Rust return pointer See also the std::ptr module. When a Pointer is not suitable, the fallback is to use a Handle. The Rust compiler uses static analysis to determine where the pointer is in scope, and handles allocating and de-allocating that memory. The resulting pointer has the address 0. Later the C code passes the exact same pointer back to the close function which tries to drop (free) it. Rust allows moving values to different memory locations, and may reuse the same memory locations for different purposes. 5 Pointers are primitive types in Rust, meaning that they implement the Copy trait, which means that the access to the data (the pointer) can be duplicated. It calls f with a and b and returns the result. I am trying to learn Rust ffi, those implementations are a frankenstein creation from different sources in internet. var += value; println!("var = {}", self. Rust allocates items on the stack by default, but the Box<T> pointer type (roughly equivalent to C++'s std::unique_ptr<T>) forces allocation to occur on the heap, which in turn means that the allocated item can outlive the scope of the current block. struct Foo { var: i32, } impl Foo { fn method(&mut self, value: i32) { self. Here, the C function "takes ownership" in the sense that we expect it to eventually return the pointer and length to Rust, for example, by calling a Rust function to deallocate it: Function pointer types, written using the fn keyword, refer to a function whose identity is not necessarily known at compile-time. I'm calling a C constructor function keyNew that allocates memory for a Key struct and returns a *mut Key to the Rust side. Therefore, the pointer points to the first item in the slice. There is nothing universal, a pointer of struct A can't be convert to void * and then convert to a pointer to struct B. *const T and *mut T. TLDR: Add to main. Advanced Functions and Closures. I can also pass a function pointer to a function which accepts an impl Fn, so I'm guessing that function pointer type in Rust is useful only as the Return String from Rust fn to ReactApp. fn-pointer. It segfaults in free(3) . Rust's vtable structure is not accessible from Rust code. 32-bit on 32-bit x86, 64-bit on 64-bit x86, etc. On the other hand, it is possible to wrap a fixed size array in a struct and return that struct. T parameter &T parameter &mut T parameter T return value Option<T> parameter Option<T> return value JavaScript representation You can call freely, but unsafely, from C to Rust and from Rust to C. §Examples. It's a nothing pointer type. c const char* hello(){ return "Hello World!"; } main. rs use wasm_bindgen::prelude::*; Instead allocate and return a raw byte pointer containing the data which has to be then encoded as a JS string on the JavaScript side. The standard library contains additional 'smart Manually manage memory through raw pointers. ; It is then called with Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company fn is a function pointer; closures cannot be converted to function pointers. Passing a function pointer from C++ to Rust is not implemented yet, only from Rust to an extern "C++" function is implemented. as_mut_ptr() and Box::into_raw is that Box::into_raw takes ownership of the box Despite pointers and references being similar to usizes in the machine code emitted on most platforms, the semantics of transmuting a reference or pointer type to a non-pointer type is currently undecided. Now, I want to see how Box<T>::new(x: T) works when the underlying memory allocator fails to allocate memory. This is “safe” per Rust's definition of safety as long as you don't use the pointer, but you eventually do so in a unsafe block. Raw, unsafe pointers, *const T, and *mut T. This is mildly overloading the term “pointer” for the sake of brevity/exposition. Raw Pointers. Modified 10 years, 5 months ago. Working with raw pointers in Rust is uncommon, typically limited to a few patterns. Follow asked Apr 27, 2023 at I believe I saw something similar in the Rust book at some point, but I have tried both of those combinations for this case, as well as tried doing the same thing with parenthesis in all the different ways, but that still evaluates the function first, then points to You can use the into_raw on a CString to turn it into a raw pointer, which you can then return from your function. If it is a pointer that might be not to critical (pointers don't implement Drop), but in other cases it might be problematic (e. We’ve talked a lot about funtions that accept various kinds of pointers, but what about returning them? Here’s the rule of thumb: only return a unique or managed pointer if Raw, unsafe pointers, *const T, and *mut T. Ouch. For First, some logical errors with the code: It is not correct to cast pointers to i32 on many platforms (like 64-bit). I