[clang-format] Fix breaking change in 20.1.0 by automatically falling back to Cpp formatting if C formatting fails

[sean-mcmanus]

@owenca The PR at #128287 broke the case where a .c file is formatted in a folder with a .clang-format with Language: Cpp (and no "Language: C").

Can you make it not be a breaking change by automatically falling back to formatting with Cpp if no C language block is specified in .clang-format instead of giving an error?

Or do you recommend a higher level tooling layer detect the failure and change use --assume-filename=file.cpp instead of file.c as a workaround or what?

i.e. command line clang-format test.c gives "Configuration file(s) do(es) not support C".

[llvmbot]

@llvm/issue-subscribers-clang-format

Author: Sean McManus (sean-mcmanus)

@owenca The PR at https://github.com//pull/128287 broke the case where a .c file is formatted in a folder with a .clang-format with Language: Cpp.

Can you make it not be a breaking change by automatically falling back to formatting with Cpp if no C language block is specified in .clang-format instead of giving an error?

Or do you recommend a higher level tooling layer detect the failure and change use --assume-filename=file.cpp instead of file.c as a workaround or what?

[bobbrow]

Additional context: We are maintainers of the Microsoft C/C++ extension for VS Code and this issue was brought to our attention by one of our users.

[owenca]

Can you provide a reproducer? It seems to work for me.

$ cat .clang-format
Language: Cpp
$ cat foo.c
int i;
$ clang-format -version
clang-format version 20.1.0
$ clang-format foo.c
int i;
$ 

[owenca]

Using the config file in microsoft/vscode-cpptools#13406 also works for me.

[MaJerle]

@owenca the code in the cpptools#13406 is a reproducible on my side.

With the latest release, if you pass the .c file and you don't have the C language in the clang-config, then you get an error for wrong language.

This breaks practically everything as now clang-format has to be updated and eventually 2 configs have to be put together for C and Cpp, respectively. For versions prior to 20, adding the C language will throw an error.

vscode cpp tools does this call:

clang-format --style=file --fallback-style=LLVM --Wno-error=unknown --assume-filename=C:\Users\file\main.c C:\USERS\FILE\MAIN.C

When you run it with clang-format 20.1, with format file

---
Language: Cpp
---

You get:

Configuration file(s) do(es) not support C:

But when you run it with clang-format as:

---
Language: Cpp
---
Language: C
---

Then no error and it produces the expected result.

Ultimately, I think what we shall have is:

• Cpp parameter remains valid for C and C++, since this is how users are used to do it up to now
• C shall be for C only while maybe CppOnly or similar parameter would be only for C++ files but no C.

[owenca]

@owenca the code in the cpptools#13406 is a reproducible on my side.

With the latest release, if you pass the .c file and you don't have the C language in the clang-config, then you get an error for wrong language.

This breaks practically everything as now clang-format has to be updated and eventually 2 configs have to be put together for C and Cpp, respectively. For versions prior to 20, adding the C language will throw an error.

vscode cpp tools does this call:

clang-format --style=file --fallback-style=LLVM --Wno-error=unknown --assume-filename=C:\Users\file\main.c C:\USERS\FILE\MAIN.C

When you run it with clang-format 20.1, with format file

---
Language: Cpp
---

You get:

Configuration file(s) do(es) not support C:

But when you run it with clang-format as:

---
Language: Cpp
---
Language: C
---

Then no error and it produces the expected result.

Ultimately, I think what we shall have is:

• Cpp parameter remains valid for C and C++, since this is how users are used to do it up to now
• C shall be for C only while maybe CppOnly or similar parameter would be only for C++ files but no C.

I still couldn't reproduce it:

$ cat .clang-format
---
Language: Cpp
---
$ cat main.c
int main() {
  return 0;
}
$ clang-format -version
clang-format version 20.1.1
$ clang-format --style=file --fallback-style=LLVM --Wno-error=unknown --assume-filename=main.c main.c
int main() { return 0; }
$ echo $?
0
$ 

[sean-mcmanus]

@owenca I may know why you're not reprong it -- adding a .clang-format with

BasedOnStyle:  LLVM

in the parent folder causes it to not repro for me -- can you check your parent folders for any .clang-format files? When I remove/that file the bug start to repro. I just have a .clang-format in my current working directory where the source file is.

repro:

My repro is on Windows, but I can check mac/Linux later today.

[owenca]

@sean-mcmanus you are right!

[owenca]

@MaJerle if you delete the Language: Cpp line in your config, it should work for both .c and .cpp files.

[MaJerle]

@MaJerle if you delete the Language: Cpp line in your config, it should work for both .c and .cpp files.

Yup, I confirm this is the case.

But this will still impact other users in general. Because C and Cpp will have to be updated in their format file.

[owenca]

@sean-mcmanus One workaround you may want to consider is to change the command to:

clang-format --style=file --fallback-style=LLVM --Wno-error=unknown < main.c

[sean-mcmanus]

@owenca What is changed by that? Removing the --assume-filename? That flag also enables the include sorting format feature to properly sort the file's header include at the top. Also, we already pass the file text to clang-format via stdin, so I'm not sure how that would be different from using <.

An example C language repo with a Cpp language in .clang-format is at https://github.com/leo-arch/clifm/blob/91e1e7f03adcc757882a310a4bac55220eb9e7d1/src/.clang-format#L4 . The comment next to it is Language: Cpp # Cpp is also used for C.

[owenca]

@owenca What is changed by that? Removing the --assume-filename? That flag also enables the include sorting format feature to properly sort the file's header include at the top. Also, we already pass the file text to clang-format via stdin, so I'm not sure how that would be different from using <.

If you need to use --assume-filename, then pass a .cpp argument, e.g. --assume-filename=test.cpp. Either way, you will need to use the < for the workaround:

$ clang-format --style=file --fallback-style=LLVM --Wno-error=unknown < test.c
int i;
$ clang-format --style=file --fallback-style=LLVM --Wno-error=unknown --assume-filename=.cpp < test.c
int i;
$ 

[bobbrow]

Thank you for the quick response @owenca!