Primitive Type fn

Function pointers, like fn(usize) -> bool.

See also the traits Fn, FnMut, and FnOnce.

Function pointers are pointers that point to code, not data. They can be called just like functions. Like references, function pointers are, among other things, assumed to not be null, so if you want to pass a function pointer over FFI and be able to accommodate null pointers, make your type Option<fn()> with your required signature.

Safety

Plain function pointers are obtained by casting either plain functions, or closures that don’t capture an environment:

fn add_one(x: usize) -> usize {
    x + 1
}

let ptr: fn(usize) -> usize = add_one;
assert_eq!(ptr(5), 6);

let clos: fn(usize) -> usize = |x| x + 5;
assert_eq!(clos(5), 10);

In addition to varying based on their signature, function pointers come in two flavors: safe and unsafe. Plain fn() function pointers can only point to safe functions, while unsafe fn() function pointers can point to safe or unsafe functions.

fn add_one(x: usize) -> usize {
    x + 1
}

unsafe fn add_one_unsafely(x: usize) -> usize {
    x + 1
}

let safe_ptr: fn(usize) -> usize = add_one;

//ERROR: mismatched types: expected normal fn, found unsafe fn
//let bad_ptr: fn(usize) -> usize = add_one_unsafely;

let unsafe_ptr: unsafe fn(usize) -> usize = add_one_unsafely;
let really_safe_ptr: unsafe fn(usize) -> usize = add_one;

ABI

On top of that, function pointers can vary based on what ABI they use. This is achieved by adding the extern keyword before the type, followed by the ABI in question. The default ABI is “Rust”, i.e., fn() is the exact same type as extern "Rust" fn(). A pointer to a function with C ABI would have type extern "C" fn().

extern "ABI" { ... } blocks declare functions with ABI “ABI”. The default here is “C”, i.e., functions declared in an extern {...} block have “C” ABI.

For more information and a list of supported ABIs, see the nomicon’s section on foreign calling conventions.

Variadic functions

Extern function declarations with the “C” or “cdecl” ABIs can also be variadic, allowing them to be called with a variable number of arguments. Normal Rust functions, even those with an extern "ABI", cannot be variadic. For more information, see the nomicon’s section on variadic functions.

Creating function pointers

When bar is the name of a function, then the expression bar is not a function pointer. Rather, it denotes a value of an unnameable type that uniquely identifies the function bar. The value is zero-sized because the type already identifies the function. This has the advantage that “calling” the value (it implements the Fn* traits) does not require dynamic dispatch.

This zero-sized type coerces to a regular function pointer. For example:

use std::mem;

fn bar(x: i32) {}

let not_bar_ptr = bar; // `not_bar_ptr` is zero-sized, uniquely identifying `bar`
assert_eq!(mem::size_of_val(&not_bar_ptr), 0);

let bar_ptr: fn(i32) = not_bar_ptr; // force coercion to function pointer
assert_eq!(mem::size_of_val(&bar_ptr), mem::size_of::<usize>());

let footgun = &bar; // this is a shared reference to the zero-sized type identifying `bar`

The last line shows that &bar is not a function pointer either. Rather, it is a reference to the function-specific ZST. &bar is basically never what you want when bar is a function.

Casting to and from integers

You can cast function pointers directly to integers:

let fnptr: fn(i32) -> i32 = |x| x+2;
let fnptr_addr = fnptr as usize;

However, a direct cast back is not possible. You need to use transmute:

let fnptr = fnptr_addr as *const ();
let fnptr: fn(i32) -> i32 = unsafe { std::mem::transmute(fnptr) };
assert_eq!(fnptr(40), 42);

Crucially, we as-cast to a raw pointer before transmuteing to a function pointer. This avoids an integer-to-pointer transmute, which can be problematic. Transmuting between raw pointers and function pointers (i.e., two pointer types) is fine.

Note that all of this is not portable to platforms where function pointers and data pointers have different sizes.

ABI compatibility

Generally, when a function is declared with one signature and called via a function pointer with a different signature, the two signatures must be ABI-compatible or else calling the function via that function pointer is Undefined Behavior. ABI compatibility is a lot stricter than merely having the same memory layout; for example, even if i32 and f32 have the same size and alignment, they might be passed in different registers and hence not be ABI-compatible.

ABI compatibility as a concern only arises in code that alters the type of function pointers, code that imports functions via extern blocks, and in code that combines #[target_feature] with extern fn. Altering the type of function pointers is wildly unsafe (as in, a lot more unsafe than even transmute_copy), and should only occur in the most exceptional circumstances. Most Rust code just imports functions via use. #[target_feature] is also used rarely. So, most likely you do not have to worry about ABI compatibility.

But assuming such circumstances, what are the rules? For this section, we are only considering the ABI of direct Rust-to-Rust calls, not linking in general – once functions are imported via extern blocks, there are more things to consider that we do not go into here.

For two signatures to be considered ABI-compatible, they must use a compatible ABI string, must take the same number of arguments, the individual argument types and the return types must be ABI-compatible, and the target feature requirements must be met (see the subsection below for the last point). The ABI string is declared via extern "ABI" fn(...) -> ...; note that fn name(...) -> ... implicitly uses the "Rust" ABI string and extern fn name(...) -> ... implicitly uses the "C" ABI string.

