diff --git a/internal/clang/clang_test.go b/internal/clang/clang_test.go
new file mode 100644
index 00000000..b998f977
--- /dev/null
+++ b/internal/clang/clang_test.go
@@ -0,0 +1,510 @@
+/*
+ * Copyright (c) 2024 The GoPlus Authors (goplus.org). All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package clang
+
+import (
+	"bytes"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestConfig(t *testing.T) {
+	t.Run("NewConfig", func(t *testing.T) {
+		ccflags := []string{"-O2", "-g"}
+		cflags := []string{"-std=c99"}
+		ldflags := []string{"-lm"}
+
+		config := NewConfig("clang++", ccflags, cflags, ldflags, "ld.lld")
+
+		if config.CC != "clang++" {
+			t.Errorf("Expected CC to be 'clang++', got %q", config.CC)
+		}
+		if !reflect.DeepEqual(config.CCFLAGS, ccflags) {
+			t.Errorf("Expected CCFLAGS %v, got %v", ccflags, config.CCFLAGS)
+		}
+		if !reflect.DeepEqual(config.CFLAGS, cflags) {
+			t.Errorf("Expected CFLAGS %v, got %v", cflags, config.CFLAGS)
+		}
+		if !reflect.DeepEqual(config.LDFLAGS, ldflags) {
+			t.Errorf("Expected LDFLAGS %v, got %v", ldflags, config.LDFLAGS)
+		}
+		if config.Linker != "ld.lld" {
+			t.Errorf("Expected Linker to be 'ld.lld', got %q", config.Linker)
+		}
+	})
+}
+
+func TestNew(t *testing.T) {
+	t.Run("WithApp", func(t *testing.T) {
+		config := Config{CC: "gcc"}
+		cmd := New("clang++", config)
+
+		if cmd.app != "clang++" {
+			t.Errorf("Expected app to be 'clang++', got %q", cmd.app)
+		}
+		if !reflect.DeepEqual(cmd.config, config) {
+			t.Errorf("Expected config %+v, got %+v", config, cmd.config)
+		}
+		if cmd.Stdout != os.Stdout {
+			t.Error("Expected Stdout to be os.Stdout")
+		}
+		if cmd.Stderr != os.Stderr {
+			t.Error("Expected Stderr to be os.Stderr")
+		}
+	})
+
+	t.Run("WithoutApp", func(t *testing.T) {
+		config := Config{}
+		cmd := New("", config)
+
+		if cmd.app != "clang" {
+			t.Errorf("Expected app to be 'clang', got %q", cmd.app)
+		}
+	})
+}
+
+func TestNewCompiler(t *testing.T) {
+	t.Run("WithCC", func(t *testing.T) {
+		config := Config{CC: "gcc"}
+		cmd := NewCompiler(config)
+
+		if cmd.app != "gcc" {
+			t.Errorf("Expected app to be 'gcc', got %q", cmd.app)
+		}
+	})
+
+	t.Run("WithoutCC", func(t *testing.T) {
+		config := Config{}
+		cmd := NewCompiler(config)
+
+		if cmd.app != "clang" {
+			t.Errorf("Expected app to be 'clang', got %q", cmd.app)
+		}
+	})
+}
+
+func TestNewLinker(t *testing.T) {
+	t.Run("WithLinker", func(t *testing.T) {
+		config := Config{Linker: "ld.lld"}
+		cmd := NewLinker(config)
+
+		if cmd.app != "ld.lld" {
+			t.Errorf("Expected app to be 'ld.lld', got %q", cmd.app)
+		}
+	})
+
+	t.Run("WithoutLinkerButWithCC", func(t *testing.T) {
+		config := Config{CC: "gcc"}
+		cmd := NewLinker(config)
+
+		if cmd.app != "gcc" {
+			t.Errorf("Expected app to be 'gcc', got %q", cmd.app)
+		}
+	})
+
+	t.Run("WithoutLinkerAndCC", func(t *testing.T) {
+		config := Config{}
+		cmd := NewLinker(config)
+
+		if cmd.app != "clang" {
+			t.Errorf("Expected app to be 'clang', got %q", cmd.app)
+		}
+	})
+}
+
+func TestMergeCompilerFlags(t *testing.T) {
+	// Save original environment
+	origCCFlags := os.Getenv("CCFLAGS")
