Add //export to cgo.

The new //export comment marks a Go function as callable from
C.  The syntax is "//export NAME" where NAME is the name of
the function as seen from C.  If such a comment is seen, cgo
will generate two new files: _cgo_export.h and _cgo_export.c.
The _cgo_export.h file provides declarations which C code may
use to call Go functions.  The _cgo_export.c file contains
wrappers, and is to be compiled with gcc.

The changes to Make.pkg support using this from a Go Makefile,
though it could probably be more convenient.

R=rsc
CC=golang-dev
http://codereview.appspot.com/853042

2 files changed, 352 insertions, 1 deletions

diff --git a/src/cmd/cgo/ast.go b/src/cmd/cgo/ast.go
index 0dfe9217a..580a72a95 100644
--- a/src/cmd/cgo/ast.go
+++ b/src/cmd/cgo/ast.go
@@ -13,6 +13,7 @@ import (
 	"go/parser"
 	"go/scanner"
 	"os"
+	"strings"
 )
 
 // A Cref refers to an expression of the form C.xxx in the AST.
@@ -25,6 +26,12 @@ type Cref struct {
 	FuncType *FuncType
 }
 
+// A ExpFunc is an exported function, callable from C.
+type ExpFunc struct {
+	Func    *ast.FuncDecl
+	ExpName string // name to use from C
+}
+
 // A Prog collects information about a cgo program.
 type Prog struct {
 	AST         *ast.File // parsed AST
@@ -37,6 +44,7 @@ type Prog struct {
 	Funcdef     map[string]*FuncType
 	Enumdef     map[string]int64
 	Constdef    map[string]string
+	ExpFuncs    []*ExpFunc
 	PtrSize     int64
 	GccOptions  []string
 	OutDefs     map[string]bool
@@ -303,6 +311,8 @@ func walk(x interface{}, p *Prog, context string) {
 			walk(n.Body, p, "stmt")
 		}
 
+		checkExpFunc(n, p)
+
 	case *ast.File:
 		walk(n.Decls, p, "decl")
 
@@ -329,3 +339,38 @@ func walk(x interface{}, p *Prog, context string) {
 		}
 	}
 }
+
+// If a function should be exported add it to ExpFuncs.
+func checkExpFunc(n *ast.FuncDecl, p *Prog) {
+	if n.Doc == nil {
+		return
+	}
+	for _, c := range n.Doc.List {
+		if string(c.Text[0:9]) != "//export " {
+			continue
+		}
+
+		name := strings.TrimSpace(string(c.Text[9:]))
+		if name == "" {
+			error(c.Position, "export missing name")
+		}
+
+		if p.ExpFuncs == nil {
+			p.ExpFuncs = make([]*ExpFunc, 0, 8)
+		}
+		i := len(p.ExpFuncs)
+		if i >= cap(p.ExpFuncs) {
+			new := make([]*ExpFunc, 2*i)
+			for j, v := range p.ExpFuncs {
+				new[j] = v
+			}
+			p.ExpFuncs = new
+		}
+		p.ExpFuncs = p.ExpFuncs[0 : i+1]
+		p.ExpFuncs[i] = &ExpFunc{
+			Func:    n,
+			ExpName: name,
+		}
+		break
+	}
+}
diff --git a/src/cmd/cgo/out.go b/src/cmd/cgo/out.go
index 31df5a0c4..2fae48fe4 100644
--- a/src/cmd/cgo/out.go
+++ b/src/cmd/cgo/out.go
@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/printer"
+	"go/token"
 	"os"
 	"strings"
 )
@@ -47,7 +48,7 @@ func (p *Prog) writeDefs() {
 	}
 	fmt.Fprintf(fgo2, "type _C_void [0]byte\n")
 
-	fmt.Fprintf(fc, cProlog, pkgroot, pkgroot, pkgroot, pkgroot)
+	fmt.Fprintf(fc, cProlog, pkgroot, pkgroot, pkgroot, pkgroot, pkgroot)
 
 	for name, def := range p.Vardef {
 		fmt.Fprintf(fc, "#pragma dynimport ·_C_%s %s \"%s.so\"\n", name, name, path)
@@ -140,6 +141,8 @@ func (p *Prog) writeDefs() {
 		fmt.Fprintf(fc, "\n")
 	}
 
+	p.writeExports(fgo2, fc)
+
 	fgo2.Close()
 	fc.Close()
 }
@@ -245,6 +248,294 @@ func (p *Prog) writeOutput(srcfile string) {
 	fgcc.Close()
 }
 
