Hi Andraeas, The documentation has a paragraph about this:
http://docs.julialang.org/en/release-0.3/manual/calling-c-and-fortran-code/ : "Currently, it is not possible to reliably pass structs and other non-primitive types by *value* from Julia to/from C libraries. However, *pointers* to structs can be passed. The simplest case is that of C functions that generate and use *opaque* pointers to struct types, which can be passed to/from Julia as Ptr{Void} (or any other Ptr type). Memory allocation and deallocation of such objects must be handled by calls to the appropriate cleanup routines in the libraries being used, just like in any C program. A more complicated approach is to declare a composite type in Julia that mirrors a C struct, which allows the structure fields to be directly accessed in Julia. Given a Julia variable x of that type, a pointer can be passed as &x to a C function expecting a pointer to the corresponding struct. If the Julia type T is immutable, then a Julia Array{T} is stored in memory identically to a C array of the corresponding struct, and can be passed to a C program expecting such an array pointer." I have never tried using the immutable approach to mirror a C structure, but it looks interesting. Instead, I use the Ptr{Void} method. So, in your C code you might have a couple of methods for building your type and setting a and b in your structure, void* builder() { return (void*) malloc(sizeof(struct mytype)); } void setter(struct mytype *t, int a, int b) { t.a=a ; t.b=b; } Then in your Julia code, you would write something like, t = ccall((:builder, "mylib"), Ptr{Void}, ()) ccall((:setter, "mylib"), Void, (Ptr{Void}, Int32,Int32), t, 1, 2) Hope it helps. Sam On Thursday, January 8, 2015 3:29:21 AM UTC-6, Andreas Lobinger wrote: > > Hello colleagues, > > i thought i understood the ccall interface but now i ran into a problem > (segmentation fault) and i cannot really track down, where actually the > problem occures. > I want/need to pass a pointer to structure (c style) to a library and the > function in the library writes entries in the structure. > > (the following i write from memory, i do not have the code on this > computer...) > > type mytype > a::Int32 > b::Int32 > end > > t = mytype(0,0) > > ccall(:flip, Void, (mytype,), t) > > > 1) is there somewhere code you would recommend to read? > 2) how can i use code_lowered or code_llvm to actually see the details. > The above example is included in a module and it looks like the code is > compiled at 'using' so code_llvm e.g. only shows the call to the compiled > function, but not the inside. > 3) other documentation (blog, FR) etc? > > Wishing a happy day, > Andreas >
