Set CC and CXX environment variables for clang toolchain #1169

betamaxbandit · 2025-05-27T22:51:29Z

Sets the C and C++ compiler environment variables so CMake uses Clang instead of the system default compiler.

Fixes #1140

betamaxbandit · 2025-05-27T22:52:43Z

Thanks to @Kummallinen for the suggestion to set cc environment variables.

I'll update this soon with details of how to test.

build/org.eclipse.cdt.build.gcc.core/src/org/eclipse/cdt/build/gcc/core/ClangToolChain.java

betamaxbandit · 2025-05-29T09:32:38Z

build/org.eclipse.cdt.build.gcc.core/src/org/eclipse/cdt/build/gcc/core/ClangToolChain.java

+		 * set(CMAKE_C_COMPILER clang)
+		 * set(CMAKE_CXX_COMPILER clang++)
+		 */
+		addIfAbsent("cc", "clang", envVarsNew); //$NON-NLS-1$ //$NON-NLS-2$


cc and cxx should be upper case!

Does envVarsNew already include the system environment, or is it just the environment set by other parts of Core Build? If it includes the system environment then we cant use addIfAbsent as they will likely already be set if you're on Linux or macOS

Does envVarsNew already include the system environment,

No; its the env from the core build and/or any env vars set by the user for a "User Defined Toolchain".

jld01 · 2025-05-30T11:55:33Z

build/org.eclipse.cdt.build.gcc.core/src/org/eclipse/cdt/build/gcc/core/ClangToolChain.java

+	private static IEnvironmentVariable[] addClangEnvVars(IEnvironmentVariable[] envVars) {
+		List<IEnvironmentVariable> envVarsNew = new ArrayList<>(Arrays.asList(envVars));
+		/*
+		 * Set CC and CXX environment variables for clang and clang++. This is equivalent to setting these in the


Do we also need to set the ASM environment variable?

Do we also need to set the ASM environment variable?

Good question. I do not think so. CMake appears to set ASM automatically to clang when CC is set to clang.

I tested this using the attached project.
cmakeASM.zip

In the screendshot below, it can be seen that the ASM is set to clang.exe.

betamaxbandit · 2025-06-02T16:52:37Z

Tests

Test cases
A) Toolchain set to clang, causes the clang compiler to be used automatically.
B) Toolchain set to gcc, uses the default gcc compiler automatically.
Tests should be performed on Linux & Windows.

Prerequisites
Make sure cmake, make and ninja and multiple toolchains are installed on the host, including clang. See [1].

To test
Test Case A) Toolchain set to clang, causes the clang compiler to be used automatically.
Create a CMake Hello World project
From the Launch Bar, open the Launch Configuration (gear icon)
Select Build Settings tab and in the Toolchain dropdown, change to clang toolchain and click OK.
From the Launch Bar, build the project.
Expected: The project builds successfully and in the Console build output it can be seen that the C and CXX compiler are identified as "Clang".

The following shows an example of Console build output:

Configuring in: C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -G Ninja C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw
-- The C compiler identification is Clang 19.1.7
-- The CXX compiler identification is Clang 19.1.7
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/msys64/ucrt64/bin/clang.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/msys64/ucrt64/bin/clang++.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (3.1s)
-- Generating done (0.1s)
-- Build files have been written to: C:/Users/a5107948/cdt-main20250110b/runtime-cdtruntime_clean/cmake_hw/build/cmake.debug.win32.x86_64.Local
Building in: C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
cmake --build . --target all
[1/2] Building CXX object CMakeFiles/cmake_hw.dir/cmake_hw.cpp.obj
[2/2] Linking CXX executable cmake_hw.exe
Build complete (0 errors, 0 warnings): C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local

Test case B) Toolchain set to gcc, uses the default gcc compiler automatically.
Continuing from test above, open the Build Settings tab and change the toolchain to gcc and click OK.
Delete the project's build folder.
From the Launch Bar, build the project.
Expected: The project builds successfully and in the Console build output it can be seen that the C and CXX compiler are identified as "GNU".

