Skip to content
Switch branches/tags

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time


Runtime Tiny C Compiler for VB6


VbRtcc is a fork of OTCC by Fabrice Bellard with some additional tweaks -- relocatable code, stdcall calling convention, short * access, _asm for inline assembly, local and global arrays.

Sample usage

First download and add src\mdRtcc.bas to your VB6 project so that all Rtcc prefixed functions are accessible (see API section below).

Here is a sample form that on button click compiles a C program that calls GetVersionEx API function and returns current OS version.

Private Declare Function CallWindowProc Lib "user32" Alias "CallWindowProcA" (ByVal lpPrevWndFunc As Long, Optional ByVal hWnd As Long, Optional ByVal Msg As Long, Optional ByVal wParam As Long, Optional ByVal lParam As Long) As Long

Private m_ctx                   As UcsRtccContextType

Private Sub Command1_Click()
    Dim pfn         As Long
    pfn = RtccCompile(m_ctx, _
        "main(n, t, wParam, lParam) {" & vbCrLf & _
        "   int v; " & vbCrLf & _
        "   v = GlobalAlloc(0x40, 148); " & vbCrLf & _
        "   *(int *)v = 148; " & vbCrLf & _
        "   GetVersionExA(v); " & vbCrLf & _
        "   return *(int *)(v + 4) * 100 + *(int *)(v + 8); " & vbCrLf & _
    Print CallWindowProc(pfn), "&H" & Hex(pfn)
    RtccFree m_ctx
End Sub

All functions are compiled with stdcall calling convention and unresolved externals like GlobalAlloc and GetVersionExA are searched in currently loaded DLLs.

Context m_ctx is reusable for additional invokations of RtccCompile with all previous symbols in scope, so already compiled functions can be called from subsequent compilations.

Returned pfn are usable until m_ctx is destroyed with RtccFree.


  • RtccCompile([in] ctx, [in] source, [in, optional] allocsize)

    If ctx is new initializes internal buffers according to the optional allocsize. Then compiles inline C source to ctx, appending generated code to previously compiled code in ctx and returns a callable pfn pointer to the entry point of the first function in source.

  • RtccFree([in] ctx)

    Deallocates ctx internal buffers, rendering previously compiled pfns in ctx invalid.

  • RtccGetSymbol([in] ctx, [in] name)

    Searches function name in ctx and returns a callable pfn pointer to its entry point or 0 (NULL) if not found. RtccGetSymbol can be used when compiling sources with multiple functions that call each other because OTCC can compile calls to forward references e.g. f calling g and g calling f in a single source code.

  • RtccPatchProto([in] addressof)

    Helper function that patches a native VB6 function to become a pfn calling trampoline. For instance a VB6 function with declaration Function MyProto(byval pfn As Long, a1 As Long, a2 As Long, ...) As Long can be patched with a call to RtccPatchProto AddressOf MyProto to jump at run-time to pfn with arguments a1, a2, ....

    This allows calling RtccCompiled pfn without resorting to CallWindowProc or similar work-arounds that are usually slower that implementing simple trampolines with RtccPatchProto.

Allowed C syntax

See README in lib/ottc for original Bellard's comments.

  • All local variables are signed int, including pointers.
  • Array indexing is not supported but global and local arrays declaration is possible.
  • Three types of dereferencing are allowed - *(char *)p for byte, *(short *)p for 16-bit word and *(int *)p for 32-bit dword.
  • Pointer arithmetic uses byte offset, e.g. use *(int *)(p + 4) to get second dword.
  • Shifts are signed e.g. i = i >> 8 extends the sign bit.
  • Function retval/parameters are always int (no types allowed) e.g. main(n, t) returns int and has two int parameters.
  • Char literals are cast to dword e.g. v = 'a' is sign-extended
  • Char/strings escape allows hex values e.g. "\x12\x5f" and \r, \n and \t
  • Unicode char/strings allowed with L"string" and L'a' and hex word escapes with L"\x1234\xabff"
  • Supported statements: if/else while for break return
  • Supported operators: + - * / % ! ~ >> << | & ^ ++ -- && || == != < > <= >=
  • Not supported operators: += (and family) ?: (ternary) , (comma in for loops too)
  • Block comments only (no // line comments)
  • #define supports constants only (no macro expansion)
  • Function alloca can be used for stack allocation of local arrays
  • Only global arrays declaration is supported w/ size in bytes e.g. int a[100] reserves 100 bytes (from ctx->glob)
  • _asm _emit <num> allows encoding custom instructions in codegen
  • _asm mov eax, <expr> supported only, where <expr> can be const/var or complex expression


  • Support short data type (for Unicode strings)
  • Support inline _asm _emit
  • Implement get symbol address after compile


Runtime Tiny C Compiler for VB6





No releases published


No packages published