os, syscall: add Unsetenv

Also address a TODO, making Clearenv pass through to cgo.

Based largely on Minux's earlier https://golang.org/cl/82040044

Fixes #6423

LGTM=iant, alex.brainman, r, rsc
R=rsc, iant, r, alex.brainman
CC=golang-codereviews
https://golang.org/cl/148370043

1 files changed, 12 insertions, 1 deletions

diff --git a/src/runtime/env_posix.go b/src/runtime/env_posix.go
index 6c04f6cc70..dd57872d7c 100644
--- a/src/runtime/env_posix.go
+++ b/src/runtime/env_posix.go
@@ -32,7 +32,8 @@ func gogetenv(key string) string {
 	return ""
 }
 
-var _cgo_setenv uintptr // pointer to C function
+var _cgo_setenv uintptr   // pointer to C function
+var _cgo_unsetenv uintptr // pointer to C function
 
 // Update the C environment if cgo is loaded.
 // Called from syscall.Setenv.
@@ -44,6 +45,16 @@ func syscall_setenv_c(k string, v string) {
 	asmcgocall(unsafe.Pointer(_cgo_setenv), unsafe.Pointer(&arg))
 }
 
+// Update the C environment if cgo is loaded.
+// Called from syscall.unsetenv.
+func syscall_unsetenv_c(k string) {
+	if _cgo_unsetenv == 0 {
+		return
+	}
+	arg := [1]unsafe.Pointer{cstring(k)}
+	asmcgocall(unsafe.Pointer(_cgo_unsetenv), unsafe.Pointer(&arg))
+}
+
 func cstring(s string) unsafe.Pointer {
 	p := make([]byte, len(s)+1)
 	sp := (*_string)(unsafe.Pointer(&s))