The ABI strings are guaranteed to be compatible if they are the same, or if the caller ABI string is $X-unwind and the callee ABI string is $X, where $X is one of the following: “C”, “aapcs”, “fastcall”, “stdcall”, “system”, “sysv64”, “thiscall”, “vectorcall”, “win64”.

The following types are guaranteed to be ABI-compatible:

• *const T, *mut T, &T, &mut T, Box<T> (specifically, only Box<T, Global>), and NonNull<T> are all ABI-compatible with each other for all T. They are also ABI-compatible with each other for different T if they have the same metadata type (<T as Pointee>::Metadata).
• usize is ABI-compatible with the uN integer type of the same size, and likewise isize is ABI-compatible with the iN integer type of the same size.
• char is ABI-compatible with u32.
• Any two fn (function pointer) types are ABI-compatible with each other if they have the same ABI string or the ABI string only differs in a trailing -unwind, independent of the rest of their signature. (This means you can pass fn() to a function expecting fn(i32), and the call will be valid ABI-wise. The callee receives the result of transmuting the function pointer from fn() to fn(i32); that transmutation is itself a well-defined operation, it’s just almost certainly UB to later call that function pointer.)
• Any two types with size 0 and alignment 1 are ABI-compatible.
• A repr(transparent) type T is ABI-compatible with its unique non-trivial field, i.e., the unique field that doesn’t have size 0 and alignment 1 (if there is such a field).
• i32 is ABI-compatible with NonZero<i32>, and similar for all other integer types.
• If T is guaranteed to be subject to the null pointer optimization, then T and Option<T> are ABI-compatible.

Furthermore, ABI compatibility satisfies the following general properties:

• Every type is ABI-compatible with itself.
• If T1 and T2 are ABI-compatible and T2 and T3 are ABI-compatible, then so are T1 and T3 (i.e., ABI-compatibility is transitive).
• If T1 and T2 are ABI-compatible, then so are T2 and T1 (i.e., ABI-compatibility is symmetric).

More signatures can be ABI-compatible on specific targets, but that should not be relied upon since it is not portable and not a stable guarantee.

Noteworthy cases of types not being ABI-compatible in general are:

• bool vs u8, i32 vs u32, char vs i32: on some targets, the calling conventions for these types differ in terms of what they guarantee for the remaining bits in the register that are not used by the value.
• i32 vs f32 are not compatible either, as has already been mentioned above.
• struct Foo(u32) and u32 are not compatible (without repr(transparent)) since structs are aggregate types and often passed in a different way than primitives like i32.

Note that these rules describe when two completely known types are ABI-compatible. When considering ABI compatibility of a type declared in another crate (including the standard library), consider that any type that has a private field or the #[non_exhaustive] attribute may change its layout as a non-breaking update unless documented otherwise – so for instance, even if such a type is a 1-ZST or repr(transparent) right now, this might change with any library version bump.

If the declared signature and the signature of the function pointer are ABI-compatible, then the function call behaves as if every argument was transmuted from the type in the function pointer to the type at the function declaration, and the return value is transmuted from the type in the declaration to the type in the pointer. All the usual caveats and concerns around transmutation apply; for instance, if the function expects a NonZero<i32> and the function pointer uses the ABI-compatible type Option<NonZero<i32>>, and the value used for the argument is None, then this call is Undefined Behavior since transmuting None::<NonZero<i32>> to NonZero<i32> violates the non-zero requirement.

Requirements concerning target features

Under some conditions, the signature used by the caller and the callee can be ABI-incompatible even if the exact same ABI string and types are being used. As an example, the std::arch::x86_64::__m256 type has a different extern "C" ABI when the avx feature is enabled vs when it is not enabled.

Therefore, to ensure ABI compatibility when code using different target features is combined (such as via #[target_feature]), we further require that one of the following conditions is met:

• The function uses the "Rust" ABI string (which is the default without extern).
• Caller and callee are using the exact same set of target features. For the callee we consider the features enabled (via #[target_feature] and -C target-feature/-C target-cpu) at the declaration site; for the caller we consider the features enabled at the call site.
• Neither any argument nor the return value involves a SIMD type (#[repr(simd)]) that is not behind a pointer indirection (i.e., *mut __m256 is fine, but (i32, __m256) is not).

Trait implementations

In this documentation the shorthand fn(T₁, T₂, …, Tₙ) is used to represent non-variadic function pointers of varying length. Note that this is a convenience notation to avoid repetitive documentation, not valid Rust syntax.

The following traits are implemented for function pointers with any number of arguments and any ABI.

• PartialEq
• Eq
• PartialOrd
• Ord
• Hash
• Pointer
• Debug
• Clone
• Copy
• Send
• Sync
• Unpin
• UnwindSafe
• RefUnwindSafe

Note that while this type implements PartialEq, comparing function pointers is unreliable: pointers to the same function can compare inequal (because functions are duplicated in multiple codegen units), and pointers to different functions can compare equal (since identical functions can be deduplicated within a codegen unit).

In addition, all safe function pointers implement Fn, FnMut, and FnOnce, because these traits are specially known to the compiler.