* Author:[Alexis Laferrière](https://github.com/xymus)

* Review Manager:[Steve Canon](https://github.com/stephentyrone)

* Status:**Active Review (September 25th...October 9th, 2025)**

* Implementation: On`main` with the experimental feature flags`CDecl` for`@c`, and`CImplementation` for`@c @implementation`. With the exception of the`@objc` support for global functions which isavailable under the name`@_cdecl`.

* Implementation: On`6.3`. With the exception of the`@objc` support for global functions which isnot yet implemented.

* Review: ([pitch](https://forums.swift.org/t/pitch-formalize-cdecl/79557))([review](https://forums.swift.org/t/se-0495-c-compatible-functions-and-enums/82365))

enumCEnum:CInt{

}

The attribute`@objc` is already accepted on enums. These enums qualify as an Objective-C representable type and are usable from`@objc` global function signatures but not from`@c` functions.

In the compatibility header, the`@c` enum is printed with the C name specified in the`@c` attribute or the Swift name by default. It defines a storage of the specified raw type with support for different dialects of C.

Each case is printed using a name composed of the enum name with the case name attached. The first letter of the case name is capitalized automatically. For the enum above, the generated cases for C are named`CEnumFirst` and`CEnumSecond`.

Extend support for the`@implementation` attribute, introduced in[SE-0436](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0436-objc-implementation.md), to global functions marked with either`@c` or`@objc`. These functions are declared in an imported C or Objective-C header, while the Swift function provides their implementation. Type-checking ensures the declaration matches the implementation signature in Swift. Functions marked`@implementation` are not printed in the compatibility header.

Marking a global function with`@c` or`@objc` makes it use the C calling convention. Adding or removing these attributes on a function is an ABI breaking change. Updating existing`@_cdecl` to`@objc` or`@c` is ABI stable.

The compiler emits a single symbol for`@c` and`@objc` functions, the symbol uses the C calling convention.

Adding or removing the attributes`@c` and`@objc` on a function is an ABI breaking change. Changing between`@c` and`@objc` is ABI stable. Changing between`@_cdecl` and either`@c` or`@objc` is an ABI breaking change since`@_cdecl` emits two symbols and Swift clients of`@_cdecl` call the one with the Swift calling convention.

Adding or removing the`@c` attribute on an enum is ABI stable, but changing its raw type is not.