could also return the raw pointer directly, but this makes for a very ugly API and is not idiomatic to Rust. void * is not a "magic" pointer, it's not "universal". I need some assistance with the ownership. They may also not appear in the return type of closure traits or function pointers, unless these are themselves part of a legal return type. Viewed 298 times 0 . Ultimately, this is about object identity: you need to pass something which allows to identify one instance of an object. bar(); // called like callback(); The allocator in Rust is not necessarily the same as malloc. See the example code on that page. Owned pointers are not garbage So we have a get_items() func in the DLL that returns a MyItem* pointer that can actually be zero-to-many elements, so the code loops over it like an array. Pointers may use all of those bits. To put it bluntly: you're violating Box::into_raw returns a pointer to the beginning of the allocated storage. In Rust it is efficient to return by value, as the value is moved, not copied. One needs to use the extern "C" fn() (a. This topic aims to correct that shortfall. However, when loading from or storing to a raw pointer, it must be valid for the given access and aligned. env. Basic usage with &i32: impl Trait may only be written within the return type of a freestanding or inherent-impl function, not in trait definitions or any non-return type position. There’s no real need to have the aggregation functions working on f64 references - they should work on iterators that return f64 values, which You return the Box, which has a pointer to heap allocated memory, as value. Truncating a pointer and then calling a function at the truncated address will lead to Really Bad Things. I have an array of enum values. You shouldn't rely on Rust using the system allocator. Under the covers, Box<T> is also a simple eight Raw, unsafe pointers, *const T, and *mut T. Young. In your case, the compiler isn't smart enough to figure out that you want the function pointers from your foo and bar instead of function items. Note that in Rust, every (stack-allocated) variable is considered a separate allocated object. You must ensure that any value received or returned is FFI friendly: basic types or repr(C) types or raw pointers to those and a few others. The length is the number of elements currently contained, and the capacity is the total size in elements of the allocated memory. Thus, it may not be valid to transmute a In part 1, we explored how to take a C library and write a crate of unsafe Rust bindings for it. [type. When you say unsafe, you are promising the compiler that you will manually uphold all of its expected invariants and then immediately breaking that promise. K. let callback = || foo. Commented Dec 28, 2020 at 20:28. 32 doesn't report the error, but thankfully Rust 1. The original array still lives in static memory but the reference you are trying to return is not pointing at that, it's pointing at the copy on the stack. Else the lookup fails. e. Rc and Arc. This engine would be given a parameter on creation. . , and they all manage separate heaps. I tried the following and it doesn't work either: No, unless by allocate you mean creation. Over the years, there has been ample discussion on the concept of a "&move"/"&own"/"move reference"/"owning reference". On Windows, for example, there's msvcrt, msvcr80, msvcr90, etc. let callback = Foo::bar; // called like callback(&foo); However, if you want it with the self variable already bound (as in, calling callback() will be the same as calling bar on the foo object), then you need to use an explicit closure:. Modified 6 years, 3 months ago. Many functions in this module take raw pointers as arguments and read from or write to them. The library will then copy its own functions to the callbacks which are then be called from Rust side. If you're confident you will never want to store a closure in foo, you can replace Box<Fn(&Test)> with fn(&Test) instead. 3: 11872: January 12, 2023 Home ; A Rust reference can refer to items that are located either on the stack or on the heap. However, in some contexts (e. When using a field expression, tuple index expression, or Methods do not bind the self argument, but you can get access to the methods via their fully qualified names and you can save them in a function pointer if you like. Here is a sample code of it: struct BigStruct { one: int, two: int, // etc one_hundred: int, } fn foo(x: Box<BigStruct>) -> Returns whether two pointers are guaranteed to be equal. You cannot return a reference to this value because it lives on the stack, and therefore references to it will be invalid after the function returns. Thus, it may not be valid to transmute a My understanding is that implementing a function that takes an impl Fn as an argument