add an optional expression which can be used to default a slice length

you can add a configuration in the yaml file for the expression to use:

```yaml
TRANSLATOR:
  LenFields:
    MyStruct.some_data: some_size
```

generator/gen_bindings.go

@@ -315,7 +315,8 @@ The caller is responsible for freeing the this memory via C.free.`, name, cgoSpe
 			}
 			return mem
 		}`, name, sizeofConst)
-	fmt.Fprintln(buf, "\n")
+	fmt.Fprintln(buf)
+	fmt.Fprintln(buf)
 	fmt.Fprintf(buf, `const %s = unsafe.Sizeof([1]%s{})`, sizeofConst, cgoSpec)
 
 	helper.Source = buf.String()
@@ -787,7 +788,7 @@ func (gen *Generator) proxyArgToGo(memTip tl.Tip, varName, ptrName string,
 }
 
 func (gen *Generator) proxyValueToGo(memTip tl.Tip, varName, ptrName string,
-	goSpec tl.GoTypeSpec, cgoSpec tl.CGoSpec) (proxy string, nillable bool) {
+	goSpec tl.GoTypeSpec, cgoSpec tl.CGoSpec, lenField string) (proxy string, nillable bool) {
 	nillable = true
 
 	if goSpec.IsGoString() {
@@ -829,7 +830,15 @@ func (gen *Generator) proxyValueToGo(memTip tl.Tip, varName, ptrName string,
 		fmt.Fprintf(buf, "hx%2x := (*sliceHeader)(unsafe.Pointer(&%s))\n", postfix, varName)
 		fmt.Fprintf(buf, "hx%2x.Data = unsafe.Pointer(%s)\n", postfix, ptrName)
 		fmt.Fprintf(buf, "hx%2x.Cap = %s\n", postfix, gen.maxMem)
-		fmt.Fprintf(buf, "// hx%2x.Len = ?\n", postfix)
+
+		// check if we have a configuration for a custom length property
+		idx := strings.LastIndex(ptrName, ".")
+		if lenField != "" && idx > 0 {
+			// replace the `x.refxxxxx.some_field` suffix with the lenField name
+			fmt.Fprintf(buf, "hx%2x.Len = int(%s)\n", postfix, ptrName[0:idx+1]+lenField)
+		} else {
+			fmt.Fprintf(buf, "// hx%2x.Len = ? %s %s\n", postfix, varName, ptrName)
+		}
 		proxy = buf.String()
 		return
 	case isPlain: // ex: byte, [4]byte

generator/gen_struct_helpers.go

@@ -208,15 +208,16 @@ func (gen *Generator) getRawStructHelpers(goStructName []byte, cStructName strin
 			goSpec.Pointers += 1
 			cgoSpec.Pointers += 1
 		}
-		fmt.Fprintf(buf,"func (s *%s) Get%s() %s {\n", goStructName, goName, goSpec)
-		toProxy, _ := gen.proxyValueToGo(memTip, "ret", "&s." + m.Name, goSpec, cgoSpec)
-		fmt.Fprintf(buf,"\tvar ret %s\n", goSpec)
-		fmt.Fprintf(buf,"\t%s\n",toProxy)
+		fmt.Fprintf(buf, "func (s *%s) Get%s() %s {\n", goStructName, goName, goSpec)
+		lenField := getLenField(gen, structSpec, m)
+		toProxy, _ := gen.proxyValueToGo(memTip, "ret", "&s."+m.Name, goSpec, cgoSpec, lenField)
+		fmt.Fprintf(buf, "\tvar ret %s\n", goSpec)
+		fmt.Fprintf(buf, "\t%s\n", toProxy)
 		fmt.Fprintf(buf, "\treturn ret\n")
 		fmt.Fprintf(buf, "}\n")
 		helpers = append(helpers, &Helper{
-			Name:	fmt.Sprintf("%s.Get%s", goStructName, goName),
-			Description: fmt.Sprintf("Get%s returns a reference to C object within a struct",goName),
+			Name:        fmt.Sprintf("%s.Get%s", goStructName, goName),
+			Description: fmt.Sprintf("Get%s returns a reference to C object within a struct", goName),
 			Source:      buf.String(),
 		})
 	}
@@ -345,8 +346,17 @@ func (gen *Generator) getDerefSource(goStructName []byte, cStructName string, sp
 		goName := "x." + string(gen.tr.TransformName(tl.TargetType, m.Name, public))
 		cgoName := fmt.Sprintf("x.ref%2x.%s", crc, m.Name)
 		cgoSpec := gen.tr.CGoSpec(m.Spec, false)
-		toProxy, _ := gen.proxyValueToGo(memTip, goName, cgoName, goSpec, cgoSpec)
+		lenField := getLenField(gen, structSpec, m)
+		toProxy, _ := gen.proxyValueToGo(memTip, goName, cgoName, goSpec, cgoSpec, lenField)
 		fmt.Fprintln(buf, toProxy)
 	}
 	return buf.Bytes()
 }
+
+func getLenField(gen *Generator, structSpec *tl.CStructSpec, m *tl.CDecl) string {
+	lenField := ""
+	if gen.tr.LenFields() != nil {
+		lenField = gen.tr.LenFields()[structSpec.Typedef+"."+m.Name]
+	}
+	return lenField
+}

