cmd/cgo: add -exportheader option

The -exportheader option tells cgo to generate a header file declaring
expoted functions.  The header file is only created if there are, in
fact, some exported functions, so it also serves as a signal as to
whether there were any.

In future CLs the go tool will use this option to install header files
for packages that use cgo and export functions.

Change-Id: I5b04357d453a9a8f0e70d37f8f18274cf40d74c9
Reviewed-on: https://go-review.googlesource.com/9796Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/cgo/doc.go

@@ -242,6 +242,10 @@ The following options are available when running cgo directly:
 	-importpath string
 		The import path for the Go package. Optional; used for
 		nicer comments in the generated files.
+	-exportheader file
+		If there are any exported functions, write the
+		generated export declarations to file.
+		C code can #include this to see the declarations.
 	-gccgo
 		Generate output for the gccgo compiler rather than the
 		gc compiler.

src/cmd/cgo/main.go

@@ -169,6 +169,7 @@ var godefs = flag.Bool("godefs", false, "for bootstrap: write Go definitions for
 
 var objDir = flag.String("objdir", "", "object directory")
 var importPath = flag.String("importpath", "", "import path of package being built (for comments in generated files)")
+var exportHeader = flag.String("exportheader", "", "where to write export header if any exported functions")
 
 var gccgo = flag.Bool("gccgo", false, "generate files for use with gccgo")
 var gccgoprefix = flag.String("gccgoprefix", "", "-fgo-prefix option used with gccgo")

src/cmd/cgo/out.go

@@ -166,10 +166,33 @@ func (p *Package) writeDefs() {
 		}
 	}
 
+	fgcc := creat(*objDir + "_cgo_export.c")
+	fgcch := creat(*objDir + "_cgo_export.h")
 	if *gccgo {
-		p.writeGccgoExports(fgo2, fm)
+		p.writeGccgoExports(fgo2, fm, fgcc, fgcch)
 	} else {
-		p.writeExports(fgo2, fm)
+		p.writeExports(fgo2, fm, fgcc, fgcch)
+	}
+	if err := fgcc.Close(); err != nil {
+		fatalf("%s", err)
+	}
+	if err := fgcch.Close(); err != nil {
+		fatalf("%s", err)
+	}
+
+	if *exportHeader != "" && len(p.ExpFunc) > 0 {
+		fexp := creat(*exportHeader)
+		fgcch, err := os.Open(*objDir + "_cgo_export.h")
+		if err != nil {
+			fatalf("%s", err)
+		}
+		_, err = io.Copy(fexp, fgcch)
+		if err != nil {
+			fatalf("%s", err)
+		}
+		if err = fexp.Close(); err != nil {
+			fatalf("%s", err)
+		}
 	}
 
 	init := gccgoInit.String()
@@ -634,10 +657,7 @@ func (p *Package) packedAttribute() string {
 
 // Write out the various stubs we need to support functions exported
 // from Go so that they are callable from C.
-func (p *Package) writeExports(fgo2, fm io.Writer) {
-	fgcc := creat(*objDir + "_cgo_export.c")
-	fgcch := creat(*objDir + "_cgo_export.h")
-
+func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 	p.writeExportHeader(fgcch)
 
 	fmt.Fprintf(fgcc, "/* Created by cgo - DO NOT EDIT. */\n")
@@ -829,10 +849,7 @@ func (p *Package) writeExports(fgo2, fm io.Writer) {
 }
 
 // Write out the C header allowing C code to call exported gccgo functions.
-func (p *Package) writeGccgoExports(fgo2, fm io.Writer) {
-	fgcc := creat(*objDir + "_cgo_export.c")
-	fgcch := creat(*objDir + "_cgo_export.h")
-
+func (p *Package) writeGccgoExports(fgo2, fm, fgcc, fgcch io.Writer) {
 	gccgoSymbolPrefix := p.gccgoSymbolPrefix()
 
 	p.writeExportHeader(fgcch)