+	origCFlags := os.Getenv("CFLAGS")
+	defer func() {
+		os.Setenv("CCFLAGS", origCCFlags)
+		os.Setenv("CFLAGS", origCFlags)
+	}()
+
+	t.Run("WithEnvironmentAndConfig", func(t *testing.T) {
+		os.Setenv("CCFLAGS", "-O2 -g")
+		os.Setenv("CFLAGS", "-std=c99")
+
+		config := Config{
+			CCFLAGS: []string{"-Wall"},
+			CFLAGS:  []string{"-pedantic"},
+		}
+		cmd := New("clang", config)
+
+		flags := cmd.mergeCompilerFlags()
+		expected := []string{"-O2", "-g", "-std=c99", "-Wall", "-pedantic"}
+
+		if !reflect.DeepEqual(flags, expected) {
+			t.Errorf("Expected flags %v, got %v", expected, flags)
+		}
+	})
+
+	t.Run("WithOnlyConfig", func(t *testing.T) {
+		os.Unsetenv("CCFLAGS")
+		os.Unsetenv("CFLAGS")
+
+		config := Config{
+			CCFLAGS: []string{"-Wall"},
+			CFLAGS:  []string{"-pedantic"},
+		}
+		cmd := New("clang", config)
+
+		flags := cmd.mergeCompilerFlags()
+		expected := []string{"-Wall", "-pedantic"}
+
+		if !reflect.DeepEqual(flags, expected) {
+			t.Errorf("Expected flags %v, got %v", expected, flags)
+		}
+	})
+
+	t.Run("WithOnlyEnvironment", func(t *testing.T) {
+		os.Setenv("CCFLAGS", "-O3")
+		os.Setenv("CFLAGS", "-fPIC")
+
+		config := Config{}
+		cmd := New("clang", config)
+
+		flags := cmd.mergeCompilerFlags()
+		expected := []string{"-O3", "-fPIC"}
+
+		if !reflect.DeepEqual(flags, expected) {
+			t.Errorf("Expected flags %v, got %v", expected, flags)
+		}
+	})
+
+	t.Run("Empty", func(t *testing.T) {
+		os.Unsetenv("CCFLAGS")
+		os.Unsetenv("CFLAGS")
+
+		config := Config{}
+		cmd := New("clang", config)
+
+		flags := cmd.mergeCompilerFlags()
+
+		if len(flags) != 0 {
+			t.Errorf("Expected empty flags, got %v", flags)
+		}
+	})
+}
+
+func TestMergeLinkerFlags(t *testing.T) {
+	// Save original environment
+	origCCFlags := os.Getenv("CCFLAGS")
+	origLDFlags := os.Getenv("LDFLAGS")
+	defer func() {
+		os.Setenv("CCFLAGS", origCCFlags)
+		os.Setenv("LDFLAGS", origLDFlags)
+	}()
+
+	t.Run("WithEnvironmentAndConfig", func(t *testing.T) {
+		os.Setenv("CCFLAGS", "-O2")
+		os.Setenv("LDFLAGS", "-lm -lpthread")
+
+		config := Config{
+			LDFLAGS: []string{"-static"},
+		}
+		cmd := New("clang", config)
+
+		flags := cmd.mergeLinkerFlags()
+		expected := []string{"-O2", "-lm", "-lpthread", "-static"}
+
+		if !reflect.DeepEqual(flags, expected) {
+			t.Errorf("Expected flags %v, got %v", expected, flags)
+		}
+	})
+
+	t.Run("WithOnlyConfig", func(t *testing.T) {
+		os.Unsetenv("CCFLAGS")
+		os.Unsetenv("LDFLAGS")
+
+		config := Config{
+			LDFLAGS: []string{"-static", "-lm"},
+		}
+		cmd := New("clang", config)
+
+		flags := cmd.mergeLinkerFlags()
+		expected := []string{"-static", "-lm"}
+
+		if !reflect.DeepEqual(flags, expected) {
+			t.Errorf("Expected flags %v, got %v", expected, flags)
+		}
+	})
+}
+
+func TestCompile(t *testing.T) {
+	// This test uses echo instead of clang to avoid dependency on clang installation
+	config := Config{
+		CCFLAGS: []string{"-Wall"},
+		CFLAGS:  []string{"-std=c99"},
+	}
+	cmd := New("echo", config)
+
+	var stdout bytes.Buffer
+	cmd.Stdout = &stdout
+
+	err := cmd.Compile("-c", "test.c")
+	if err != nil {
+		t.Errorf("Compile failed: %v", err)
+	}
+
