inst eecs berkeley edu cs61c CS61C Machine Structures Lecture 6 C Memory Management 2010 02 01 Lecturer SOE Dan Garcia www cs berkeley edu ddgarcia Tape ogre awakens IBM Zurich has made a new tape material that can store 29 5 gigabits in2 i e a cartridge that can hold 35 terabytes of data more than 40 times the current capacity www technologyreview com computing 24406 CS61C L06 C Memory Management 1 Garcia Spring 2010 UCB Review Use handles to change pointers Create abstractions and your own data structures with structures Dynamically allocated heap memory must be manually deallocated in C Use malloc and free to allocate and de allocate persistent storage CS61C L06 C Memory Management 2 Garcia Spring 2010 UCB Don t forget the globals Remember Structure declaration does not allocate memory Variable declaration does allocate memory So far we have talked about several different ways to allocate memory for data 1 2 Declaration of a local variable int i struct Node list char string int ar n Dynamic allocation at runtime by calling allocation function alloc ptr struct Node malloc sizeof struct Node n One more possibility exists Data declared outside of any procedure i e before main Similar to 1 above but has global scope CS61C L06 C Memory Management 3 int myGlobal main Garcia Spring 2010 UCB C Memory Management C has 3 pools of memory Static storage global variable storage basically permanent entire program run The Stack local variable storage parameters return address location of activation records in Java or stack frame in C The Heap dynamic malloc storage data lives until deallocated by programmer C requires knowing where objects are in memory otherwise things don t work as expected Java hides location of objects CS61C L06 C Memory Management 4 Garcia Spring 2010 UCB Normal C Memory FFFF FFFF Management A program s address space contains 4 regions hex stack stack local variables grows downward heap space requested for pointers via malloc resizes dynamically grows upward static data variables declared outside main does not grow or shrink code loaded when program starts does not change CS61C L06 C Memory Management 5 heap static data code 0hex For now OS somehow prevents accesses between stack and heap gray hash lines Wait for virtual memory Garcia Spring 2010 UCB Where are variables allocated If declared outside a procedure allocated in static storage If declared inside procedure allocated on the stack and freed when procedure returns NB main is a procedure int myGlobal main int myTemp CS61C L06 C Memory Management 6 Garcia Spring 2010 UCB The Stack Stack frame includes Return instruction address Parameters Space for other local variables SP Stack frames contiguous blocks of memory stack pointer tells where top stack frame is When procedure ends stack frame is tossed off the stack frees memory for future stack frames CS61C L06 C Memory Management 7 frame frame frame frame Garcia Spring 2010 UCB Stac k Last In First Out LIFO data structure stack main a 0 void a int m b 1 void b int n c 2 void c int o d 3 void d int p CS61C L06 C Memory Management 8 Stack Stack Pointer grows down Stack Pointer Stack Pointer Stack Pointer Stack Pointer Garcia Spring 2010 UCB Who cares about stack management Pointers in C allow access to deallocated memory leading to hard to find bugs int ptr main main main SP int y y 3 ptr printf return y y 3 y SP SP main int stackAddr content stackAddr ptr content stackAddr printf d content 3 content stackAddr printf d content 13451514 CS61C L06 C Memory Management 9 Garcia Spring 2010 UCB The Heap Dynamic memory Large pool of memory not allocated in contiguous order back to back requests for heap memory could result blocks very far apart where Java new command allocates memory In C specify number of bytes of memory explicitly to allocate item int ptr ptr int malloc sizeof int malloc returns type void so need to cast to right type malloc Allocates raw uninitialized memory from heap CS61C L06 C Memory Management 10 Garcia Spring 2010 UCB Memory Management How do we manage memory Code Static storage are easy they never grow or shrink Stack space is also easy stack frames are created and destroyed in last in first out LIFO order Managing the heap is tricky memory can be allocated deallocated at any time CS61C L06 C Memory Management 11 Garcia Spring 2010 UCB Heap Management Requirements Want malloc and free to run quickly Want minimal memory overhead Want to avoid fragmentation when most of our free memory is in many small chunks In this case we might have many free bytes but not be able to satisfy a large request since the free bytes are not contiguous in memory This is technically called external fragmention CS61C L06 C Memory Management 12 Garcia Spring 2010 UCB Heap Management An example Request R1 for 100 bytes Request R2 for 1 byte Memory from R1 is freed R1 100 bytes R2 1 byte Request R3 for 50 bytes CS61C L06 C Memory Management 13 Garcia Spring 2010 UCB Heap Management An example R3 Request R1 for 100 bytes Request R2 for 1 byte Memory from R1 is freed Request R3 for 50 bytes CS61C L06 C Memory Management 14 R2 1 byte R3 Garcia Spring 2010 UCB K R Malloc Free Implementation From Section 8 7 of K R Code in the book uses some C language features we haven t discussed and is written in a very terse style don t worry if you can t decipher the code Each block of memory is preceded by a header that has two fields size of the block and a pointer to the next block All free blocks are kept in a circular linked list the pointer field is unused in an allocated block CS61C L06 C Memory Management 15 Garcia Spring 2010 UCB K R Implementation malloc searches the free list for a block that is big enough If none is found more memory is requested from the operating system If what it gets can t satisfy the request it fails free checks if the blocks adjacent to the freed block are also free If so adjacent free blocks are merged coalesced into a single larger free block Otherwise the freed block is just added to the free list CS61C L06 C Memory Management 16 Garcia Spring 2010 UCB Choosing a block in malloc If there are multiple free blocks of memory that are big enough for some request how do we choose which one to use best fit choose the smallest block that is big enough for the request first fit choose the first block we see that is big enough next fit like first fit but remember where we finished searching and resume searching from there CS61C L06 C Memory Management 17 Garcia
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