lightware_laser_serial: fix pointer for enabling serial mode

[MaEtUgR]

Solved Problem

When trying to fix #21737 (review) and compiling make beaglebone_blue locally with arm-linux-gnueabihf-gcc (Ubuntu 11.3.0-1ubuntu1~22.04.1) 11.3.0 I got a compile warning -> error:

PX4-Autopilot/src/drivers/distance_sensor/lightware_laser_serial/lightware_laser_serial.cpp: In member function ‘virtual void LightwareLaserSerial::Run()’:
PX4-Autopilot/src/drivers/distance_sensor/lightware_laser_serial/lightware_laser_serial.cpp:311:39: error: ‘ssize_t write(int, const void*, size_t)’ reading 5 bytes from a region of size 4 [-Werror=stringop-overread]
  311 |                         (void)!::write(_fd, &data, strlen(data));
      |                                ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated due to -Wfatal-errors.

Solution

const char *data = "www\r\n";
Defines a cstring of 6 bytes: {'w', 'w', 'w', '\r', '\n', '\0'}

type of data: char const*
type of &data: char const**

So when we call write(_fd, &data, strlen(data)); then strlen(data) == 5 (without the terminating null character) and we send the 4 byte memory address of data + some additional random byte to the distance sensor.

Correct is write(_fd, data, strlen(data)); where char const* gets cast to const void * and we pass the pointer to the content of data so the 5 bytes w, w, w, \r, \n get sent out via serial.

The fundamental problem here is write() not being typesafe.

Changelog Entry

For release notes:

Bugfix: lightware distance sensor serial initialization

Alternatives

const char data[] = "www\r\n";
write(_fd, data, strlen(data));

would be correct as well but creates slightly more overhead since it makes a copy of the cstring on the stack and passes the pointer of that to write().

Test coverage

• Compiles without error
• I verified the findings with https://onlinegdb.com/GA21Q3LHp