Can't open a serial port in g++ on Ubuntu

Question

I am trying to run the sample programs that came with a development lidar unit (RPLIDAR A1M8 360 Degree Laser Scanner Kit). The sample code for a Linux target compiles without error using g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0 however when I run it, it stops with the message 'Error, cannot bind to the specified serial port /dev/ttyUSB0'. Using gdb I can trace the code down to the serial port open call to __libc_open64 where I don't have source anymore...

__libc_open64 (file=0x5555557832a9 "/dev/ttyUSB0", oflag=2306) at ../sysdeps/unix/sysv/linux/open64.c:36}

36 ../sysdeps/unix/sysv/linux/open64.c: No such file or directory.

Here is what I tried so far to eliminate obvious failure modes:

1. The serial port, a USB-to-serial converter on ttyUSB0, works just fine from Putty (115200, 8, none, 1, software flow control only). I connect it to a Raspberry Pi as the console to get a large amount of data without issue and can bidirectionally interact with the console. Therefore I conclude the port and converter work fine
2. The section of code that calls __libc_open64 is...

bool raw_serial::open(const char * portname, uint32_t baudrate, uint32_t flags) 
{ 
if (isOpened()) close(); 
serial_fd = ::open(portname, O_RDWR | O_NOCTTY | O_NDELAY);

I looked up the constants that are ORed together and hard coded the value (04403) just in case there was some issue with the version of the header files. Oddly the value is off by 1 from the oflag value in the gdb line. Compiled and ran it, no difference

3. I verified the call to ::open returned -1 which is treated as a failure in the code immediately after

4. I can see in dmesg that ttyUSB0 is open and available

I am not a c++ guy. This looks to me like an issue with the g++ __libc_open64 code but that also seems very unlikely. I don't know where to go next. Any advice would be greatly appreciated.

The first comments received pointed to permissions. I chmoded /dev/ttyUSB0 wide open before starting this exercise.

I ran strace and see the line...

openat(AT_FDCWD, "/dev/ttyUSB0", O_ACCMODE|O_NOCTTY|O_NONBLOCK) = -1 ENOENT (No such file or directory)

---

Well, that's embarrassing! It returns Permission Denied! Yes, I chmoded permissions wide open, then promptly forgot that being a USB device, it goes away and gets redefined when I unplug/plug it in again. Thank you for your help!

Answer 1

The prediction that strace would show "Permission denied" was correct. I was forgetting this is not a fixed serial port but rather a USB-to-serial converter. Even though I chmoded the permissions and verified it use with Putty, I forgot that as soon as I rebooted, or unplugged the USB, the /dev/ttyUSB0 device goes away and is recreated again when I rebooted or plugged it in, requiring that I set the permissions again.