diff --git a/cmd/tools/modules/testing/common.v b/cmd/tools/modules/testing/common.v
index cd20ba80a1..6eddb5b140 100644
--- a/cmd/tools/modules/testing/common.v
+++ b/cmd/tools/modules/testing/common.v
@@ -1,6 +1,7 @@
 module testing
 
 import os
+import time
 import term
 import benchmark
 import sync
@@ -59,6 +60,13 @@ pub fn (mut ts TestSession) init() {
 }
 
 pub fn (mut ts TestSession) test() {
+	// Ensure that .tmp.c files generated from compiling _test.v files,
+	// are easy to delete at the end, *without* affecting the existing ones.
+	now := time.sys_mono_now()
+	new_vtmp_dir := os.join_path(os.temp_dir(), 'v', 'test_session_$now')
+	os.mkdir_all(new_vtmp_dir)
+	os.setenv('VTMP', new_vtmp_dir, true)
+	//
 	ts.init()
 	mut remaining_files := []string{}
 	for dot_relative_file in ts.files {
@@ -99,6 +107,10 @@ pub fn (mut ts TestSession) test() {
 	pool_of_test_runners.work_on_pointers(remaining_files.pointers())
 	ts.benchmark.stop()
 	eprintln(term.h_divider('-'))
+	// cleanup generated .tmp.c files after successfull tests:
+	if ts.benchmark.nfail == 0 {
+		os.rmdir_all(new_vtmp_dir)
+	}
 }
 
 pub fn (mut m TestMessageHandler) display_message() {
diff --git a/vlib/os/os.v b/vlib/os/os.v
index 41651bf32f..ea31a71831 100644
--- a/vlib/os/os.v
+++ b/vlib/os/os.v
@@ -1225,12 +1225,14 @@ pub fn flush() {
 
 pub fn mkdir_all(path string) {
 	mut p := if path.starts_with(os.path_separator) { os.path_separator } else { '' }
-	for subdir in path.split(os.path_separator) {
+	path_parts := path.trim_left(os.path_separator).split(os.path_separator)
+	for subdir in path_parts {
 		p += subdir + os.path_separator
-		if !os.is_dir(p) {
-			os.mkdir(p) or {
-				panic(err)
-			}
+		if os.exists(p) && os.is_dir(p) {
+			continue
+		}
+		os.mkdir(p) or {
+			panic('folder: $p, error: $err')
 		}
 	}
 }
diff --git a/vlib/v/builder/compile.v b/vlib/v/builder/compile.v
index 863a1473ae..dac716be64 100644
--- a/vlib/v/builder/compile.v
+++ b/vlib/v/builder/compile.v
@@ -8,12 +8,15 @@ import os
 import v.pref
 
 fn get_vtmp_folder() string {
-	vtmp := os.join_path(os.temp_dir(), 'v')
-	if !os.is_dir(vtmp) {
-		os.mkdir(vtmp) or {
-			verror(err)
-		}
+	mut vtmp := os.getenv('VTMP')
+	if vtmp.len > 0 {
+		return vtmp
 	}
+	vtmp = os.join_path(os.temp_dir(), 'v')
+	if !os.exists(vtmp) || !os.is_dir(vtmp) {
+		os.mkdir_all(vtmp)
+	}
+	os.setenv('VTMP', vtmp, true)
 	return vtmp
 }