allows the rust compiler to perform more optimization, but using an impl Fn instead of a function pointer shouldn't be more restrictive. One pattern that I see very often is to get a C pointer that points to a Rust structure, make it usable by rust and then forgetting it to avoid the drop. For alloc APIs this is literally the pointer the call returns, and for local variables and statics, this is the name of the variable/static. I'd like to have a function use a MemWriter to write some bytes and then return a pointer to the buffer. The printed pointer values are not guaranteed to be stable nor unique identifiers of objects. With fully-qualified syntax, Foo::bar will work, yielding a fn(&Foo) -> (similar to Python). At runtime this function behaves like Some(self == other). The CString is temporary, hence the as_ptr() call returns the inner pointer of that temporary, which will likely be dangling when you use it. Function Pointers. In my personal example, I have a Rust library which can receive maybe-null pointers from FFI callers, and react accordingly. To model pointers to opaque types in FFI, until extern type is stabilized, it is recommended to use a newtype wrapper around an empty byte Function pointers with a Result return type are not implemented yet. I'm reading the return pointers part of Rust Guide. We’ve talked about how to pass closures to functions; you can also pass regular functions to functions! Rust doesn't have constructors, just functions that return values like any other. The pointer is implicit in the fn type, and they have no lifetime of their own; therefore, function pointers are assumed The into_raw function consumes a box and returns the raw pointer. Here's the small Rust library contains the static struct, out_rust_stdout_plugin. For I want to call a Rust library which will create an object increment_engine for me and return a pointer to it (or anything persistent). Currently I am with two approaches: a) Remove the array from rust GC and return the point. After that point, any reference to a local variable will be pointing to some useless data. If Box::into_raw returned anything else, it wouldn't be really useful. Later I could call this object's increment(a) method and it would add the remembered parameter to a and return the result. I have googled for an hour to no avail. This struct is to be initialized in Rust side and the pointer is to be sent to C++ side. You should bind the string to a variable and use If the type can be dereferenced, do so, then return to step 1. Here is the code: The `wasm-bindgen` Guide. Commented Nov 4, the return type is Successors<f64, fn(&f64) -> Option<f64>>, Rust cloned closures expected closure, found different closure. Here is why: References are pointers to memory locations. That said, this is not the same as C’s void return type, which is Rust’s type. A slice is a contiguous sequence of items in memory. The callback must be in some sense exposed to the user of my library (probably using closures), and I want to provide a Rust interface as convenient as possible (meaning accepting a String output if possible). Example. – C. This requires the f64s to be created prior to where the closure is created. Doing so is undefined behaviour, and the compiler is completely free to do whatever it wants. Any help would be highly appreciated. The compiler will once again complain about mismatched types as setter_function in MyTrait::do_some_setting is a Rust Lifetimes: Returning a pointer [duplicate] Ask Question Asked 6 years, 3 months ago. They're not directly compatible (e. help. Some C code calls into the Rust open call below which returns a pointer. callee returns pointer to Struct1 result in RAX. Rust is correct here: cannot assign to values, as it is not declared as mutable It seems so far the WebAssembly backend in Rust doesn't provide a way import or export the (function) table; the index f is just fine, but imports. Example 1: Raw Pointer. This works because B can never be all I'd suggest to return a raw C-pointer from your library instead and own it in Rust. Common ways to create raw pointers 1. Thus, it may not be valid to transmute a Creates a null raw pointer. In the second edition of The Rust Programming Language (emphasis mine): Function pointers implement all three of the closure traits (Fn, FnMut, and FnOnce), so you can always pass a function pointer as an argument for a function that expects a closure. In the main function:. You were unlucky that Rust 1. So take it with a grain of salt. coercion] They can be created via a coercion from both function items and non-capturing, non-async closures . Rust MemWriter return pointer to Buffer. @Jimmay, you should really read Rust guide, especially the part on pointers. In fact, they compile down to the same C / C++ pointer you know. a. This