The following shows an example of Console build output:

Configuring in: C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -G Ninja C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw
-- The C compiler identification is GNU 14.2.0
-- The CXX compiler identification is GNU 14.2.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/msys64/ucrt64/bin/cc.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/msys64/ucrt64/bin/c++.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (3.2s)
-- Generating done (0.1s)
-- Build files have been written to: C:/Users/a5107948/cdt-main20250110b/runtime-cdtruntime_clean/cmake_hw/build/cmake.debug.win32.x86_64.Local
Building in: C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
cmake --build . --target all
[1/2] Building CXX object CMakeFiles/cmake_hw.dir/cmake_hw.cpp.obj
[2/2] Linking CXX executable cmake_hw.exe
Build complete (0 errors, 0 warnings): C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local

[1]: Before you begin
C/C++ Development User Guide > Before you begin
https://github.com/eclipse-cdt/cdt/blob/abe4d1b8e16b03ceafbeb25f063cb728c7487862/doc/org.eclipse.cdt.doc.user/concepts/cdt_c_before_you_begin.htm

jonahgraham · 2025-06-05T14:04:28Z

build/org.eclipse.cdt.build.gcc.core/src/org/eclipse/cdt/build/gcc/core/ClangToolChain.java

+	 * However, on Windows, existing cc/cxx envVars with lower-case names will have their names changed to upper case;
+	 * the value remains unchanged.<br>


Why do this - can you simply use same logic as on Linux? See code comment for why I am asking the question

In our meeting on Th 5 Jun, we discussed this and agreed on windows any lower case cc/cxx env vars should be changed to upper case.
Do you no longer think that is a good idea?

Otherwise, yes, the same logic as used on Linux can be used for Windows.

jonahgraham · 2025-06-05T14:07:44Z

build/org.eclipse.cdt.build.gcc.core/src/org/eclipse/cdt/build/gcc/core/ClangToolChain.java

+						iterator.remove();
+						envVars.add(new EnvironmentVariable(name, evOldValue));


Goes with comment on line 73 and I have a suggestion attached to line 81-111

When you do the remove + add you may be changing the type and logic of the variable. The removed one may not be an EnvironmentVariable but one of the other implementations of of IEnvironmentVariable

Yes I see that now. Renaming the variable name is tricky. Perhaps we should forget about it.

jonahgraham · 2025-06-05T14:15:03Z

build/org.eclipse.cdt.build.gcc.core/src/org/eclipse/cdt/build/gcc/core/ClangToolChain.java

+		if (Platform.OS_WIN32.equals(Platform.getOS())) {
+			// On Windows: case-insensitive check ignores case.
+			Iterator<IEnvironmentVariable> iterator = envVars.iterator();
+			boolean found = false;
+
+			while (iterator.hasNext()) {
+				IEnvironmentVariable ev = iterator.next();
+				if (ev.getName().equalsIgnoreCase(name)) {
+					found = true;
+					// Replace if existing name is in lower-case and new name is in upper-case
+					if (!ev.getName().equals(name) && name.equals(name.toUpperCase())) {
+						// Original value is respected.
+						String evOldValue = ev.getValue();
+						iterator.remove();
+						envVars.add(new EnvironmentVariable(name, evOldValue));
+					}
+					break;
+				}
+			}
+
+			if (!found) {
+				envVars.add(new EnvironmentVariable(name, value));
+			}
+
+		} else {
+			// On non-Windows: case-sensitive check respects case (exact match).
+			boolean isAbsent = envVars.stream().noneMatch(ev -> ev.getName().equals(name));
+			if (isAbsent) {
+				envVars.add(new EnvironmentVariable(name, value));
+			}
+		}


Suggested change

if (Platform.OS_WIN32.equals(Platform.getOS())) {

// On Windows: case-insensitive check ignores case.

Iterator<IEnvironmentVariable> iterator = envVars.iterator();

boolean found = false;

while (iterator.hasNext()) {

IEnvironmentVariable ev = iterator.next();

if (ev.getName().equalsIgnoreCase(name)) {

found = true;

// Replace if existing name is in lower-case and new name is in upper-case

if (!ev.getName().equals(name) && name.equals(name.toUpperCase())) {

// Original value is respected.

