cmd/cgo: omit misaligned struct fields, like we omit bitfields

Fixes #7560.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/96300045

misc/cgo/test/cgo_test.go

@@ -52,5 +52,6 @@ func Test6390(t *testing.T)                { test6390(t) }
 func Test5986(t *testing.T)                { test5986(t) }
 func Test7665(t *testing.T)                { test7665(t) }
 func TestNaming(t *testing.T)              { testNaming(t) }
+func Test7560(t *testing.T)                { test7560(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue7560.go

+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cgotest
+
+/*
+#include <stdint.h>
+
+typedef struct {
+	char x;
+	long y;
+} __attribute__((__packed__)) misaligned;
+
+int
+offset7560(void)
+{
+	return (uintptr_t)&((misaligned*)0)->y;
+}
+*/
+import "C"
+
+import (
+	"reflect"
+	"testing"
+)
+
+func test7560(t *testing.T) {
+	// some mingw don't implement __packed__ correctly.
+	if C.offset7560() != 1 {
+		t.Skip("C compiler did not pack struct")
+	}
+
+	// C.misaligned should have x but then a padding field to get to the end of the struct.
+	// There should not be a field named 'y'.
+	var v C.misaligned
+	rt := reflect.TypeOf(&v).Elem()
+	if rt.NumField() != 2 || rt.Field(0).Name != "x" || rt.Field(1).Name != "_" {
+		t.Errorf("unexpected fields in C.misaligned:\n")
+		for i := 0; i < rt.NumField(); i++ {
+			t.Logf("%+v\n", rt.Field(i))
+		}
+	}
+}

src/cmd/cgo/doc.go

@@ -68,6 +68,9 @@ Go references to C
 Within the Go file, C's struct field names that are keywords in Go
 can be accessed by prefixing them with an underscore: if x points at a C
 struct with a field named "type", x._type accesses the field.
+C struct fields that cannot be expressed in Go, such as bit fields
+or misaligned data, are omitted in the Go struct, replaced by
+appropriate padding to reach the next field or the end of the struct.
 
 The standard C numeric types are available under the names
 C.char, C.schar (signed char), C.uchar (unsigned char),

src/cmd/cgo/gcc.go

@@ -1499,7 +1499,7 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 		t := c.Type(f.Type, pos)
 		tgo := t.Go
 		size := t.Size
-
+		talign := t.Align
 		if f.BitSize > 0 {
 			if f.BitSize%8 != 0 {
 				continue
@@ -1512,8 +1512,17 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 				name = "uint"
 			}
 			tgo = ast.NewIdent(name + fmt.Sprint(f.BitSize))
+			talign = size
 		}
 
+		if talign > 0 && f.ByteOffset%talign != 0 {
+			// Drop misaligned fields, the same way we drop integer bit fields.
+			// The goal is to make available what can be made available.
+			// Otherwise one bad and unneeded field in an otherwise okay struct
+			// makes the whole program not compile. Much of the time these
+			// structs are in system headers that cannot be corrected.
+			continue
+		}
 		n := len(fld)
 		fld = fld[0 : n+1]
 		name := f.Name
@@ -1528,8 +1537,8 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 		buf.WriteString(" ")
 		buf.WriteString(name)
 		buf.WriteString("; ")
-		if t.Align > align {
-			align = t.Align
+		if talign > align {
+			align = talign
 		}
 	}
 	if off < dt.ByteSize {