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
https://golang.org/cl/853042

src/Make.pkg

@@ -44,7 +44,7 @@ coverage:
 	$(QUOTED_GOBIN)/gotest
 	$(QUOTED_GOBIN)/6cov -g $(shell pwd) $O.out | grep -v '_test\.go:'
 
-CLEANFILES+=*.cgo1.go *.cgo2.c _cgo_defun.c _cgo_gotypes.go *.so _obj _test _testmain.go
+CLEANFILES+=*.cgo1.go *.cgo2.c _cgo_defun.c _cgo_gotypes.go _cgo_export.* *.so _obj _test _testmain.go
 
 test:
 	$(QUOTED_GOBIN)/gotest
@@ -92,9 +92,10 @@ dir:
 #
 # to the main Makefile.  This signals that cgo should process x.go
 # and y.go when building the package.
-# There are two optional variables to set, CGO_CFLAGS and CGO_LDFLAGS,
-# which specify compiler and linker flags to use when compiling
-# (using gcc) the C support for x.go and y.go.
+# There are three optional variables to set, CGO_CFLAGS, CGO_LDFLAGS,
+# and CGO_DEPS, which specify compiler flags, linker flags, and linker
+# dependencies to use when compiling (using gcc) the C support for
+# x.go and y.go.
 
 # Cgo translates each x.go file listed in $(CGOFILES) into a basic
 # translation of x.go, called x.cgo1.go. Additionally, three other
@@ -118,6 +119,9 @@ _cgo_defun.c _cgo_gotypes.go: $(CGOFILES)
 %.cgo2.o: %.cgo2.c
 	gcc $(_CGO_CFLAGS_$(GOARCH)) -fPIC -O2 -o $@ -c $(CGO_CFLAGS) $*.cgo2.c
 
+_cgo_export.o: _cgo_export.c _cgo_export.h
+	gcc $(_CGO_CFLAGS_$(GOARCH)) -fPIC -O2 -o $@ -c $(CGO_CFLAGS) _cgo_export.c
+
 # The rules above added x.cgo1.go and _cgo_gotypes.go to $(GOFILES),
 # added _cgo_defun.$O to $OFILES, and added the installed copy of
 # package_x.so (built from x.cgo2.c) to $(INSTALLFILES).
@@ -139,7 +143,7 @@ _CGO_LDFLAGS_darwin=-dynamiclib -Wl,-undefined,dynamic_lookup
 _cgo_defun.$O: _cgo_defun.c
 	$(QUOTED_GOBIN)/$(CC) $(CFLAGS) $(RUNTIME_CFLAGS) _cgo_defun.c
 
-_cgo_.so: $(GCC_OFILES)
+_cgo_.so: $(GCC_OFILES) $(CGO_DEPS)
 	gcc $(_CGO_CFLAGS_$(GOARCH)) -o $@ $(GCC_OFILES) $(CGO_LDFLAGS)  $(_CGO_LDFLAGS_$(GOOS))
 
 $(pkgdir)/$(TARG).so: _cgo_.so

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
+	}
+}