Lecture 4 - Memory Management PDF

Space: bytes in memory ​ number/index of memory location is its address ​ address value used as a pointer ○​ references data occupying addressed location ​ allocate space: assign location for data item to occupy ​ can pass array name as a string ○​ resolve...

Space: bytes in memory ​ number/index of memory location is its address ​ address value used as a pointer ○​ references data occupying addressed location ​ allocate space: assign location for data item to occupy ​ can pass array name as a string ○​ resolves to a pointer ​ pointer allocates space from constant pool (readonly) ○​ allocated on launch ​ can use pointer as array ○​ can use [] to index ​ attempt to modify readonly memory results in a fault ​ can declare pointer as constant ○​ const means that the data being referred to is readonly ○​ const char* cp = “nifty”; cp = ‘a’; ​ uninitialized pointers have undefined value: could raise fault or could clobber Null pointers, wild pointers, faults ​ not all numbers a valid address ​ invalid pointer causes a fault (pointer value not valid address) ​ wild pointer: pointer value is valid but not pointing to right space ​ special pointer NULL guaranteed to segfault ○​ easier to debug Strings ​ strcat(dest_string, src_string) ○​ concatenates source string onto destination string ○​ #define N 16 void f() { ​ char ca[N] = “nifty”; ​ strcat(ca, “ish”); ​ make sure enough space  ○​ not enough room results in buffer overflow ​ could overwrite existing data ○​ strn variants more safe ○​ size_t an integer type for the length of something (bytes of space) Data lifetimes ​ constants and static/globals live forever ○​ code: function instructions ○​ data: global variables ​ locals die on return ​ dynamic data: allocate space at runtime in stack, heap ○​ stack for local variables and parameters ○​ heap for dynamic memory explicitly allocated and released Heap ​ Java deletes objects when not used anymore ○​ garbage collection: deletes object if no references to it ​ In C must manually use operators malloc and free ○​ no builtin new() ​ malloc returns raw void* pointer to allocated space (heap block) ○​ must know how much space needed in bytes ○​ must convert pointer to T* type ○​ must free() space when done ○​ void* malloc(size_t size) ​ allocates block os space size bytes ​ returns pointer (address) of that space ​ returns NULL if allocation failed ○​ myPtr = (int*) malloc(10 * sizeof(int));