cmd/cgo: define exported Go func as function

src/cmd/cgo/gcc.go

@@ -1614,6 +1614,8 @@ func (p *Package) gccCmd() []string {
 		c = append(c, "-maix64")
 		c = append(c, "-mcmodel=large")
 	}
+	// disable LTO so we get a real object
+	c = append(c, "-fno-lto")
 	c = append(c, "-") //read input from standard input
 	return c
 }

src/cmd/cgo/out.go

@@ -168,8 +168,18 @@ func (p *Package) writeDefs() {
 			if *gccgo {
 				fmt.Fprintf(fc, "extern byte *%s;\n", n.C)
 			} else {
-				fmt.Fprintf(fm, "extern char %s[];\n", n.C)
-				fmt.Fprintf(fm, "void *_cgohack_%s = %s;\n\n", n.C, n.C)
+				// We must ensure the executable cgo is creating here has a
+				// reference to all symbols Go code is referring
+				// to -- because otherwise the system linker might decline
+				// to add DT_NEEDED entries for the library that has the symbol definition.
+				// We treat function pointer types differently to avoid link-time failures
+				// when using LTO with an external linker.
+				if n.Kind == "fpvar" {
+					fmt.Fprintf(fm, "extern void %s();\n", n.C)
+				} else {
+					fmt.Fprintf(fm, "extern char %s[];\n", n.C)
+					fmt.Fprintf(fm, "void *_cgohack_%s = %s;\n\n", n.C, n.C)
+				}
 				fmt.Fprintf(fgo2, "//go:linkname __cgo_%s %s\n", n.C, n.C)
 				fmt.Fprintf(fgo2, "//go:cgo_import_static %s\n", n.C)
 				fmt.Fprintf(fgo2, "var __cgo_%s byte\n", n.C)
@@ -1028,7 +1038,7 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 		fmt.Fprintf(fgo2, "//go:cgo_export_static _cgoexp%s_%s\n", cPrefix, exp.ExpName)
 		fmt.Fprintf(fgo2, "func _cgoexp%s_%s(a *%s) {\n", cPrefix, exp.ExpName, gotype)
 
-		fmt.Fprintf(fm, "int _cgoexp%s_%s;\n", cPrefix, exp.ExpName)
+		fmt.Fprintf(fm, "void _cgoexp%s_%s(void* p){}\n", cPrefix, exp.ExpName)
 
 		if gccResult != "void" {
 			// Write results back to frame.

src/cmd/go/testdata/script/issue_43830.txt

@@ -0,0 +1,38 @@
+# tests golang.org/issue/43830
+
+[!cgo] skip 'skipping test without cgo'
+[!exec:gcc] skip 'skipping test without gcc present'
+[openbsd] env CC='clang'
+[!openbsd] env CC='gcc'
+
+env CGO_CFLAGS='-Wno-ignored-optimization-argument -flto -ffat-lto-objects'
+
+go build main.go add.go
+
+-- main.go --
+
+package main
+
+/*
+int c_add(int a, int b) {
+	return myadd(a, b);
+}
+*/
+import "C"
+
+func main() {
+	println(C.c_add(1, 2))
+}
+
+-- add.go --
+
+package main
+
+import "C"
+
+/* test */
+
+//export myadd
+func myadd(a C.int, b C.int) C.int {
+	return a + b
+}

src/cmd/go/testdata/script/issue_43830_2.txt

@@ -0,0 +1,32 @@
+# tests golang.org/issue/43830
+
+[!cgo] skip 'skipping test without cgo'
+[!exec:gcc] skip 'skipping test without gcc present'
+[openbsd] env CC='clang'
+[!openbsd] env CC='gcc'
+
+env CGO_CFLAGS='-Wno-ignored-optimization-argument -flto -ffat-lto-objects'
+
+go build main.go
+
+-- main.go --
+
+package main
+
+import "fmt"
+
+// #include "hello.h"
+import "C"
+
+func main() {
+	hello := C.hello
+	fmt.Printf("%v\n", hello)
+}
+
+-- hello.h --
+
+#include <stdio.h>
+
+void hello(void) {
+  printf("hello\n");
+}