question already has answers here: rust [mockall return_once] borrowed value does not live long enough. C++ struct test{ string data; assocc *el; } Function operate takes another function f and two integers a and b as inputs. Using this crate allows direct access to functions and other symbols in the library via unsafe, but I know the answer is "you shouldn't" but for the sake of argument, how should you do it?. var); The Rust Programming Language. The *const T type also defines the offset method, for pointer math. Raw pointers can be created directly using core::ptr::addr_of! for *const pointers and core::ptr::addr_of_mut! for *mut pointers. I see that you can make 'something that compiles and runs' by transmuting * mut T values into u64 and adding to them, then transmuting them back into * mut T and reading the value at the pointer (see Hey Rust community! I'm having trouble re-assigning pointer from null (set on the Rust side) to a new valid address (on the C side). There is no guarantee that the printed value can be converted back to a pointer. You can either derive Clone and return the struct after cloning it, or you can change the return type in the signature to &mut Self. Best practices. When using a field expression, tuple index expression, or You have two alternatives, either return the value or use a static variable. g. Regardless, it is true that pointers are generally harder to use in Rust, that's because they are unsafe and rust is also quite explicit about any casts. table is not the same table used by the wasm instance (i. I'm struggling to I want to do Rust bindings to a C library which requires a callback, and this callback must return a C-style char* pointer to the C library which will then free it. The dereference expression produces an lvalue, but that lvalue is not actually read from, we're just doing pointer math on it, so in theory, it should be well defined. My solution involves using a null pointer to retrieve the offset to the field, so it's a bit simpler than yours as it avoids one subtraction (we'd be subtracting 0). ): Despite pointers and references being similar to usizes in the machine code emitted on most platforms, the semantics of transmuting a reference or pointer type to a non-pointer type is currently undecided. However, there's one extra point: fun is a C function, but the type fn() is a Rust function. Returning Pointers. However, after the plug-in callback returns (thus giving back control to the host), the pointer can become stale. Alignment Experimental A type storing a usize which is a power of two, and thus represents a possible alignment in the Rust Equivalent to C’s void type when used as a pointer. In short, I have a Rust struct that exists in a share library, which I would love to exposed to my C main program through FFI. For example, if you wanted to write an alternative to Vec<T> that worked differently. Also you should use extern fn in FFI. Functions add and subtract are simple arithmetic functions for addition and subtraction, respectively. References in Rust aren't C++ "names for another place in memory," They absolutely are names for a place in memory. Once functions are executed, local variables are popped off the execution stack and resources are de-allocated. addr_ of_ mut Creates a mut raw pointer to a place, without creating an intermediate reference. Ask Question Asked 10 years, 5 months ago. This function then hands back pointers into Rust memory which are known good; sibling functions in the library receive that pointer back from FFI later, and must still go through the motions of checking validity when, as long as the caller is not broken, Raw, unsafe pointers, *const T, and *mut T. However, if we have a special kind of enum: enum Foo { A, B(ContainsANonNullPtr), } the null pointer optimization kicks in, which eliminates the space needed for the tag. When you store a boxed value, you create a thin pointer . , compile-time evaluation), it is not Pass a mutable reference as an argument to your function and use that to copy the value you want to create into the caller's memory space. 