compiler: __offsetof keyword for C offsetof() macro

vlib/builtin/cfns.v

@@ -400,4 +400,3 @@ fn C.WaitForSingleObject(voidptr, int) int
 
 
 fn C.ReleaseMutex(voidptr) bool
-

vlib/compiler/cheaders.v

@@ -20,6 +20,10 @@ const (
 #define TCCSKIP(x)
 #endif
 
+// for __offset_of
+#define __offsetof(s,memb) \\
+    ((size_t)((char *)&((s *)0)->memb - (char *)0))
+
 #define OPTION_CAST(x) (x)
 '
 	c_headers = '

vlib/compiler/expression.v

@@ -227,6 +227,7 @@ fn (p mut Parser) name_expr() string {
 		p.string_expr()
 		return 'charptr'
 	}
+
 	// known_type := p.table.known_type(name)
 	orig_name := name
 	is_c := name == 'C' && p.peek() == .dot
@@ -718,6 +719,21 @@ fn (p mut Parser) factor() string {
 			// p.fgen('$sizeof_typ)')
 			return 'int'
 		}
+		.key_offsetof {
+			p.next()
+			p.check(.lpar)
+
+			offsetof_typ := p.get_type()
+			p.check(.comma)
+
+			member := p.check_name()
+			p.check(.rpar)
+
+			p.gen('__offsetof($offsetof_typ, $member)')
+			
+			return 'int'
+		}
+
 		.amp, .dot, .mul {
 			// (dot is for enum vals: `.green`)
 			return p.name_expr()

vlib/compiler/token.v

@@ -108,6 +108,7 @@ enum TokenKind {
 	key_return
 	key_select
 	key_sizeof
+	key_offsetof
 	key_struct
 	key_switch
 	key_true
@@ -237,11 +238,12 @@ fn build_token_str() []string {
 	s[TokenKind.key_match] = 'match'
 	s[TokenKind.key_select] = 'select'
 	s[TokenKind.key_none] = 'none'
+	s[TokenKind.key_offsetof] = '__offsetof'
 	return s
 }
 
 const (
-	NrTokens = 140
+	NrTokens = 141
 	TokenStr = build_token_str()
 	KEYWORDS = build_keys()
 )