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This choice is definitely one that you will see both ways in embedded, and is arguable the most debatable one here. In the end, we this choice was made because:

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Also, this rule is almost a direct contradiction to the rule described in “Movement of Objects”, but since these are not being treated as objects, I’m allowing it.

All embedded base drivers should be made independent of any HAL or other driver. Instead of relying on hard set driversa specific HAL, function pointers should be used to maintain this. to allow any HAL to work with the driver

Instead of this:

int lsmdso_read_reg(uint8_t reg)
{
  return HAL_i2c_Mem_read(...); // THIS IS A FUNCTION FROM STM ONLY FOR CERTAIN STM32 DEVICES
}

Which relies on STM’s HAL i2c driver,

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typedef void (*I2C_WriteFuncPtr)(int device_addr, int reg_addr, int data);
typedef int (*I2C_ReadFuncPtr)(int device_addr, int reg_addr);

typedef staticstruct {
I2C_WriteFuncPtr local_I2C_Write = NULL;
static
I2C_ReadFuncPtr local_I2C_Read = NULL;
} lsm6dso_init(I2C_WriteFuncPtr write_func, I2C_ReadFuncPtr read_func)
{
    local_I2C_Write = write_func;
    local_I2C_Read = read_func;
}

t;

lsm6dso_t imu; //DEFINE OBJECT HERE

int lsmdso_read_reg(uint8_t reg)
{
  return local_I2C_Read(...);
}


(main.c)

lsm6dso_t imu;
imu = {.I2C_WriteFuncPtr = HAL_i2c_MemWrite, .I2C_ReadFuncPtr = HAL_i2c_MemRead};

While this is obviously more work, it means that these drivers should succeed for the foreseeable future and for all different chip sets in use right now, without having to change.

Also note the method of updating these values does utilize extern. For devices where more than one of these devices may be present on the bus, have the driver and its APIs have a device select parameter to dynamically update to a different device, do not make multiple instances of the device struct. In the above example, you may notice the first parameter is “device address”. This would allow you to have multiple lsm6dso’s on the bus (as they’d have the different i2c addresses), but they both use the same, local lsm6dso object, which works because this object only contains function pointers to i2c read and write, which will be the same no matter which instance of the hardware you are refencing.