Question

I’m having some beginner problems setting an FFI struct in Ruby. What I want to do is pass a pointer to a C string by setting a string property in an FFI::Struct object:

class SpSessionConfig < FFI::Struct   layout :api_version,          :int,            :cache_location,       :string,            :settings_location,    :string,            :application_key,      :pointer,            :application_key_size, :int,            :user_agent,           :string,            :sp_session_callbacks, :pointer,            :user_data,            :pointer    end end   sessionConf = SpotifyLibrary::SpSessionConfig.new()  puts sessionConf # => '#<SpotifyLibrary::SpSessionConfig:0x9acc00c>'  sessionConf[:api_version] = 1 puts 'Api Version: #{sessionConf[:api_version]}'  myTempDir = 'tmp' sessionConf[:cache_location] = myTempDir # !Error! 

But when I run the code I get this error:

jukebox.rb:44:in `[]=': Cannot set :string fields (ArgumentError) from jukebox.rb:44:in `<main>' 

So I don’t really know where to go from here.

Also, if you know of any good documtation or tutorials on this subject please leave a response! So far I have found the wiki documentation on Project Kenai very useful but the more the merrier!

Thanks!

I have tried to declare the string data members as [:char, 5] but that gives another error:

jukebox.rb:44:in `put': put not supported for FFI::StructLayoutBuilder::ArrayField_Signed8_3 (ArgumentError)     from jukebox.rb:44:in `[]='     from jukebox.rb:44:in `<main> 

There is a good suggestion to try out the memory pointer type and I will try that after work today.

Answer 1

FFI automatically rejects setting strings. Try changing it from :string to :char_array, as mentioned on this page:

:char_array – used ONLY in a struct layout where struct has a C-style string (char []) as a member

If that doesn’t work, you’re going to have to use a :pointer and convert it back into a string. It’s not well documented, but MemoryPointer has a bunch of available functions, such as write_string, that should help.