docs/c23.md

# C23 language changes

## Breaking changes

### `void foo()` now means `void foo(void)`
In C17 and earlier, `void foo()` means "I haven't yet told you how many
arguments this function has". In C23, it's equivalent to C++ and means "this
function has no arguments". This may surface as a function pointer type
mismatch, because previously `()` matched functions taking any arguments,
whereas in C23 it only matches functions taking no arguments.

Fix: in cases where your function does have arguments, declare them.

### Undeclared identifiers are now errors
In C17 and earlier, calling `foo(123)` without a declaration for `foo()`
produced a warning. In C23 this is an error instead. One common special case of
this is code that's explicitly ignoring such warnings to call functions that are
GNU extensions; such code should be fixed to ensure that `_GNU_SOURCE` is
defined before any header is included instead (often by adding `-D_GNU_SOURCE`
to the cflags in the build file).

Fix: add the missing forward declaration or `#include` (or `-D_GNU_SOURCE`).

### `bool`/`true`/`false` are now keywords
In C17 and earlier, only code that included `<stdbool.h>` would have standard
definitions for these (typically macros for `_Bool`/`1`/`0`). In C23 these are
keywords and should no longer be defined in your code.

Fix: delete any definitions of `bool`/`true`/`false` if you only need to build
as C23, or switch to `#include <stdbool.h>` for compatibility back to C99.

### `false` is no longer `0`
In C17 and earlier, it was common for true and false to be defined as 1 and 0
(either by `<stdbool.h>` or by user-provided `#define`/`enum`). This meant that
`false` (as 0) could be implicitly converted to `NULL`. In C23, a function that
returns (or takes) a pointer can no longer return `false` (or be passed
`false`).

Fix: return/pass `NULL` (or `nullptr` for C23-only code) instead of `false`
in pointer contexts.

### `unreachable()` is now a predefined function-like macro in `<stddef.h>`
In C17 and earlier, `unreachable()` was available for your own macros/functions.
In C23 there's a standard definition.

Fix: delete your `unreachable()` if it was just equivalent to
`__builtin_unreachable()` or rename it if it had different behavior.

### K&R prototypes are no longer valid
In C17 and earlier, K&R function prototypes were deprecated but still allowed.
In C23 K&R prototypes are no longer allowed.

Fix: rewrite any K&R prototypes as ANSI/ISO prototypes.


## Non-breaking changes

### Unused function parameters can now be anonymous
In C17 and earlier you'd have to use `__attribute__((unused))` on an unused
function parameter. In C23 you can just omit the parameter name instead,
like `void* pthread_callback_fn(void*) {` (as in C++).

### New standard attributes
C23 adds `[[deprecated("reason")]]`, `[[fallthrough]]`, `[[nodiscard]]` (the
equivalent of the clang attribute `warn_unused_result`),
`[[maybe_unused]]` (the equivalent of the clang attribute `unused`),
and `[[noreturn]]` (equivalent to C11 `_Noreturn`).
Most of these have been available before via `__attribute__` or other syntax,
but are now standard.

### `#embed`
You can now include binary data directly into an array or string: https://en.cppreference.com/w/c/preprocessor/embed

### `void foo(...)` is now allowed
In C17 and earlier, a varargs function needed a non-varargs argument.
In C23 this is allowed (as in C++).

### `enum` base types
You can now say `enum E : long { ... }` to explicitly choose the base type of
your enum (as in C++, and already supported by clang as an extension).

### `nullptr` constant
There is now a `nullptr` constant (as in C++),
and a corresponding `nullptr_t` type for that constant.

### `constexpr`
There is now a limited form of `constexpr` for defining `const` variables
(similar to, but much more limited than C++ constexpr).


## Library changes

Library changes are not covered here because bionic does not make library
functionality available based on target C version, since the target API level
distinctions are confusing enough already.

See [status.md](status.md) for what functionality went into which API level.