+// Write out the various stubs we need to support functions exported
+// from Go so that they are callable from C.
+func (p *Prog) writeExports(fgo2, fc *os.File) {
+	if len(p.ExpFuncs) == 0 {
+		return
+	}
+
+	fgcc := creat("_cgo_export.c")
+	fgcch := creat("_cgo_export.h")
+
+	fmt.Fprintf(fgcch, "/* Created by cgo - DO NOT EDIT. */\n")
+	fmt.Fprintf(fgcch, "%s\n", gccExportHeaderProlog)
+
+	fmt.Fprintf(fgcc, "/* Created by cgo - DO NOT EDIT. */\n")
+	fmt.Fprintf(fgcc, "#include \"_cgo_export.h\"\n")
+
+	for _, exp := range p.ExpFuncs {
+		fn := exp.Func
+
+		// Construct a gcc struct matching the 6c argument and
+		// result frame.
+		structType := "struct {\n"
+		off := int64(0)
+		npad := 0
+		if fn.Recv != nil {
+			t := p.cgoType(fn.Recv.List[0].Type)
+			structType += fmt.Sprintf("\t\t%s recv;\n", t.C)
+			off += t.Size
+		}
+		fntype := fn.Type
+		forFieldList(fntype.Params,
+			func(i int, atype ast.Expr) {
+				t := p.cgoType(atype)
+				if off%t.Align != 0 {
+					pad := t.Align - off%t.Align
+					structType += fmt.Sprintf("\t\tchar __pad%d[%d];\n", npad, pad)
+					off += pad
+					npad++
+				}
+				structType += fmt.Sprintf("\t\t%s p%d;\n", t.C, i)
+				off += t.Size
+			})
+		if off%p.PtrSize != 0 {
+			pad := p.PtrSize - off%p.PtrSize
+			structType += fmt.Sprintf("\t\tchar __pad%d[%d];\n", npad, pad)
+			off += pad
+			npad++
+		}
+		forFieldList(fntype.Results,
+			func(i int, atype ast.Expr) {
+				t := p.cgoType(atype)
+				if off%t.Align != 0 {
+					pad := t.Align - off%t.Align
+					structType += fmt.Sprintf("\t\tchar __pad%d[%d]\n", npad, pad)
+					off += pad
+					npad++
+				}
+				structType += fmt.Sprintf("\t\t%s r%d;\n", t.C, i)
+				off += t.Size
+			})
+		if off%p.PtrSize != 0 {
+			pad := p.PtrSize - off%p.PtrSize
+			structType += fmt.Sprintf("\t\tchar __pad%d[%d];\n", npad, pad)
+			off += pad
+			npad++
+		}
+		if structType == "struct {\n" {
+			structType += "\t\tchar unused;\n" // avoid empty struct
+			off++
+		}
+		structType += "\t}"
+
+		// Get the return type of the wrapper function
+		// compiled by gcc.
+		gccResult := ""
+		if fntype.Results == nil || len(fntype.Results.List) == 0 {
+			gccResult = "void"
+		} else if len(fntype.Results.List) == 1 && len(fntype.Results.List[0].Names) <= 1 {
+			gccResult = p.cgoType(fntype.Results.List[0].Type).C
+		} else {
+			fmt.Fprintf(fgcch, "\n/* Return type for %s */\n", exp.ExpName)
+			fmt.Fprintf(fgcch, "struct %s_return {\n", exp.ExpName)
+			forFieldList(fntype.Results,
+				func(i int, atype ast.Expr) {
+					fmt.Fprintf(fgcch, "\t%s r%d;\n", p.cgoType(atype).C, i)
+				})
+			fmt.Fprintf(fgcch, "};\n")
+			gccResult = "struct " + exp.ExpName + "_return"
+		}
+
+		// Build the wrapper function compiled by gcc.
+		s := fmt.Sprintf("%s %s(", gccResult, exp.ExpName)
+		if fn.Recv != nil {
+			s += p.cgoType(fn.Recv.List[0].Type).C
+			s += " recv"
+		}
+		forFieldList(fntype.Params,
+			func(i int, atype ast.Expr) {
+				if i > 0 || fn.Recv != nil {
+					s += ", "
+				}
+				s += fmt.Sprintf("%s p%d", p.cgoType(atype).C, i)
+			})
+		s += ")"
+		fmt.Fprintf(fgcch, "\nextern %s;\n", s)
+
+		fmt.Fprintf(fgcc, "extern _cgoexp_%s(void *, int);\n", exp.ExpName)
