Raspberry Pi RS232 read RFID Transponder - c

Question

I want to open my door with a RFID Transponder. For this I use a Raspberry Pi and an 125Khz RFID Reader with UART. So now I have written a little C programm, wich sets up the RS232 ('ttyAMA0'). This works all fine, and I can read the Transponder, but it reads some sh**
Here is my Code:
char read_rfid(char* rfid_num)
{
fd_set input_fdset;
ssize_t length;
while(1)
{
FD_ZERO(&input_fdset);
FD_SET(fd,&input_fdset);
if(select(fd+1 ,&input_fdset, NULL,NULL,NULL)==-1)
perror("Terminal select() failed");
if(FD_ISSET(fd,&input_fdset))
{
if((length = read(fd,rfid_num,14)) ==-1)
perror("Terminal: read() failed");
else
{
write(STDOUT_FILENO,rfid_num,length);
return;
}
}
}
}
int setupRS232()
{
struct termios term_attr;
if((fd = open(RFID,O_RDWR)) == -1)
{
perror("Can't open Device");
return(1);
}
if(tcgetattr(fd,&term_attr) != 0)
{
perror("terminal: tcgetattr() failed");
return(1);
}
term_attr.c_cflag = BAUD|CS8|CRTSCTS|CLOCAL|CREAD;
term_attr.c_iflag = 0;
term_attr.c_oflag = 0;
term_attr.c_lflag = 0;
if(tcsetattr(fd,TCSAFLUSH,&term_attr) != 0)
{
perror("terminal: tcsetattr() failed");
return(1);
}
}
int main(int argc, char** argv)
{
MYSQL *mysql = NULL;
char rfid_num[14];
int i;
if(init_mysql(mysql) == 1)
return(1);
if(setupRS232() == 1)
return(1);
puts("Warte auf Transponder...");
read_rfid(rfid_num);
for(i=0;i<20;i++)
{
printf("%x\n",rfid_num[i]);
}
}
PS: Sorry for my bad English

Answer 1

Minimal approach to buffering. You should probably check the contents of the buffer before returning valid (is there a final \n ? )
char read_rfid(char* rfid_num) {
fd_set input_fdset;
ssize_t length;
int done;
for(done=0; done < 14; ) {
FD_ZERO(&input_fdset);
FD_SET(fd,&input_fdset);
if(select(fd+1 ,&input_fdset, NULL,NULL,NULL) == -1) {
if (errno == EAGAIN) continue;
perror("Terminal select() failed");
return -1;
}
if(FD_ISSET(fd,&input_fdset)) {
if((length = read(fd,rfid_num+done,14-done)) == -1) {
if (errno == EAGAIN) continue;
perror("Terminal: read() failed");
return -1;
}
write(STDOUT_FILENO,rfid_num+done,length);
done += length;
}
}
return 0;
}
Note: I don't understand why this function returns char.