clang/docs/BoundsSafetyAdoptionGuide.md

# Adoption Guide for `-fbounds-safety`

```{contents}
:local: true
```

## Where to get `-fbounds-safety`

The open sourcing to llvm.org's `llvm-project` is still on going and the
feature is not available yet. In the mean time, the preview implementation is
available
[here](https://github.com/swiftlang/llvm-project/tree/stable/20240723) in a
fork of `llvm-project`. Please follow
[Building LLVM with CMake](https://llvm.org/docs/CMake.html) to build the
compiler.

## Feature flag

Pass `-fbounds-safety` as a Clang compilation flag for the C file that you
want to adopt. We recommend adopting the model file by file, because adoption
requires some effort to add bounds annotations and fix compiler diagnostics.

## Include `ptrcheck.h`

`ptrcheck.h` is a Clang toolchain header to provide definition of the bounds
annotations such as `__counted_by`, `__counted_by_or_null`, `__sized_by`,
etc. In the LLVM source tree, the header is located in
`llvm-project/clang/lib/Headers/ptrcheck.h`.

## Add bounds annotations on pointers as necessary

Annotate pointers on struct fields and function parameters if they are pointing
to an array of object, with appropriate bounds annotations. Please see
{doc}`BoundsSafety` to learn what kind of bounds annotations are available and
their semantics. Note that local pointer variables typically don't need bounds
annotations because they are implicitly a wide pointer (`__bidi_indexable`)
that automatically carries the bounds information.

## Address compiler diagnostics

Once you pass `-fbounds-safety` to compile a C file, you will see some new
compiler warnings and errors, which guide adoption of `-fbounds-safety`.
Consider the following example:

```c
#include <ptrcheck.h>

void init_buf(int *p, int n) {
   for (int i = 0; i < n; ++i)
      p[i] = 0; // error: array subscript on single pointer 'p' must use a constant index of 0 to be in bounds
}
```

The parameter `int *p` doesn't have a bounds annotation, so the compiler will
complain about the code indexing into it (`p[i]`) as it assumes that `p` is
pointing to a single `int` object or null. To address the diagnostics, you
should add a bounds annotation on `int *p` so that the compiler can reason
about the safety of the array subscript. In the following example, `p` is now
`int *__counted_by(n)`, so the compiler will allow the array subscript with
additional run-time checks as necessary.

```c
#include <ptrcheck.h>

void init_buf(int *__counted_by(n) p, int n) {
   for (int i = 0; i < n; ++i)
      p[i] = 0; // ok; `p` is now has a type with bounds annotation.
}
```

## Run test suites to fix new run-time traps

Adopting `-fbounds-safety` may cause your program to trap if it violates
bounds safety or it has incorrect adoption. Thus, it is necessary to perform
run-time testing of your program to gain confidence that it won't trap at
run time.

## Repeat the process for each remaining file

Once you've done with adopting a single C file, please repeat the same process
for each remaining C file that you want to adopt.