Moving the implementation of the snprintf wrapper functions to the
platform module allows to re-use the same implementation throughout the
entire codebase.
Replace the small helper function to retrieve the function pointer and
then call the function, with another helper function to call the filter
function directly. This way the function pointer doesn't need to be
exposed at all.
The set_latency function is the perfect example of a feature that should
be implemented as an ioctl: it's only implemented by a single driver,
and the functionality is also highly platform specific.
This new ioctl function allows to perform I/O stream specific requests
through a generic interface. This provides an easy way to extend the I/O
interface with some driver specific features, without having to modify
the public api.
The Linux implementation is very straighforward and just a lightweight
wrapper around the select function. But the Windows implementation is
much more complex, because the Windows event notification mechanism
behaves very different:
The WaitCommEvent function does not support a timeout and is always a
blocking call. The only way to implement a timeout is to use
asynchronous I/O (or overlapped I/O as it's called in the Windows API),
to run the operation in the background. This requires some additional
book keeping to keep track of the pending background operation.
The event mechanism is also edge triggered instead of level triggered,
and reading the event with the WaitCommEvent function clears the pending
event. Therefore, the state of the input buffer needs to be checked with
the ClearCommError function before and after the WaitCommEvent call.
The check before is necessary in case the event is already cleared by a
previous WaitCommEvent call, while there is still data present in the
input buffer. In this case, WaitCommEvent should not be called at all,
because it would wait until more data arrives.
The check afterwards is necessary in case WaitCommEvent reports a
pending event, while the data in the input buffer has already been
consumed. In this case, the current event must be ignored and
WaitCommEvent needs to be called again, to wait for the next event.
On Windows, the WSAStartup() function needs to be called, to initialize
the socket library, before using any of the other WSA socket functions.
This includes the functions used for the bluetooth device discovery.
For most I/O stream implementations the serial communication specific
functions are meaningless. Implementing them as no-ops allows the dive
computer backends the call the I/O stream functions unconditionally.
However, implementing the no-op with a dummy function returning
DC_STATUS_SUCCESS, does not only add some (small) overhead at runtime,
but also requires many such functions. This is inconvenient and the same
result can easily be obtained by using a NULL pointer instead.
The consequence is that the logic is reversed now. To obtain the
previous behaviour of returning the DC_STATUS_UNSUPPORTED error code
again, you'll need to implement a dummy function. But that's fine
because it's the less common case.
Add a function to query the underlying transport type. This allows the
dive computer backends to implement transport specific behaviour where
necessary.
For the built-in I/O implementations, the transport type is obviously
always hardcoded, but for a custom I/O implementation the application
needs to provide the correct type. Hence the transport type can't be
hardcoded in the vtable and needs to be passed as a parameter.
There is no need to expose the two step connection setup of the
underlying socket interface in the public api. Doing so may complicate
the implementation on platforms where the native api is not based on the
socket interface (e.g. Mac OS X).
Note that the function to connect based on the IrDA service name is
removed. It's not used anywhere in libdivecomputer and since IrDA is an
outdated technology nowadays, it's unlikely we'll need it in the future.
The device descriptors are extended with a filter function. During the
device discovery, this filter function is used to return only devices
that match a known dive computer.
The filtering is optional, and can be disabled by passing a NULL pointer
for the device descriptor when creating the iterator with one of the
dc_xxx_iterator_new() functions.
Replacing the callback based interface with an iterator based interface,
results in a more extensible abstraction with a common interface for
each of the built-in I/O implementations (serial, usbhid, irda and
bluetooth).
When the port number is set to zero (which is an invalid value), detect
the port number automatically. On Windows, we can simply supply the UUID
of the serial port service, and the Windows api will take care of the
discovery. On Linux (bluez), the SDP discovery needs to be performed
manually to retrieve the port number.
For the socket based I/O stream implementations (IrDA and bluetooth) the
serial communication specific functions are meaningless. Implementing
them as no-ops allows the dive computer backends the call the I/O stream
functions unconditionally.
This is important for the bluetooth implementation, because bluetooth
enabled dive computers will be able to use both the native bluetooth
communication and the legacy bluetooth serial port emulation.
For the time being, the bluetooth communication code is of very limited
use. It's not used anywhere in the library, and as an internal api it's
also not available to applications. It serves mainly as a reference
implementation for future use.
The implementation supports Windows and Linux.