String evOldValue = ev.getValue();

iterator.remove();

envVars.add(new EnvironmentVariable(name, evOldValue));

}

break;

}

}

if (!found) {

envVars.add(new EnvironmentVariable(name, value));

}

} else {

// On non-Windows: case-sensitive check respects case (exact match).

boolean isAbsent = envVars.stream().noneMatch(ev -> ev.getName().equals(name));

if (isAbsent) {

envVars.add(new EnvironmentVariable(name, value));

}

}

boolean isAbsent = envVars.stream().noneMatch(ev -> {

if (Platform.OS_WIN32.equals(Platform.getOS())) {

return ev.getName().equalsIgnoreCase(name);

} else {

return ev.getName().equals(name);

}

});

if (isAbsent) {

envVars.add(new EnvironmentVariable(name, value));

}

build/org.eclipse.cdt.build.gcc.core/src/org/eclipse/cdt/build/gcc/core/ClangToolChain.java

betamaxbandit · 2025-06-17T20:24:46Z

Hi @jonahgraham ,
I have pushed an update which addresses your comments.

Sets the C (CC) and C++ (CXX) compiler environment variables so the choice of toolchain selected in the Build Settings tab is respected. Toolchains supported are GNU GCC and Clang. Added JUnit tests for GCCToolChain and ClangToolChain. Added test document describing manual test steps. Fixes eclipse-cdt#1140

jonahgraham

LGTM

jonahgraham · 2025-06-18T16:52:44Z

...se.cdt.build.gcc.core.tests/src/org/eclipse/cdt/build/gcc/core/tests/TestClangToolChain.java

+	public void clangExe() throws Exception {
+		IToolChain tc = new ClangToolChain(null, Paths.get("a/clang.exe"), null, null);
+		IEnvironmentVariable[] variables = tc.getVariables();
+		assertThat(Arrays.asList(variables), hasItems( //


Nice use of assertions here 😄

jonahgraham · 2025-06-18T16:54:16Z

...se.cdt.build.gcc.core.tests/src/org/eclipse/cdt/build/gcc/core/tests/TestClangToolChain.java

+		IEnvironmentVariable[] envVars = { new EnvironmentVariable("CC", "CCtestvalue") };
+		IToolChain tc = new ClangToolChain(null, Paths.get("a/clang.exe"), null, envVars);


This is certainly an interesting case. FTR if an integrator is doing this and they aren't happy with the decision you made here it is on them to resolve it in their code by providing CC and CXX environment variables.

jld01 reviewed May 28, 2025

View reviewed changes

build/org.eclipse.cdt.build.gcc.core/src/org/eclipse/cdt/build/gcc/core/ClangToolChain.java Show resolved Hide resolved

betamaxbandit commented May 29, 2025

View reviewed changes

jld01 reviewed May 30, 2025

View reviewed changes

betamaxbandit force-pushed the clang_1140 branch from fb47754 to 680f662 Compare June 2, 2025 16:27

jonahgraham reviewed Jun 5, 2025

View reviewed changes

betamaxbandit force-pushed the clang_1140 branch from 680f662 to 88bbfc0 Compare June 17, 2025 20:22

betamaxbandit force-pushed the clang_1140 branch from 88bbfc0 to 8aa05d0 Compare June 18, 2025 09:40

jonahgraham approved these changes Jun 18, 2025

View reviewed changes

		* However, on Windows, existing cc/cxx envVars with lower-case names will have their names changed to upper case;
		* the value remains unchanged.<br>

		iterator.remove();
		envVars.add(new EnvironmentVariable(name, evOldValue));

		IEnvironmentVariable[] envVars = { new EnvironmentVariable("CC", "CCtestvalue") };
		IToolChain tc = new ClangToolChain(null, Paths.get("a/clang.exe"), null, envVars);

Set CC and CXX environment variables for clang toolchain #1169

Are you sure you want to change the base?

Set CC and CXX environment variables for clang toolchain #1169

Uh oh!

Conversation

betamaxbandit commented May 27, 2025

Uh oh!

betamaxbandit commented May 27, 2025

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

betamaxbandit Jun 2, 2025 • edited by jonahgraham Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

betamaxbandit Jun 2, 2025 • edited by jonahgraham Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

betamaxbandit commented Jun 2, 2025

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

betamaxbandit commented Jun 17, 2025

Uh oh!

jonahgraham left a comment

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

betamaxbandit Jun 2, 2025 •

edited by jonahgraham

Loading

betamaxbandit Jun 2, 2025 •

edited by jonahgraham

Loading