Currently in the entire C API of WPILib we have ~8 different ways of handling strings. The C API actually isn't built for pure C callers (We don't actually have any of those). Instead, they're built for interop between languages like LabVIEW and C# which can talk to C API's directly.
For output parameters, the choice was fairly obvious. An output struct containing a const string pointer and a length makes the most sense. Its easy to use these from most other languages, and doesn't require special null termination handling. Freeing these is also easy, as if you ever receive one of these string structures, theres just a single function call to free it.
Input parameters are a bit more complex. To be used from pure C, and from LabVIEW, a null terminated string is the best in most cases. However, null terminated strings in general have a lot of downsides. Additionally, from LabVIEW there are other considerations around encoding that having a wrapper struct helps make a bit easier. From a language like C#, a wrapper struct is by far the easiest, as custom marshalling can make it trivial to marshal both UTF8 and UTF16 strings down.
The final consideration is its nice to have an identical concept for both input and output. It makes the rules fairly easy to understand.
WPILib will not have any APIs that manipulate a string allocated externally. This means WPI_String can be const, as across the boundary it is always const.
If a WPILib API takes a const WPI_String*, WPILib will not manipulate or attempt to free that string, and that string is treated as an input. It is up to the caller to handle that memory, WPILib will never hold onto that memory longer than the call.
If a WPILib API takes a WPI_String*, that string is an output. WPILib will allocate that API with WPI_AllocateString(), fill in the string, and return to the caller. When the caller is done with the string, they must free it with WPI_FreeString().
If an output struct contains a WPI_String member, that member is considered read only, and should not be explicitly freed. The caller should call the free function for that struct.
If an array of WPI_Strings are returned, each individual string is considered read only, and should not be explicitly freed. The free function for that array should be called by the caller.
If an input struct containing a WPI_String, or an input array of WPI_Strings is passed to WPILib, the individual strings will not be manipulated or freed by WPILib, and the caller owns and should free that memory.
Callbacks also follow these rules. The most common is a callback either getting passed a const WPI_String* or a struct containing a WPI_String. In both of these cases, the callback target should consider these strings read only, and not attempt to free them or manipulate them.
DataLog is now a base class, with DataLogBackgroundWriter being the
background thread version and DataLogWriter being a non-threaded version.
Also split the C header into a separate file to make it more wpiformat friendly.
We now use a wrapper (wpi::print) to catch exceptions since we can't patch
std::print() to not throw when we ultimately migrate to it.
fmtlib and std format/print throw the same exceptions and always have. We previously patched fmt::print() to not throw a write failure exception, but we can't do that for std::print(); wpi::print() is the migration plan.
Restarting a stopped log results in creating a new log file with fresh copies of the same start records and schema data records.
Also check to see if the file has been deleted or if the log file exceeds 1.8 GB, and start a new one.
- In both C++ and Java, add listener functions to Instance class (same as NT3 provided)
- Add WaitForListenerQueue functions (same as NT3 provided)
- Move Java non-poller implementation to Instance (previously only handled single instance)
- Change C++ listeners to take non-const references for subscribers etc to help avoid footguns from use of temporary objects (also add doc comment)
- Fix Preferences making .type persistent
* TopicListener: Fix Add() return values
* Update PubSubOption poll storage documentation
* Update NetworkTableEntry::GetValue() doc
* Add documentation regarding asynchronous callbacks
* Unpublish entry: set publisher to nullptr
* Implement ValueListenerPoller default constructor
* Remove SetNetworkIdentity, make parameter to StartClient
* URI-escape client ID, improve error message
* Add connected message with client id; also improve disconnected message a bit
* Handle SetServers either before or after StartClient
* Fix client use-after-free; also delay reconnect after disconnect to rate limit
* Don't re-announce to already subscribed client; we especially don't want to send the last value again
* Always accept in-order sets, only use timestamp for tiebreak
* Fix LocalStorage::StartNetwork race
* Remove unused/unimplemented function
Also:
* [glass] Remove debug print
* [glass] Fix mpack string decoding
* [cameraserver] Fix up startclient
