_IOCTL_SET_FREQ, freq);//？？？ioctl函数的功能和参数的含义（第二个和第三个参数）？
if(ret < 0) {
perror("set the frequency of the buzzer");
exit(1);
}
}
static void stop_buzzer(void)
{
int ret = ioctl(fd, PWM_IOCTL_STOP);//功能？？？？
if(ret < 0) {
perror("stop the buzzer");
exit(1);
}
}

int main(int argc, char **argv)
{
int freq = 1000 ;

open_buzzer();

printf( "\nBUZZER TEST ( PWM Control )\n" );
printf( "Press +/- to increase/reduce the frequency of the BUZZER\n" ) ;
printf( "Press 'ESC' key to Exit this program\n\n" );

while( 1 )
{
int key;

set_buzzer_freq(freq);
printf( "\tFreq = %d\n", freq );

key = getch();

switch(key) {
case '+':
if( freq < 20000 )
freq += 10;
break;

case '-':
if( freq > 11 )
freq -= 10 ;
break;

case ESC_KEY:
case EOF:
stop_buzzer();
exit(0);

default:
break;
}
}
}