Failed repeated reading from a file in HITL mode

PX4 Autopilot OS / NuttX 🌰
1

The following px4 module reads next 100 bytes of a text file (several kilobytes) when user command received:

class SampleModule final : public ModuleBase<SampleModule>, public ModuleParams
{
    int _fd = -1;
public:
    static int custom_command(int argc, char *argv[])
    {
    	if (argc == 1 && !strcmp(argv[0], "test"))
            return get_instance()->test_command();
        return print_usage("unknown command");
    }

    int test_command()
    {
        if (_fd < 0) {
            _fd = px4_open("/fs/microsd/1.txt", O_RDONLY);
            if (_fd < 0) {
                PX4_ERR("can't open file, %d", errno);
                return -1;
            }
        }

        char buf[100];
        int st = px4_read(_fd, buf, 100);
        if (st < 0) {
            PX4_ERR("can't read from %d, because %d", _fd, errno);
            return -1;
        } else {
            PX4_INFO("read %d bytes", st);
        }

        return 0;
    }
};

The problem is the function read (or px4_read) works at the first call only. On the second call I am getting the reading error

nsh> sample_module start
nsh> sample_module status
INFO  [sample_module] running
nsh> sample_module test
INFO  [sample_module] read 100 bytes
nsh> sample_module test
ERROR [sample_module] can't read from 3, because 13

It seems, that the file descriptor _fd is not valid after the method custom_command completes.
But the function read works fine if I call it several times consecutive at one custom_command call. Probably, the posix file functions have specific behavior and I should use them in a different manner, should I?

I launch this code in HITL mode on the holybro durandal v1 board.

2

I found the reason of the bug. The difference between Unix POSIX file functions and NuttX is that in NuttX all the descriptors are thread-specific. NuttX creates a unique list of opened files for every thread (called task or process, but actually this is just a thread). When my code is called in custom_command callback, it happens in a newly created thread, so this thread possesses its own array of opened files.