AXL Create function signature

[gonsie]

What is the best way to design the AXL API w.r.t. getting the transfer type from the caller.

Currently we take a string and AXL does the conversion axl_xfer_t type.

Should we change this so that AXL Create takes in an axl_xfer_t parameter instead?

Pro:

• simpler api
• the enum provides the set of xfer options
• matches how other tools work

Cons:

• as the API develops, the enum will change, possibly making incompatibility issues. We could avoid this by only appending to the enum, but this may lead to a bunch of cruft for xfer types that are no longer supported. Otherwise, users will have to recompile for what would be ABI compatible versions.

[gonsie]

@becker33 says:

Changing the enum would thus be a new major version in AXL... which will be true. It represents either new technology support or deprecation.
No user should try to use a new version of a library without recompiling. Some people do this, but that's wrong.

[becker33]

In addition to what I said above:

• a typo in a string is a runtime error and a typo in an enum is a compile time error.
• We can maintain the gap in an enum if we delete an entry (by manually specifying the gap) so there will not be a recompiling issue for those who are wrong and want to swap in libraries without recompiling.