On 2018-09-26 19:40, Linus Torvalds wrote:
Jef, what are the semantics forret = dc_iostream_read(iostream, buf, sizeof(buf), NULL); meant to be when there is only a partial read? Right now it returns DC_STATUS_SUCCESS and garbage in the end of "buf". Which really does seem wrong. The caller has no way to know that it got a just a partial reply..
The semantics depends on the transport type, and not whether the actual parameter is NULL or not:
For a stream based transport (e.g. serial, irda and rfcomm), the implementation should *always* try to read the requested size. Thus if the operation is successful (DC_STATUS_SUCCESS), the value of the actual parameter is guaranteed to be equal to the requested size, and hence you don't really need it. But if the operation fails for some reason, for example due to a timeout or some other I/O error, then an error should be returned. However, a failure does not necessary mean nothing got transferred at all. Therefore, the actual value should contain the number of bytes that were transferred successfully. This allows the caller to resume the operation after a partially failed transfer. (Note that most dive computer backends treat all errors as fatal errors, and in that case passing NULL is fine.)
The main reason why the iostream code always passes a non NULL pointer to the underlying implementation is because it needs the size for the logging.
For a packet based transport (e.g. ble and usbhid), the implementation should return a packet and indicate the size of that packet in the actual parameter. Thus the actual parameter is mandatory, and passing in a NULL pointer is indeed a very bad idea here.
Jef _______________________________________________ subsurface mailing list subsurface@subsurface-divelog.org http://lists.subsurface-divelog.org/cgi-bin/mailman/listinfo/subsurface
