Can clang-format break my code?

Question

As clang-format is a tool to only reformat code, is it possible that such formatting can break working code or at least change how it works? Is there some kind of contract that it will/can not change how code works?

We have a lot of code that we want to format with clang-format. This means, many lines of code will change. Not having to review every single line of code that only changed due to a clang-format would be a big simplification of this process.

I would say that clang-format will not change how code works. On the other hand I am not 100% sure, if this can be guaranteed.

Answer 1

Short answer: YES.

---

The clang-format tool has a -sort-includes option. Changing the order of #include directives can definitely change the behavior of existing code, and may break existing code.

Since the corresponding SortIncludes option is set to true by several of the built-in styles, it might not be obvious that clang-format is going to reorder your includes.

MyStruct.h:

struct MyStruct {
    uint8_t value;
};

original.c:

#include <stdint.h>
#include <stddef.h>
#include "MyStruct.h"

int main (int argc, char **argv) {
    struct MyStruct s = { 0 };
    return s.value;
}

Now let's say we run clang-format -style=llvm original.c > restyled.c.

restyled.c:

#include "MyStruct.h"
#include <stddef.h>
#include <stdint.h>

int main(int argc, char **argv) {
  struct MyStruct s = {0};
  return s.value;
}

Due to the reordering of the header files, I get the following error when compiling restyled.c:

In file included from restyled.c:1:
./MyStruct.h:2:5: error: unknown type name 'uint8_t'
    uint8_t value;
    ^
1 error generated.

However, this issue should be easy to work around. It's unlikely that you have order-dependent includes like this, but if you do, you can fix the problem by putting a blank line between groups of headers that require a specific order, since apparently clang-format only sorts groups of #include directives with no non-#include lines in between.

fixed-original.c:

#include <stdint.h>
#include <stddef.h>

#include "MyStruct.h"

int main (int argc, char **argv) {
    struct MyStruct s = { 0 };
    return s.value;
}

fixed-restyled.c:

#include <stddef.h>
#include <stdint.h>

#include "MyStruct.h"

int main(int argc, char **argv) {
  struct MyStruct s = {0};
  return s.value;
}

Note that stdint.h and stddef.h were still reordered since their includes are still "grouped", but that the new blank line prevented MyStruct.h from being moved before the standard library includes.

---

However...

If reordering your #include directives breaks your code, you should probably do one of the following anyway:

1. Explicitly include the dependencies for each header in the header file. In my example, I'd need to include stdint.h in MyStruct.h.

2. Add a comment line between the include groups that explicitly states the ordering dependency. Remember that any non-#include line should break up a group, so comment lines work as well. The comment line in the following code also prevents clang-format from including MyStruct.h before the standard library headers.

alternate-original.c:

#include <stdint.h>
#include <stddef.h>
// must come after stdint.h
#include "MyStruct.h"

int main (int argc, char **argv) {
    struct MyStruct s = { 0 };
    return s.value;
}