Question

How can I return a struct array from a .Call to a function in a C shared library and subsequently use that array in R?

For example:

typedef struct{
    int    thing1;
    int    thing2;
    float  thing3;
    float  thing4;
    char   thing5;
    //... many more things of various simple types
} MY_STRUCT;

SEXP R_Calls_Me(SEXP args) {

    // Do stuff with args...

    // Create arrayOfMyStructs as what type??

    return arrayOfMyStructs;
}

What type is arrayOfMyStructs such that R can use it?

It seems like a common thing one would want to do, but I cannot find any examples of this in the documentation for writing R extensions.

Answer 1

Typically you create list (generic vector) with the components you want to return. In your case something like

SEXP res = PROTECT(allocVector(VECSXP, 5));
SET_VECTOR_ELT(res, 0, ScalarInteger(a.thing1));
SET_VECTOR_ELT(res, 1, ScalarInteger(a.thing2));
SET_VECTOR_ELT(res, 2, ScalarReal(a.thing3));
...
UNPROTECT(1)
return res;

It is also customary to assign names to the vector, e.g.:

const char *names[5] = { "thing1", "thing2", "thing3", "thing4", "thing5" };
SEXP sNames = PROTECT(allocVector(STRSXP, 5));
for (int i = 0; i < 5; i++) SET_STRING_ELT(res, i, mkString(names[i]));
setAttrib(res, R_NamesSymbol, sNames);
UNPROTECT(1);

Note that what you are describing is not an array but a structure. Arrays are typically much easier to pass as vectors.