+	output := strings.TrimSpace(stdout.String())
+	expectedArgs := "-Wall -std=c99 -c test.c"
+	if output != expectedArgs {
+		t.Errorf("Expected output %q, got %q", expectedArgs, output)
+	}
+}
+
+func TestLink(t *testing.T) {
+	// This test uses echo instead of clang to avoid dependency on clang installation
+	config := Config{
+		LDFLAGS: []string{"-lm"},
+	}
+	cmd := New("echo", config)
+
+	var stdout bytes.Buffer
+	cmd.Stdout = &stdout
+
+	err := cmd.Link("test.o", "-o", "test")
+	if err != nil {
+		t.Errorf("Link failed: %v", err)
+	}
+
+	output := strings.TrimSpace(stdout.String())
+	expectedArgs := "-lm test.o -o test"
+	if output != expectedArgs {
+		t.Errorf("Expected output %q, got %q", expectedArgs, output)
+	}
+}
+
+func TestVerboseMode(t *testing.T) {
+	config := Config{}
+	cmd := New("echo", config)
+	cmd.Verbose = true
+
+	// Since verbose output goes to os.Stderr directly, we'll test
+	// that verbose flag is set and command executes successfully
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
+	cmd.Stdout = &bytes.Buffer{} // Suppress stdout
+
+	err := cmd.Compile("-c", "test.c")
+	if err != nil {
+		t.Errorf("Compile failed: %v", err)
+	}
+
+	// The test passes if verbose is set and no error occurred
+	if !cmd.Verbose {
+		t.Error("Expected Verbose to be true")
+	}
+}
+
+func TestCmdEnvironment(t *testing.T) {
+	config := Config{}
+	cmd := New("echo", config)
+	cmd.Env = []string{"TEST_VAR=test_value"}
+
+	var stdout bytes.Buffer
+	cmd.Stdout = &stdout
+
+	// Use a command that will show environment variables
+	// Note: This is a simplified test that just ensures the Env field is set
+	err := cmd.Compile("-c", "test.c")
+	if err != nil {
+		t.Errorf("Compile failed: %v", err)
+	}
+
+	if len(cmd.Env) != 1 || cmd.Env[0] != "TEST_VAR=test_value" {
+		t.Errorf("Expected environment to be set correctly")
+	}
+}
+
+func TestCmdIO(t *testing.T) {
+	config := Config{}
+	cmd := New("cat", config) // Use cat to test stdin/stdout
+
+	input := "test input"
+	cmd.Stdin = strings.NewReader(input)
+
+	var stdout bytes.Buffer
+	cmd.Stdout = &stdout
+
+	// cat will read from stdin and write to stdout
+	err := cmd.exec() // Call exec directly with no args
+	if err != nil {
+		t.Errorf("exec failed: %v", err)
+	}
+
+	output := stdout.String()
+	if output != input {
+		t.Errorf("Expected output %q, got %q", input, output)
+	}
+}
+
+func TestCheckLinkArgs(t *testing.T) {
+	t.Run("BasicTest", func(t *testing.T) {
+		// Use echo instead of clang to avoid dependency
+		config := Config{}
+		cmd := New("echo", config)
+
+		// Redirect output to avoid clutter
+		cmd.Stdout = &bytes.Buffer{}
+		cmd.Stderr = &bytes.Buffer{}
+
+		// This should succeed with echo
+		err := cmd.CheckLinkArgs([]string{"-o"}, false)
+		if err != nil {
+			t.Errorf("CheckLinkArgs failed: %v", err)
+		}
+	})
+
+	t.Run("WasmExtension", func(t *testing.T) {
+		config := Config{}
+		cmd := New("echo", config)
+
+		cmd.Stdout = &bytes.Buffer{}
+		cmd.Stderr = &bytes.Buffer{}
+
+		// Test with wasm=true
+		err := cmd.CheckLinkArgs([]string{"-o"}, true)
+		if err != nil {
+			t.Errorf("CheckLinkArgs with wasm failed: %v", err)
+		}
