请高手帮忙解释一下这些 串行端口程序设计 的代码，最好每句都注释一下，急用，谢谢！

#include <termios.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/signal.h>
#include <pthread.h>
#define BAUDRATE B115200
#define COM1 "/dev/ttyS0"
#define COM2 "/dev/ttyS1"
#define ENDMINITERM 27 /* ESC to quit miniterm */
#define FALSE 0
#define TRUE 1
volatile int STOP=FALSE;
volatile int fd;
void child_handler(int s)
{
printf("stop!!!\n");
STOP=TRUE;
}
/*--------------------------------------------------------*/
void* keyboard(void * data)
{
int c;
for (;;){
c=getchar();
if( c== ENDMINITERM){
STOP=TRUE;
break ;
}
}
return NULL;
}
/*--------------------------------------------------------*/
/* modem input handler */
void* receive(void * data)
{
int c;
printf("read modem\n");
while (STOP==FALSE)
{
read(fd,&c,1); /* com port */
write(1,&c,1); /* stdout */
}
printf("exit from reading modem\n");
return NULL;
}
/*--------------------------------------------------------*/
void* send(void * data)
{
int c='0';
printf("send data\n");
while (STOP==FALSE) /* modem input handler */
{
c++;
c %= 255;
write(fd,&c,1); /* stdout */
usleep(100000);
}
return NULL; }
/*--------------------------------------------------------*/
int main(int argc,char** argv)
{
struct termios oldtio,newtio,oldstdtio,newstdtio;
struct sigaction sa;
int ok;
pthread_t th_a, th_b, th_c;
void * retval;
if( argc > 1)
fd = open(COM2, O_RDWR );
else
fd = open(COM1, O_RDWR ); //| O_NOCTTY |O_NONBLOCK);
if (fd <0) {
error(COM1);
exit(-1);
}
tcgetattr(0,&oldstdtio);
tcgetattr(fd,&oldtio); /* save current modem settings */
tcgetattr(fd,&newstdtio); /* get working stdtio */
newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD; /*ctrol flag*/
newtio.c_iflag = IGNPAR; /*input flag*/
newtio.c_oflag = 0; /*output flag*/
newtio.c_lflag = 0;
newtio.c_cc[VMIN]=1;
newtio.c_cc[VTIME]=0;
/* now clean the modem line and activate the settings for modem */
tcflush(fd, TCIFLUSH);
tcsetattr(fd,TCSANOW,&newtio); /*set attrib*/
sa.sa_handler = child_handler;
sa.sa_flags = 0;
sigaction(SIGCHLD,&sa,NULL); /* handle dying child */
pthread_create(&th_a, NULL, keyboard, 0);
pthread_create(&th_b, NULL, receive, 0);
pthread_create(&th_c, NULL, send, 0);
pthread_join(th_a, &retval);
pthread_join(th_b, &retval);
pthread_join(th_c, &retval);
tcsetattr(fd,TCSANOW,&oldtio); /* restore old modem setings */
tcsetattr(0,TCSANOW,&oldstdtio); /* restore old tty setings */
close(fd);
exit(0);
}