os: implement and smoketest os.Unsetenv

src/runtime/env_linux.go

@@ -13,6 +13,16 @@ func syscall_setenv_c(key string, val string) {
 	return
 }
 
+// Update the C environment if cgo is loaded.
+// Called from syscall.Unsetenv.
+//go:linkname syscall_unsetenv_c syscall.unsetenv_c
+func syscall_unsetenv_c(key string) {
+	keydata := cstring(key)
+	// ignore any errors
+	libc_unsetenv(&keydata[0])
+	return
+}
+
 // cstring converts a Go string to a C string.
 // borrowed from syscall
 func cstring(s string) []byte {
@@ -25,3 +35,7 @@ func cstring(s string) []byte {
 // int setenv(const char *name, const char *val, int replace);
 //export setenv
 func libc_setenv(name *byte, val *byte, replace int32) int32
+
+// int unsetenv(const char *name);
+//export unsetenv
+func libc_unsetenv(name *byte) int32