translator/translator.go

@@ -24,9 +24,10 @@ type Translator struct {
 	builtinTypemap2 CTypeMap
 	ignoredFiles    map[string]struct{}
 
-	valueMap map[string]Value
-	exprMap  map[string]string
-	tagMap   map[string]*CDecl
+	valueMap  map[string]Value
+	exprMap   map[string]string
+	tagMap    map[string]*CDecl
+	lenFields map[string]string
 
 	defines  []*CDecl
 	typedefs []*CDecl
@@ -78,14 +79,15 @@ func (t TipSpecRx) Self() Tip {
 }
 
 type Config struct {
-	Rules              Rules      `yaml:"Rules"`
-	ConstRules         ConstRules `yaml:"ConstRules"`
-	PtrTips            PtrTips    `yaml:"PtrTips"`
-	TypeTips           TypeTips   `yaml:"TypeTips"`
-	MemTips            MemTips    `yaml:"MemTips"`
-	Typemap            CTypeMap   `yaml:"Typemap"`
-	ConstCharIsString  *bool      `yaml:"ConstCharIsString"`
-	ConstUCharIsString *bool      `yaml:"ConstUCharIsString"`
+	Rules              Rules             `yaml:"Rules"`
+	ConstRules         ConstRules        `yaml:"ConstRules"`
+	PtrTips            PtrTips           `yaml:"PtrTips"`
+	TypeTips           TypeTips          `yaml:"TypeTips"`
+	MemTips            MemTips           `yaml:"MemTips"`
+	Typemap            CTypeMap          `yaml:"Typemap"`
+	ConstCharIsString  *bool             `yaml:"ConstCharIsString"`
+	ConstUCharIsString *bool             `yaml:"ConstUCharIsString"`
+	LenFields          map[string]string `yaml:"LenFields"`
 
 	IgnoredFiles []string `yaml:"-"`
 	LongIs64Bit  bool     `yaml:"-"`
@@ -122,6 +124,7 @@ func New(cfg *Config) (*Translator, error) {
 		valueMap:           make(map[string]Value),
 		exprMap:            make(map[string]string),
 		tagMap:             make(map[string]*CDecl),
+		lenFields:          cfg.LenFields,
 		typedefsSet:        make(map[string]struct{}),
 		typedefKinds:       make(map[string]CTypeKind),
 		ignoredFiles:       make(map[string]struct{}),
@@ -1016,6 +1019,10 @@ func (t *Translator) TagMap() map[string]*CDecl {
 	return t.tagMap
 }
 
+func (t *Translator) LenFields() map[string]string {
+	return t.lenFields
+}
+
 func (t *Translator) ExpressionMap() map[string]string {
 	return t.exprMap
 }