34 does. It has a capacity as well, but that's beside the point). rust; ownership; Share. And then it complains that the second argument doesn't have the In Learning Rust With Entirely Too Many Linked Lists, the author mentions:. Structs§ NonNull *mut T but non-zero and covariant. If the variant is A, the whole enum is set to all 0's. In Rust, pointers are replaced by references, which are similar in concept but have additional safety features to prevent common errors like null pointer dereferences and dangling pointers. The main difference between boxed_slice. their calling conventions differ). Function pointers are commonly useful for implementing async functions over FFI. Bug #1: Destructors running after assignment (double free)). assume_init(). When you call call_both(foo, bar) the compiler sets the generic type F to be fn(u32) -> bool {foo}, because that's the type of the first argument. Right after that function returns, the pointer is guaranteed to be a valid C string, so it is safe to dereference it. For &mut (reference to value you don't own but Wrapping the functions which expect buffers involves using the slice::raw module to manipulate Rust vectors as pointers to memory. Vecs, unless created with a nonzero capacity, only ever allocate when they need to grow, and when you create a vec with a capacity of zero, it doesn’t allocate until you start inserting items into it, hence why Vec::new can be run within a const context, since it doesn’t actually allocate then, it allocates when you say call Vec::push Use this method when we don't know how long the C code would like to be able to access the string. There's no guarantee that your Rust code will link to the same free as the C/C++ code. The provenance of the pointer is used to determine which allocated object it is derived from; a pointer is dereferenceable if the memory range of the given size starting at the pointer is entirely contained within the bounds of that allocated object. If you do not allocate memory on the stack you can return references to the result which already lives in memory. CString::new(""). §Examples As @DanielKeep said, the created closure creates a scope, 'a, for which the eventual borrow (when the closure is called) must be valid. Given the following simple da I'm writing an application that is basically a simple C++ shell calling into a Rust library. Furthermore, when using an array, all elements must be initialized, otherwise a Its starting position, as a pointer; Its length; Rust follows the same convention - a Vec<_>, below the hood, shares the same structure (well, almost. Improve this question. I am learning Rust, so apologies if this is a trivial question. So it works A function pointer is the address of a function, and has function pointer type. This part is coming soon. Despite pointers and references being similar to usizes in the machine code emitted on most platforms, the semantics of transmuting a reference or pointer type to a non-pointer type is currently undecided. In this example, we have a pointer to an integer variable integer not allocated on the heap. – JohnTortugo. unwrap(). FFI, Boxes, and returning references to the Boxed data. From C to Rust: write the Rust function with a #[no_mangle] extern "C" prefix. When using a field expression, tuple index expression, or How do I create null pointer properties in struct like in C++? I do not quite understand how to make a pointer to an empty space in the memory. This is not a duplicate of this Function pointers in Rust using constrained generics as I'd like to get a function pointer to the concrete instantiation of std::fs::remove_file with &Path as a type parameter, but not to have a generic function pointer type. operate is first called with add and the integers 10 and 5, storing the sum (15) in result_add. Edit: In fact, in your case, you are served very nicely by the semantics of the Option type: you never return true, so you may equate the None result with the false result your function returns, and this captures the idea you're trying to express: either your linear search finds the target and returns it (Some(found)), or it does not, and has All computing would happen in Rust; however I got stuck returning a complex data type from Rust. If you just want to understand how this works, then we can use little unsafe magic (but none of this is guaranteed!fn what_data(a: &dyn Trait) -> u32 { unsafe { // `&dyn Trait` is stored as two references, the data pointer coming first and then the vtable // pointer. In many languages with pointers, you'd return a pointer from a function so as to avoid copying a large data structure. Later on, when you invoke the function pointer you have to provide the self argument yourself:. This section explores some advanced features related to functions and closures, including function pointers and returning closures. I am using a u64 handle from a previously call and passing it in to the function. 179. I wish to find a random location within that array that matches a specific pattern and return a mutable reference to it, with the intent of modifying the element in that location. mylib. It's then up to you to ensure it's not the case by either creating the pointer inside a Rust wrapper directly (which wouldn't implement the Copy trait ). k. Use the null function to create null pointers, and the is_null method of the *const T type to check for null. It doesn’t destroy T or deallocate any memory. Viewed 468 times 0 . Pointers in Rust, a guide Home Blog Oct 18 2013. However, the comparison returns false when one of the objects (r) was returned by the function Engine::run, which is defined in another crate. impl<T> Box<T> { /// Allocates How is it possible that the comparison operator == returns false although the pointers seem to be the same? A few observations: The comparison returns true when both objects (a and b) live in the same crate. 