Dealing with overflow

In the previous post we have seen that C23 will provide us with tools for efficient overflow check for arithmetic. When discussing this, several people have asked why these tools do not provide a specific model to handle overflow, such as aborting or raising a signal. The answer to this is actually quite simple:

There is no commonly agreed strategy to handle overflow.

So for the C committee (and for the gcc feature this is based upon) there was no obvious choice for a strategy. Instead, the features provide the basics to implement any such strategy based on policy decisions that will be dependent on application specific settings, be they technical or cultural

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Checked integer arithmetic in the prospect of C23

As you might have noticed, C23 is scheduled to come out in November 2023 and will have a lot of improvements and new features, in particular for integers. One of the most controversial properties of integer arithmetic in C is the overflow behavior. C23 will have a new header <stdckdint.h> for “checked integer operations”, that helps to deal with overflow and puts the responsibility in your hands, the programmer. In addition to the result of an arithmetic operation, the interfaces provide an extra bool value that tells if the operation has been erroneous or not.

The addition that has been integrated into C23 is the core proposal and
some amendments. The history of this looks a bit confusing because later editions of the paper have removed parts that had already been adopted.

Anyhow, for many of you it is even possible to use these features in a C23 compatible way just now, because they are closely modeled after similar gcc and clang features. Since overflow still is an important source of bugs and security issues, you should just

start using checked integer operations, now!

C23 implications for C libraries

The upcoming standard C23 has a lot of additions to the C library clause. Most of them are small, some of them are big but optional. I have now finished a first version of a document that summarizes many of the changes. It has some general discussions about the following subjects

• Unicode support
• Thread safety of the C library
• Const-contract of the C library
• Changes to integer types
• Attributes

and then lists changes to individual header files of the C library.

This does not contain an detailed description of the changes to the math.h header. First, I am really not an expert on that, and second the changes there are quite invasive and we don’t have a diff-file that would clearly list them.