+		fmt.Fprintf(fgcc, "\n%s\n", s)
+		fmt.Fprintf(fgcc, "{\n")
+		fmt.Fprintf(fgcc, "\t%s a;\n", structType)
+		if gccResult != "void" && (len(fntype.Results.List) > 1 || len(fntype.Results.List[0].Names) > 1) {
+			fmt.Fprintf(fgcc, "\t%s r;\n", gccResult)
+		}
+		if fn.Recv != nil {
+			fmt.Fprintf(fgcc, "\ta.recv = recv;\n")
+		}
+		forFieldList(fntype.Params,
+			func(i int, atype ast.Expr) {
+				fmt.Fprintf(fgcc, "\ta.p%d = p%d;\n", i, i)
+			})
+		fmt.Fprintf(fgcc, "\tcrosscall2(_cgoexp_%s, &a, (int) sizeof a);\n", exp.ExpName)
+		if gccResult != "void" {
+			if len(fntype.Results.List) == 1 && len(fntype.Results.List[0].Names) <= 1 {
+				fmt.Fprintf(fgcc, "\treturn a.r0;\n")
+			} else {
+				forFieldList(fntype.Results,
+					func(i int, atype ast.Expr) {
+						fmt.Fprintf(fgcc, "\tr.r%d = a.r%d;\n", i, i)
+					})
+				fmt.Fprintf(fgcc, "\treturn r;\n")
+			}
+		}
+		fmt.Fprintf(fgcc, "}\n")
+
+		// Build the wrapper function compiled by 6c/8c
+		goname := exp.Func.Name.Name()
+		if fn.Recv != nil {
+			goname = "_cgoexpwrap_" + fn.Recv.List[0].Names[0].Name() + "_" + goname
+		}
+		fmt.Fprintf(fc, "#pragma dynexport _cgoexp_%s _cgoexp_%s\n", exp.ExpName, exp.ExpName)
+		fmt.Fprintf(fc, "extern void ·%s();\n", goname)
+		fmt.Fprintf(fc, "\nvoid\n")
+		fmt.Fprintf(fc, "_cgoexp_%s(void *a, int32 n)\n", exp.ExpName)
+		fmt.Fprintf(fc, "{\n")
+		fmt.Fprintf(fc, "\tcgocallback(·%s, a, n);\n", goname)
+		fmt.Fprintf(fc, "}\n")
+
+		// Calling a function with a receiver from C requires
+		// a Go wrapper function.
+		if fn.Recv != nil {
+			fmt.Fprintf(fgo2, "func %s(recv ", goname)
+			printer.Fprint(fgo2, fn.Recv.List[0].Type)
+			forFieldList(fntype.Params,
+				func(i int, atype ast.Expr) {
+					fmt.Fprintf(fgo2, ", p%d", i)
+					printer.Fprint(fgo2, atype)
+				})
+			fmt.Fprintf(fgo2, ")")
+			if gccResult != "void" {
+				fmt.Fprint(fgo2, " (")
+				forFieldList(fntype.Results,
+					func(i int, atype ast.Expr) {
+						if i > 0 {
+							fmt.Fprint(fgo2, ", ")
+						}
+						printer.Fprint(fgo2, atype)
+					})
+				fmt.Fprint(fgo2, ")")
+			}
+			fmt.Fprint(fgo2, " {\n")
+			fmt.Fprint(fgo2, "\t")
+			if gccResult != "void" {
+				fmt.Fprint(fgo2, "return ")
+			}
+			fmt.Fprintf(fgo2, "recv.%s(", exp.Func.Name)
+			forFieldList(fntype.Params,
+				func(i int, atype ast.Expr) {
+					if i > 0 {
+						fmt.Fprint(fgo2, ", ")
+					}
+					fmt.Fprintf(fgo2, "p%d", i)
+				})
+			fmt.Fprint(fgo2, ")\n")
+			fmt.Fprint(fgo2, "}\n")
+		}
+	}
+}
+
+// Call a function for each entry in an ast.FieldList, passing the
+// index into the list and the type.
+func forFieldList(fl *ast.FieldList, fn func(int, ast.Expr)) {
+	if fl == nil {
+		return
+	}
+	i := 0
+	for _, r := range fl.List {
+		if r.Names == nil {
+			fn(i, r.Type)
+			i++
+		} else {
+			for _ = range r.Names {
+				fn(i, r.Type)
+				i++
+			}
+		}
+	}
+}
+
+// Map predeclared Go types to Type.
+var goTypes = map[string]*Type{
+	"int":        &Type{Size: 4, Align: 4, C: "int"},
+	"uint":       &Type{Size: 4, Align: 4, C: "uint"},
+	"int8":       &Type{Size: 1, Align: 1, C: "schar"},
+	"uint8":      &Type{Size: 1, Align: 1, C: "uchar"},
+	"int16":      &Type{Size: 2, Align: 2, C: "short"},
