Read & write AT command to serial port in C

Question

I am trying to switch modem to command mode, write AT command and parse the response, but I am not too good with C and I do not understand what is wrong. The code I am calling is:

void switch_to_command_mode(int uart)
{
    current_command_type = BTCommandTypeSwitchToATMode;
    char switchCmd[] = "\x2F\x2F\x2F";
    char emptySymbol[] = " ";
    char checkAT[] = "AT\r";
    ssize_t len = 0;
    write(uart, emptySymbol, strlen(emptySymbol));
    sleep(1);
    write(uart, switchCmd, strlen(switchCmd));
    sleep(1);
    write(uart, checkAT, strlen(checkAT));

    char buffer[255];  /* Input buffer */
    char *bufptr;      /* Current char in buffer */
    int  nbytes;

    bufptr = buffer;

    while (ioctl(uart, FIONREAD, (unsigned long)&len) == 0 && len < 2) {
        warnx("waiting");
        usleep(1000);
    }

    while ((nbytes = read(uart, bufptr, buffer + sizeof(buffer) - bufptr - 1)) > 0)
    {
        bufptr += nbytes;
        warnx("buffer %s", buffer);
        if (bufptr[-1] == '\n' || bufptr[-1] == '\r')
            break;
    }

    warnx("Final buffer: %s", buffer);
    if (strncmp(buffer, "OK", 2) == 0) {
        warnx("Great success!");
    }
}

What I get as console output is:

waiting
waiting
buffer AT
O      p �0
buffer AT
OK
 �0
Final buffer: AT
OK
 �0

According to documentation for the BT Module, the response should be in form of <CR><LF>OK<CR><LF>.

Any guidance on why this is possibly happening and what can I do with that would be highly appreciated.

P.S. I also already went through Serial programming for POSIX OS.

Answer 1

In C a "string" needs to be 0-terminated.

If they are not before being processed (printed here) the program runs into undefined behaviour. Which could also lead to "garbage" being printed.

To fix this easily initialise the buffer used to all 0s before using it, by doing:

char buffer[255] = "";

---

Also read() returns ssize_t not int. So you better do:

ssize_t nbytes;