# Using Data\_Make\_Struct for incomplete type **URL:** \ **Category:** Ruby\ **Created:** [September 12, 2009, 6:45pm UTC](https://www.ruby-forum.com/t/using-data-make-struct-for-incomplete-type/175397 "2009-09-12T18:45:52Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![makire](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@makire](https://www.ruby-forum.com/u/makire)\ **Post date:** [September 12, 2009, 6:45pm UTC](https://www.ruby-forum.com/t/using-data-make-struct-for-incomplete-type/175397/1 "2009-09-12T18:45:52Z")
Hi all I am writing a Ruby wrapper for a C library. I can wrap some of the C structs with Data\_Make\_Struct but for one of them I get “invalid application of sizeof to incomplete type”. I expect this has something to do with that the compiler cannot find a definitive size of the struct. Am I on the right track and if so, is there any way around it? Thanks in advance. Regards Erik ---
**Author:** ![makire](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@makire](https://www.ruby-forum.com/u/makire)\ **Post date:** [September 12, 2009, 9:39pm UTC](https://www.ruby-forum.com/t/using-data-make-struct-for-incomplete-type/175397/2 "2009-09-12T21:39:40Z")
Erik L. wrote: > Hi all > > I am writing a Ruby wrapper for a C library. I can wrap some of the C > structs with Data\_Make\_Struct but for one of them I get “invalid > application of sizeof to incomplete type”. I expect this has something > to do with that the compiler cannot find a definitive size of the > struct. Have you included all the relevant headers? ---
**Author:** ![makire](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@makire](https://www.ruby-forum.com/u/makire)\ **Post date:** [September 13, 2009, 1:15am UTC](https://www.ruby-forum.com/t/using-data-make-struct-for-incomplete-type/175397/3 "2009-09-13T01:15:39Z")
On 12 Sep, 21:39, Joel VanderWerf [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > – > vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407 Yes, I can do both Data\_Wrap\_Struct for other structs in the same library, and create “instances” of the struct in question. The struct is defined in a lib that I only have as a binary so I cannot experiment with the source. I have tried both with Data\_Make\_Struct and with a construct I found in a blog post: struct\_name\* s = (struct\_name\*)malloc(sizeof(struct\_name)); VALUE obj; obj = Data\_Wrap\_Struct(klass, 0, terminate, s); Both result in the same compile message: invalid application of sizeof to incomplete type s The struct contains arrays of pointers to other structs so my guess is that it has something to do with that the compiler cannot figure out a definitive size to use for the struct. If that is the case I have no idea how to work around that. Thankful for any input. Regards Erik ---
**Author:** ![makire](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@makire](https://www.ruby-forum.com/u/makire)\ **Post date:** [September 13, 2009, 3:26am UTC](https://www.ruby-forum.com/t/using-data-make-struct-for-incomplete-type/175397/4 "2009-09-13T03:26:47Z")
Erik L. wrote: > > – > > struct\_name\* s = (struct\_name\*)malloc(sizeof(struct\_name)); > > idea how to work around that. > > If you don’t at least have the headers[1] it will be hard to do much > > with the struct, except pass it back in to the functions of the library. > > Maybe that’s enough. In that case, if you can at least determine the > > size of the struct, you can define a dummy struct of that size, and that > > may work, in a fragile sort of way. [1] Having the headers is not necessarily the same as having the full source. I’m assuming that in your case the problem will not be solved by installing the -dev package of the library? ---
**Author:** ![makire](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@makire](https://www.ruby-forum.com/u/makire)\ **Post date:** [September 13, 2009, 7:15pm UTC](https://www.ruby-forum.com/t/using-data-make-struct-for-incomplete-type/175397/5 "2009-09-13T19:15:32Z")
Erik L. wrote: > I have read up a bit on the problem and it is like I suspected. The > struct contains arrays of undeclared size and naturally the compiler > cannot guess the size. For now I have worked around it bu transferring > the struct data to VALUE instance variable, but that solution is not > as pretty as the option of wrapping the full struct. One more suggestion (maybe the same as what you just said): since the struct seems to be variable length, what about wrapping a pointer to the struct instead of the struct itself? In other words, the struct you wrap is of this type typedef struct { struct SomeOpaqueStruct \*s; } MyStruct; where SomeOpaqueStruct is from the library. It’s a bit less efficient, because of two dereferences (ruby object -\> MyStruct -\> s) and the extra allocation, but that’s probably relatively small. ---
**Author:** ![makire](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@makire](https://www.ruby-forum.com/u/makire)\ **Post date:** [September 13, 2009, 10:22am UTC](https://www.ruby-forum.com/t/using-data-make-struct-for-incomplete-type/175397/6 "2009-09-13T10:22:20Z")
The wrapped package is a closed source package so there is no equivalent to a -dev package available. I have read up a bit on the problem and it is like I suspected. The struct contains arrays of undeclared size and naturally the compiler cannot guess the size. For now I have worked around it bu transferring the struct data to VALUE instance variable, but that solution is not as pretty as the option of wrapping the full struct. On the other hand there are advantages of doing one “import” step at creation and then have the struct data as Ruby VALUEs, so it is kind of ok. I will hopefully release an alpha version of the bindings within a week or so and I’d be happy to link to the source on github at that time for input on how the incomplete struct might be completed in a “best practice” way. Thanks for the input so far. Regards Erik ---
**Author:** ![makire](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@makire](https://www.ruby-forum.com/u/makire)\ **Post date:** [September 14, 2009, 6:43pm UTC](https://www.ruby-forum.com/t/using-data-make-struct-for-incomplete-type/175397/7 "2009-09-14T18:43:54Z")
Erik L. wrote: > However there is one problem left. All the stuff above is only > possible within the wrapper C code. Once the library call exits and I > am back in Ruby land, the wrapped struct (SomeOpaqueStruct in your > example) is freed and I get a segmentation fault. I realize this is > because of how the wrapped library works but do you know of any way to > prevent this or how to copy the struct, thereby saving it for the > afterworld? Ouch. Bad library, bad! ---
**Author:** ![makire](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/fbc32d/32.png) [@makire](https://www.ruby-forum.com/u/makire)\ **Post date:** [September 14, 2009, 10:17am UTC](https://www.ruby-forum.com/t/using-data-make-struct-for-incomplete-type/175397/8 "2009-09-14T10:17:32Z")
On 13 Sep, 19:13, Joel VanderWerf [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > ## is of this type > > ``` > vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407 > > ``` Hmmm, getting very close now. The idea to wrap SomeOpaqueStruct worked like a charm. Now I can allocate, initialize and retreive the DATA through this wrapped construction. So many thanks for that one. However there is one problem left. All the stuff above is only possible within the wrapper C code. Once the library call exits and I am back in Ruby land, the wrapped struct (SomeOpaqueStruct in your example) is freed and I get a segmentation fault. I realize this is because of how the wrapped library works but do you know of any way to prevent this or how to copy the struct, thereby saving it for the afterworld? Regards Erik