+	"uint16":     &Type{Size: 2, Align: 2, C: "ushort"},
+	"int32":      &Type{Size: 4, Align: 4, C: "int"},
+	"uint32":     &Type{Size: 4, Align: 4, C: "uint"},
+	"int64":      &Type{Size: 8, Align: 8, C: "int64"},
+	"uint64":     &Type{Size: 8, Align: 8, C: "uint64"},
+	"float":      &Type{Size: 4, Align: 4, C: "float"},
+	"float32":    &Type{Size: 4, Align: 4, C: "float"},
+	"float64":    &Type{Size: 8, Align: 8, C: "double"},
+	"complex":    &Type{Size: 8, Align: 8, C: "__complex float"},
+	"complex64":  &Type{Size: 8, Align: 8, C: "__complex float"},
+	"complex128": &Type{Size: 16, Align: 16, C: "__complex double"},
+}
+
+// Map an ast type to a Type.
+func (p *Prog) cgoType(e ast.Expr) *Type {
+	switch t := e.(type) {
+	case *ast.StarExpr:
+		x := p.cgoType(t.X)
+		return &Type{Size: p.PtrSize, Align: p.PtrSize, C: x.C + "*"}
+	case *ast.ArrayType:
+		if t.Len == nil {
+			return &Type{Size: p.PtrSize + 8, Align: p.PtrSize, C: "GoSlice"}
+		}
+	case *ast.StructType:
+		// TODO
+	case *ast.FuncType:
+		return &Type{Size: p.PtrSize, Align: p.PtrSize, C: "void*"}
+	case *ast.InterfaceType:
+		return &Type{Size: 3 * p.PtrSize, Align: p.PtrSize, C: "GoInterface"}
+	case *ast.MapType:
+		return &Type{Size: p.PtrSize, Align: p.PtrSize, C: "GoMap"}
+	case *ast.ChanType:
+		return &Type{Size: p.PtrSize, Align: p.PtrSize, C: "GoChan"}
+	case *ast.Ident:
+		// Look up the type in the top level declarations.
+		// TODO: Handle types defined within a function.
+		for _, d := range p.AST.Decls {
+			gd, ok := d.(*ast.GenDecl)
+			if !ok || gd.Tok != token.TYPE {
+				continue
+			}
+			for _, spec := range gd.Specs {
+				ts, ok := spec.(*ast.TypeSpec)
+				if !ok {
+					continue
+				}
+				if ts.Name.Name() == t.Name() {
+					return p.cgoType(ts.Type)
+				}
+			}
+		}
+		for name, def := range p.Typedef {
+			if name == t.Name() {
+				return p.cgoType(def)
+			}
+		}
+		if t.Name() == "uintptr" {
+			return &Type{Size: p.PtrSize, Align: p.PtrSize, C: "uintptr"}
+		}
+		if t.Name() == "string" {
+			return &Type{Size: p.PtrSize + 4, Align: p.PtrSize, C: "GoString"}
+		}
+		if r, ok := goTypes[t.Name()]; ok {
+			if r.Align > p.PtrSize {
+				r.Align = p.PtrSize
+			}
+			return r
+		}
+	}
+	error(e.Pos(), "unrecognized Go type %v", e)
+	return &Type{Size: 4, Align: 4, C: "int"}
+}
+
 const gccProlog = `
 // Usual nonsense: if x and y are not equal, the type will be invalid
 // (have a negative array count) and an inscrutable error will come
@@ -275,6 +566,7 @@ const cProlog = `
 
 #pragma dynimport initcgo initcgo "%s/libcgo.so"
 #pragma dynimport libcgo_thread_start libcgo_thread_start "%s/libcgo.so"
+#pragma dynimport libcgo_set_scheduler libcgo_set_scheduler "%s/libcgo.so"
 #pragma dynimport _cgo_malloc _cgo_malloc "%s/libcgo.so"
 #pragma dynimport _cgo_free free "%s/libcgo.so"
 
@@ -294,3 +586,17 @@ void
 	FLUSH(&p);
 }
 `
+
+const gccExportHeaderProlog = `
+typedef unsigned int uint;
+typedef signed char schar;
+typedef unsigned char uchar;
+typedef unsigned short ushort;
+typedef long long int64;
+typedef unsigned long long uint64;
+
+typedef struct { char *p; int n; } GoString;
+typedef void *GoMap;
+typedef void *GoChan;
+typedef struct { void *t; void *v; } GoInterface;
+`