5: 3217: January 12, 2023 How to address raw pointer as array. Solution to your problem. Dereferencing will try to read the previous value and drop it. extern "C" { pub fn get_some_data() -> *const SomeStruct; } Now, for a higher level wrapper, I would like to convert this to a Option<&'a SomeStruct> with an appropriate lifetime. This function is equivalent to zero-initializing the pointer: MaybeUninit::<*const T>::zeroed(). Even in C++, to load a C++ library you provide C-ABI functions (via extern C ) to gain access to a pointer of your type and then you use it in C++ as how as you want. User need to promise to call free later. rs You are then required to get the pointer back and free it in Rust; the Rust allocator is unlikely to be I know that this is a very "C" thing to do and Rust can return strings, but that's what I need to do here. From Rust to C: write a extern "C" block with the prototypes of the C functions. See Returning Pointers below for more. Sorry ;). I'm not sure how to solve this problem. For & (reference to value you don't own and can't modify), we have *const. You shouldn't be returning a pointer to a local stack variable at all. Hello 🦀, I read an article about the importance of null-pointer checking return values of malloc/realloc/calloc functions in C and C++ (these functions return a null-pointer when they fail to allocate the requested amount of memory). If you haven't already, I strongly urge you to read the Rust Book. This limits you to function pointers, but avoids the extra allocation. Why can't Rust just return a reference (pointer) to that heap location then? I'm really confused by this point. empty). See also the pointer primitive types. If you want to take a look under the hood the Playground provides some nice WebAssembly optimizations, check out this One C function which I want to wrap returns a raw pointer. The Original Pointer for an allocation is guaranteed to have unique access to the entire allocation and only that I'm trying to get a C string returned by a C library and convert it to a Rust string via FFI. Is it appropriate to use Box::from_raw to wrap the pointer and take ownership of it?. Some functions return nullable pointers to structs, which bindgen represents as. However, its raw pointer cousin has been comparatively neglected. A good indication of it is that we would like to return a pointer out of the rust method. I tried: pub extern fn get_string(file: but then I really need to pass the reference through the pointer. That's just my interpretation though. Raw pointers can be out-of-bounds, unaligned, or null. Store the data you created on the I am struggling using ffi to call an external C function and return a struct allocated in the function. "Rust C FFI universal pointer type, analog of “void *”?" no no no and no. Rust's vectors are guaranteed to be a contiguous block of memory. First of all, while C has the concept of fixed-size arrays (int a[5] has type int[5] and sizeof(a) will return 5 * sizeof(int)), it is not possible to directly pass an array to a function or return an array from it. Thank you Pointers or Handles. This takes up the platform-native size of an integer (e. The simplest interface is to return a Pointer. extern fn()) type when interacting with C function pointers: Macros§ addr_of Creates a const raw pointer to a place, without creating an intermediate reference. In essence, *const c_void is equivalent to C’s const void* and *mut c_void is equivalent to C’s void*. One must know it's true type to be able to use it. 2 @JohnTortugo yes, references are like a pointer in many ways (and not, in others). More information on references is present in a separate guide . – Vladimir Matveev I'm using rust-bindgen to access a C library from Rust. We would transfer the ownership to C by constructing the CString object and casting it to the pointer using into_raw. For instance, malloc will always return a pointer that is 8 bytes aligned but in rust, the alignment may be specified. You can take a look at the SHA1 example here. It is the most performant interface, albeit requires trust between the parties, and a clear ownership. Notice that the proxy is a null Never dereference a pointer like that. Passing the boxed vector on top of a pointer is not only overkill, but extremely unwise. Otherwise, the variant is B. as_ptr() does not work. fdmx pkeuug oglb tjebhx jpiiarn nrd xyua xtpgvs koa thnwk huimqp kben chkjvm hnszgm ycjj