+	})
+}
+
+// Additional table-driven test for flag merging scenarios
+func TestFlagMergingScenarios(t *testing.T) {
+	tests := []struct {
+		name       string
+		envCCFlags string
+		envCFlags  string
+		envLDFlags string
+		config     Config
+		expectComp []string
+		expectLink []string
+	}{
+		{
+			name:       "empty everything",
+			envCCFlags: "",
+			envCFlags:  "",
+			envLDFlags: "",
+			config:     Config{},
+			expectComp: []string{},
+			expectLink: []string{},
+		},
+		{
+			name:       "only environment flags",
+			envCCFlags: "-O2",
+			envCFlags:  "-std=c99",
+			envLDFlags: "-lm",
+			config:     Config{},
+			expectComp: []string{"-O2", "-std=c99"},
+			expectLink: []string{"-O2", "-lm"},
+		},
+		{
+			name:       "only config flags",
+			envCCFlags: "",
+			envCFlags:  "",
+			envLDFlags: "",
+			config: Config{
+				CCFLAGS: []string{"-Wall"},
+				CFLAGS:  []string{"-pedantic"},
+				LDFLAGS: []string{"-static"},
+			},
+			expectComp: []string{"-Wall", "-pedantic"},
+			expectLink: []string{"-static"},
+		},
+		{
+			name:       "mixed environment and config",
+			envCCFlags: "-O3",
+			envCFlags:  "-fPIC",
+			envLDFlags: "-lm -lpthread",
+			config: Config{
+				CCFLAGS: []string{"-Wall", "-Wextra"},
+				CFLAGS:  []string{"-std=c11"},
+				LDFLAGS: []string{"-static"},
+			},
+			expectComp: []string{"-O3", "-fPIC", "-Wall", "-Wextra", "-std=c11"},
+			expectLink: []string{"-O3", "-lm", "-lpthread", "-static"},
+		},
+	}
+
+	// Save original environment
+	origCCFlags := os.Getenv("CCFLAGS")
+	origCFlags := os.Getenv("CFLAGS")
+	origLDFlags := os.Getenv("LDFLAGS")
+	defer func() {
+		os.Setenv("CCFLAGS", origCCFlags)
+		os.Setenv("CFLAGS", origCFlags)
+		os.Setenv("LDFLAGS", origLDFlags)
+	}()
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Set environment variables
+			os.Setenv("CCFLAGS", tt.envCCFlags)
+			os.Setenv("CFLAGS", tt.envCFlags)
+			os.Setenv("LDFLAGS", tt.envLDFlags)
+
+			cmd := New("clang", tt.config)
+
+			// Test compiler flags
+			compFlags := cmd.mergeCompilerFlags()
+			if len(tt.expectComp) == 0 && len(compFlags) == 0 {
+				// Both are empty, consider them equal
+			} else if !reflect.DeepEqual(compFlags, tt.expectComp) {
+				t.Errorf("mergeCompilerFlags() = %v, want %v", compFlags, tt.expectComp)
+			}
+
+			// Test linker flags
+			linkFlags := cmd.mergeLinkerFlags()
+			if len(tt.expectLink) == 0 && len(linkFlags) == 0 {
+				// Both are empty, consider them equal
+			} else if !reflect.DeepEqual(linkFlags, tt.expectLink) {
+				t.Errorf("mergeLinkerFlags() = %v, want %v", linkFlags, tt.expectLink)
+			}
+		})
+	}
+}
+
+// Benchmark tests
+func BenchmarkMergeCompilerFlags(b *testing.B) {
+	config := Config{
+		CCFLAGS: []string{"-Wall", "-O2"},
+		CFLAGS:  []string{"-std=c99", "-pedantic"},
+	}
+	cmd := New("clang", config)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = cmd.mergeCompilerFlags()
+	}
+}
+
+func BenchmarkMergeLinkerFlags(b *testing.B) {
+	config := Config{
+		LDFLAGS: []string{"-lm", "-lpthread", "-static"},
+	}
+	cmd := New("clang", config)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = cmd.mergeLinkerFlags()
+	}
+}