1	Practical Foundations of Debugging Chapter 10 Arrays and Structures in Memory – Part 2 Dynamic Arrays and Heap
2	Static vs. Dynamic Memory
3	Memory Managers General-purpose (variable-size) Performance Fragmentation Fixed-size Flexibility Mixed (combination of previous two) Garbage collection Compatibility
4	Layered Memory Management Kernel OS (high-level abstractions) Compiler’s default run-time C++ standard library containers User-defined containers
5	Fragmentation
6	General-purpose allocation Housekeeping information Padding, alignment How ‘free’ frees a pointer?
7	Checking for memory allocation failures Useless! DoS attack on buffer lenghts
8	Rules for reading declarations Brackets take precedence If you see ‘[]’ read to the write and add ‘array of’ Read to the left and if you see ‘’ add ‘pointer to’ else add ‘of’ and put type name. Goto #1 int p; // pointer to int int a[N]; // array of ints int ap[N]; // array of pointers to int int pp; // pointer to pointer to int int app[N]; // array of pointers to pointer to int int (pa)[N]; // pointer to array of ints int (apap[N])[N]; // array of pointers to array of pointers to int
9	Pointers to array int (*pa)[10]; // pointer to array with 10 integers int array10[10]; int array11[11]; pa = &array10; pa = &array11; // ERROR, two different objects printf("%p - %p\n", pa, array10); Conclusion: array10 and &array10 have the same value (r-value)
10	Why do we need dynamic arrays? Example: reading information from file File has the following layout: DWORD: number of elements DWORD[]: an array of integers File: 2, 0, 1 File: 10, 0, 1, 0, 1 , 0, 2, 2, 2, 2, 2 const int MAXDATA = 1000; int data[MAX_DATA]; int numElements; void ReadData() { ReadFile(…, &numElements, sizeof(numElements), …); if (numElements > sizeof(data)/sizeof(data[0])) { numElements = sizeof(data)/sizeof(data[0]); } ReadFile(…, data, numElements*sizeof(data[0]), …); // reading bytes } So we have a dilemma: Small file – waste of memory Big file (more than 1000 elements) – not enough space, data is truncated
11	Solution: dynamic arrays File has the following layout: DWORD: number of elements DWORD[]: an array of integers File: 2, 0, 1 File: 10, 0, 1, 0, 1 , 0, 2, 2, 2, 2, 2 File: 0 // C/C++ int data; int numElements; void ReadData() { free(data); // NULL can be passed ReadFile(…, &numElements, sizeof(numElements), …); data = (int )malloc(numElements); // you can pass 0 here, needs a cast in C++ if (data) { ReadFile(…, data, numElements*sizeof(data[0]), …); } }
12	dynamic arrays in C++ // C++ int data; int numElements; void ReadData() { delete [] data; // [] are important here, NULL can be passed ReadFile(…, &numElements, sizeof(numElements), …); data = new int[numElements]; // you can pass 0 here if (data) { ReadFile(…, data, numElementssizeof(data[0]), …); } }
13	Compiling the same file as C or C++ code in VC++.NET
14	Example #include <stdlib.h> int dynArray; int pInteger; int main(int argc, char* argv[]) { // allocating an integer in malloc style pInteger = (int )malloc(1); int integer = pInteger; free(pInteger); // allocating an array in malloc style dynArray = (int )malloc(100); integer = dynArray[10]; free(dynArray); // allocating an integer in C++ style pInteger = new int[1]; integer = pInteger; delete pInteger; // allocating an array in C++ style dynArray = new int[100]; integer = dynArray[10]; delete [] dynArray; return 0; }
15	What’s next? Arrays and structures – parts 3 and 4 Virtual memory and paging Multithreading, memory and stacks Calling Windows functions (stdcall vs. cdecl) Classes and Objects Templates Strings Pointers to pointers (LPSTR *)