CHARLES UNIVERSITY IN PRAGUE faculty of mathematics and physics Advanced.NET Programming II 3 rd Lecture Pavel Ježek Some of the slides are based on University of Linz.NET presentations. © University of Linz, Institute for System Software, 2004 published under the Microsoft Curriculum License (

CLI Type System All types Reference types (allocated on managed heap) PointersValue types (allocated in-place [with exceptions] ) ClassesInterfaces ArraysDelegates Simple types (Int32, Int64, Double, Boolean, Char, …) Nullables Enumerations Structures User defined structures

Pointer Types Examples int*ip;// pointer to an int cell MyStruct*sp;// pointer to a MyStruct object void*vp;// pointer to any memory location int**ipp;// pointer to a pointer to an int cell PointerType=UnmanagedType * |void *. UnmanagedType=ValueType |PointerType. pointers to arbitrary types if a struct type, all fields must be of an UnmanagedType Syntax pointers are not traced by the garbage collector (referenced objects are not collected) pointer types are not compatible with System.Object pointer variables can have the value null pointers of different types can be compared with each other (==, !=,, >=) ip sp int MyStruct vp ipp int

Unsafe Code Code that works with pointers is potentially unsafe (can break type rules and destroy arbitrary memory locations) must be enclosed in an unsafe block or an unsafe method unsafe { int* p;... } unsafe void foo() { int* p;... } must be compiled with the unsafe option csc -unsafe MyProg.cs system administrator must assign FullTrust rights to the program

Using Pointers Dereferencing a pointer int var; int* ip = &var; ip *ip = 5; int x = *ip; if v is of type T*, *v is of type T void* cannot be dereferenced var Access to struct fields struct Block { int x, y, z; } Block b; Block* bp = &b; bp->x = 1;// pointer notation (*bp).y = 2// alternatively bp x y z bp must be of type Block* works also for method calls b Access to array elements int[] a = new int[3]; int* ap = a; ap[1] = 1;// array notation *(ap+1) = 2// alternatively ap no index bound checks! ap[3] would be accepted a 0 1 2

Address Arithmetic T* p =...; T p p++; p--; T p T // increases p by sizeof(T) T* q, r; q = p + 2; r = p - 1; T p T // q = p + 2*sizeof(T) // r = p - sizeof(T) rq T Pointers can be used in calculations No pointer arithmetic with void*

Type Casts On Pointers int i; int* ip; Block*bp; void*vp; Implicit Casts (without cast operator) T*  null ip = null; bp = null; vp = null; void*  T* vp = ip; vp = bp; Explicit Casts (with cast operator) T1*  T2* ip = (int*)vp; vp = (void*)ip; bp = (Block*)vp; T*  intType ip = (int*)i; bp = (Block*)i; vp = (void*)i; i = (int)ip; i = (int)bp; i = (int)vp; unchecked !

Pinned and Unpinned Variables Pinned cannot be moved by the garbage collector local variables value parameters fields of structs which are pinned themselves Unpinned can be moved by the garbage collector fields of classes (also static fields) array elements ref and out parameters

Adress Operator &designator Note designator must denote a pinned variable (e.g. &x, &s.f) the type of the variable must not be managed (i.e. no class, no array) if designator is of type T, &designator is of type T* yields the address of designator

fixed Statement Pins an unpinned variable during the execution of this statement (i.e. the garbage collector cannot move it during this statement) Syntax FixedStat= "fixed" "(" PointerType FixedVarDecl {"," FixedVarDecl} ")" Statement. FixedVarDecl= ident "=" ("&" UnpinnedVar |ArrayVar |StringVar ). UnpinnedVar must be of an unmanaged type T T* must be assignable to PointerType array elements must be of an unmanaged type T T* must be asignable to PointerType char* must be assignable to PointerType Examples int field; int[] a = new int[10]; Person person = new Person(); string s = "Hello"; fixed (byte* p = &field) { Console.Write(*(p+1)); } fixed (int* p = &a[1]) {...} fixed (int* p = &person.id) {...} fixed (int* p = a) {...} fixed (char* p = s) {...} Variables that are declared in the header of a fixed statement are read-only

Examples Printing the bytes of an int variable int x = 3; unsafe { byte* p = (byte*) &x; for (int i = 0; i < 4; i++) { Console.Write("{0:x2} ", *p); p++; } } x 3000 p String processing string s = "Hello"; unsafe { fixed (char* p = s) { for (int i = 0; p[i] != '\0'; i++) Console.Write(p[i]); } s Hell p o\0

Examples (continued) Overlaying a byte array with a struct type struct Block { int x; float f; int y; }... byte[] buffer = new byte[1024];... unsafe { fixed (byte* bp = &buffer[3]) { Block* b = (Block*) bp; Console.WriteLine(b->x + " " + b->f + " " + b->y); } xfy b buffer

Dangers of Pointer Processing Can destroy arbitrary memory locations int[] a = new int[3]; unsafe { fixed (int* p = a) { p[5] = 0; } } One can be left with pointers to objects that are moved by the garbage collector int[] a = new int[5]; int* p; unsafe { fixed (int* q = a) { p = q+2; }......// GC can move array now *p = 5;// destroys data } p p q a

Dangers of Pointer Processing (cont.) One can be left with pointers to non-existing local variables static unsafe int* Foo() { int local = 3; return &local; } static unsafe void Main() { int* p = Foo();... *p = 5;// accesses non-existing local variable! } Therefore Avoid pointer processing! Unless you really need it for system-level programming

StructLayout Attribute using System.Runtime.InteropServices; [StructLayout(LayoutKind.Sequential, Pack=4)]// members are allocated sequentially class Time {// and are aligned at 4 byte boundaries public ushort year; public ushort month;... } [StructLayout(LayoutKind.Explicit)]// members are allocated at specified offsets struct S { [FieldOffset(0)] ushort x; [FieldOffset(2)] int y; }

StructLayout Attribute [StructLayout ( layout-enum// Sequential | Explicit | Auto [, Pack=packing-size]// 0, 1, 2, 4, 8, 16,... [, CharSet=charset-enum]// Ansi | Unicode | Auto )]