Sign-extending on serial communication with termios and uint8_t

Question

I'm trying to write bytes using the write() Unix system call, but any 8-bit value I send that starts with 1 is turned into a 32-bit value. I've tried looking to see if there are any termios flags to fix this, but I can't seem to find any.

struct termios config;

if(!isatty(fd)) { std::cout << "Not a TTY" << std::endl; }
if(tcgetattr(fd, &config) < 0) {std::cout << "Termios structure is broken" << std::endl;}

config.c_iflag  = 0;//&= ~(IGNBRK | BRKINT | ICRNL | INLCR | PARMRK | INPCK | ISTRIP | IXON);
config.c_oflag = 0; //&= ~(OCRNL | ONLCR | ONLRET | ONOCR | CSTOPB | ONOEOT| OFILL | OPOST);
config.c_cflag &= ~(CSIZE | PARENB | CSTOPB); // 8bits, no parity, 1 stop
config.c_cflag |= CS8 | CREAD | HUPCL;
config.c_cc[VMIN]  = 0;
config.c_cc[VTIME] = 1;

// Communication speed (simple version, using the predefined constants)
if(cfsetispeed(&config, B9600) < 0 || cfsetospeed(&config, B9600) < 0) {
    std::cout << "Speed is messed up" << std::endl;
}

// Finally, apply the configuration
if(tcsetattr(fd, TCSAFLUSH, &config) < 0) { std::cout << "Applied Configuration is broken" << std::endl;}

// uint8_t c[2] = {'D','A'};
// write(fd,&c,2);
 write(fd,&command[6],2);

Sample of the input:

BIN6: 10111110 -------- HEX6:be -------- DEC6: 190 -------- ASCII6: ?

BIN7: 11101100 -------- HEX7: ec -------- DEC7: 236 -------- ASCII7: ?

Sample of the output:

BIN6: 000000011111111111111111111111110111110 -------- HEX: FFFFFFBE -------- DEC: -66 -------- ASCII: ¾

BIN7: 000000011111111111111111111111111101100 -------- HEX: FFFFFFEC -------- DEC: -20

Any idea how to fix this? Thanks!

Answer 1

This was a simple, stupid error – turns out I was reading it as char instead of unsigned char.

Answer 2

The problem is not the terminal setting, but how characters are sign extended into 16-bit values. In your code snippet, you write two bytes. If you want to write only one byte, change the byte count to 1:

write (fd, &command[6], 1);