The fixed keyword
The fixed keyword allows you to "pin" a local onto the stack to prevent it from being collected or moved during garbage-collection. It is used for low-level programming scenarios.
Syntax
use ptr = fixed expression
Remarks
This extends the syntax of expressions to allow extracting a pointer and binding it to a name which is prevented from being collected or moved during garbage-collection.
A pointer from an expression is fixed via the fixed keyword and is bound to an identifier via the use keyword. The semantics of this are similar to resource management via the use keyword. The pointer is fixed while it is in scope, and once it is out of scope, it is no longer fixed. fixed cannot be used outside the context of a use binding. You must bind the pointer to a name with use.
Use of fixed must occur within an expression in a function or a method. It cannot be used at a script-level or module-level scope.
Like all pointer code, this is an unsafe feature and will emit a warning when used.
Example
open Microsoft.FSharp.NativeInterop
type Point = { mutable X: int; mutable Y: int}
let squareWithPointer (p: nativeptr<int>) =
// Dereference the pointer at the 0th address.
let mutable value = NativePtr.get p 0
// Perform some work
value <- value * value
// Set the value in the pointer at the 0th address.
NativePtr.set p 0 value
let pnt = { X = 1; Y = 2 }
printfn $"pnt before - X: %d{pnt.X} Y: %d{pnt.Y}" // prints 1 and 2
// Note that the use of 'fixed' is inside a function.
// You cannot fix a pointer at a script-level or module-level scope.
let doPointerWork() =
use ptr = fixed &pnt.Y
// Square the Y value
squareWithPointer ptr
printfn $"pnt after - X: %d{pnt.X} Y: %d{pnt.Y}" // prints 1 and 4
doPointerWork()
