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cmd/link: use a .def file to mark exported symbols on Windows

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Binutils defaults to exporting all symbols when building a Windows DLL.
To avoid that we were marking symbols with __declspec(dllexport) in
the cgo-generated headers, which instructs ld to export only those
symbols. However, that approach makes the headers hard to reuse when
importing the resulting DLL into other projects, as imported symbols
should be marked with __declspec(dllimport).

A better approach is to generate a .def file listing the symbols to
export, which gets the same effect without having to modify the headers.

Updates #30674
Fixes #56994

Change-Id: I22bd0aa079e2be4ae43b13d893f6b804eaeddabf
Reviewed-on: https://go-review.googlesource.com/c/go/+/705776
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Than McIntosh <thanm@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
qmuntal 2025-09-22 15:48:36 +02:00 • committed by Quim Muntal
parent 4b77733565
commit eaf2345256
8 changed files with 118 additions and 65 deletions
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75
src/cmd/cgo/internal/testcshared/cshared_test.go
View file

@ -8,6 +8,7 @@ import (
"bufio"
"bytes"
"cmd/cgo/internal/cgotest"
"cmp"
"debug/elf"
"debug/pe"
"encoding/binary"
@ -272,7 +273,7 @@ func createHeaders() error {
// which results in the linkers output implib getting overwritten at each step. So instead build the
// import library the traditional way, using a def file.
err = os.WriteFile("libgo.def",
[]byte("LIBRARY libgo.dll\nEXPORTS\n\tDidInitRun\n\tDidMainRun\n\tDivu\n\tFromPkg\n\t_cgo_dummy_export\n"),
[]byte("LIBRARY libgo.dll\nEXPORTS\n\tDidInitRun\n\tDidMainRun\n\tDivu\n\tFromPkg\n"),
0644)
if err != nil {
return fmt.Errorf("unable to write def file: %v", err)
@ -375,9 +376,23 @@ func TestExportedSymbols(t *testing.T) {
}
}
func checkNumberOfExportedFunctionsWindows(t *testing.T, prog string, exportedFunctions int, wantAll bool) {
func checkNumberOfExportedSymbolsWindows(t *testing.T, exportedSymbols int, wantAll bool) {
t.Parallel()
tmpdir := t.TempDir()
prog := `
package main
import "C"
func main() {}
`
for i := range exportedSymbols {
prog += fmt.Sprintf(`
//export GoFunc%d
func GoFunc%d() {}
`, i, i)
}
srcfile := filepath.Join(tmpdir, "test.go")
objfile := filepath.Join(tmpdir, "test.dll")
if err := os.WriteFile(srcfile, []byte(prog), 0666); err != nil {
@ -443,18 +458,19 @@ func checkNumberOfExportedFunctionsWindows(t *testing.T, prog string, exportedFu
t.Fatalf("binary.Read failed: %v", err)
}
// Only the two exported functions and _cgo_dummy_export should be exported.
exportedSymbols = cmp.Or(exportedSymbols, 1) // _cgo_stub_export is exported if there are no other symbols exported
// NumberOfNames is the number of functions exported with a unique name.
// NumberOfFunctions can be higher than that because it also counts
// functions exported only by ordinal, a unique number asigned by the linker,
// and linkers might add an unknown number of their own ordinal-only functions.
if wantAll {
if e.NumberOfNames <= uint32(exportedFunctions) {
t.Errorf("got %d exported names, want > %d", e.NumberOfNames, exportedFunctions)
if e.NumberOfNames <= uint32(exportedSymbols) {
t.Errorf("got %d exported names, want > %d", e.NumberOfNames, exportedSymbols)
}
} else {
if e.NumberOfNames > uint32(exportedFunctions) {
t.Errorf("got %d exported names, want <= %d", e.NumberOfNames, exportedFunctions)
if e.NumberOfNames != uint32(exportedSymbols) {
t.Errorf("got %d exported names, want %d", e.NumberOfNames, exportedSymbols)
}
}
}
@ -470,43 +486,14 @@ func TestNumberOfExportedFunctions(t *testing.T) {
t.Parallel()
const prog0 = `
package main
import "C"
func main() {
}
`
const prog2 = `
package main
import "C"
//export GoFunc
func GoFunc() {
println(42)
}
//export GoFunc2
func GoFunc2() {
println(24)
}
func main() {
}
`
// All programs export _cgo_dummy_export, so add 1 to the expected counts.
t.Run("OnlyExported/0", func(t *testing.T) {
checkNumberOfExportedFunctionsWindows(t, prog0, 0+1, false)
})
t.Run("OnlyExported/2", func(t *testing.T) {
checkNumberOfExportedFunctionsWindows(t, prog2, 2+1, false)
})
t.Run("All", func(t *testing.T) {
checkNumberOfExportedFunctionsWindows(t, prog2, 2+1, true)
})
for i := range 3 {
t.Run(fmt.Sprintf("OnlyExported/%d", i), func(t *testing.T) {
checkNumberOfExportedSymbolsWindows(t, i, false)
})
t.Run(fmt.Sprintf("All/%d", i), func(t *testing.T) {
checkNumberOfExportedSymbolsWindows(t, i, true)
})
}
}
// test1: shared library can be dynamically loaded and exported symbols are accessible.