IDA 6.8 SDK change calling convention

Question

Is there any way to change the calling convention programmatically? I found some C++ code in this repo but wasn't able to successfully update it to 6.8.

static bool idaapi convert_to_usercall(void *ud)
{
    vdui_t &vu = *(vdui_t *)ud;
    if (!vu.cfunc)
        return false;
    if ( vu.cfunc->entry_ea == BADADDR )
        return false;
    tinfo_t type;
    qtype fields;       
    if (!vu.cfunc->get_func_type(type, fields))
        return false;
    func_type_info_t fti;
    int a = build_funcarg_info(idati, type.c_str(), fields.c_str(), &fti, 0);
    if (!convert_cc_to_special(fti))
        return false;
    fields.clear();
    type.clear();
    build_func_type(&type, &fields, fti);
    if ( !apply_tinfo(idati, vu.cfunc->entry_ea, type.c_str(), fields.c_str(), 1) )
        return false;
    vu.refresh_view(true);
    return true;
}

The code seems to use apis that are now deprecated or changed up. Does anybody know a way to fix up this code or some other approach.

Any help would be appreciated, whether it is C++ or Python.

Answer 1

SetType? An example in Python could be this:

SetType(func_ea, "int x(int arg1, char *arg2);")

Answer 2

Ok, I found a solution in Python and it should work the same in C++. For anyone interested, here's the code. It changes the calling convention from __usercall to __fastcall:

old_func_type = idaapi.tinfo_t()
cfunc.get_func_type(old_func_type)

fi = idaapi.func_type_data_t()
if old_func_type.get_func_details(fi):
    if (fi.cc == idaapi.CM_CC_SPECIAL) or (fi.cc == idaapi.CM_CC_SPECIALE) or (fi.cc == idaapi.CM_CC_SPECIALP):
        fi.cc = idaapi.CM_CC_FASTCALL

        new_func_type = idaapi.tinfo_t()
        new_func_type.create_func(fi)

        idaapi.apply_tinfo2(ea, new_func_type, idaapi.TINFO_DEFINITE)