15-213 Recitation 7 – 3/5/01Outline• Control Flow• Memory Allocation– Lab 3 DetailsShaheen Gandhie-mail:[email protected] Hours:Wednesday 1:30 – 2:30Wean 3108Reminders• Lab 3: Conservative Garbage Collector• Checkpoint Due 3/13• Lab Due 3/21• At least there’s nothing over Spring Break (I think)Exceptional Control Flow• Higher level abstractions for dealing with miscellaneous conditions:– Error conditions that require errors to be thrown up many stack frames (functions)– Interrupt handling (I/O – Keyboard, Mouse, Network, etc.)– Familiarize yourself with wait(2), exec(3), fork(2), signal(2), etc.sigsetjmp(3) & siglongjmp(3)sigsetjmp(3)• Saves state about:– Stack Context– Registers– Program Counter– Blocked Signalssiglongjmp(3)• Starts executing by immediately executing code from sigsetjmp()• Man page says: “setjmp() and sigsetjmp make programs hard to understand and maintain. If possible an alternative should be used.”– But we make you do it anywayDynamic Memory Allocation• Applications need variable amounts of memory (unknown at compile time).• Use dynamic memory allocation to reserve chunks of memory at run-time.• Equivalent to the new operator in Java is malloc(3) in C• free(3) un-reserves (‘frees’) malloc’d memory.– No equivalent in Java, since Java does nifty garbage collectionDynamic Memory Allocation:How it’s doneFirst a picturekernel virtual memoryMemory mapped region forshared librariesrun-time heap (via malloc)program text (.text)initialized data (.data)uninitialized data (.bss)stack0the “brk” ptrDynamic Memory Allocation:How it’s done• The malloc package maintains the state of the run-time heap– Basically tons of grungy pointer arithmetic– The “Heap” is just the area between two addresses: dseg_lo and dseg_hi• malloc must find a valid contiguous block of memory in the heap and return this to the application• free must return the unused space to the heap for further allocations• How the functions do this is entirely up to you– Generally, fast and inefficient is preferrable to complex, efficient designs, although both are primary concernsConservative Garbage Collection• Reclaim unused space from the application– So we can use it to fill future allocations• How to do it?– Mark and Sweep• Find everything you (might) need• Reclaim the rest• Where do you find everything you need?– Start with the “roots”• Current registers• Stack• Heap (yes, the same one malloc manages)– Then do a depth first search on the data you findConservative Garbage Collection:An Exampletypedef struct node{struct node *next;} Node;void taste(Node *h){Node *n = h;h = malloc(sizeof(Node));h->next = n;}void srees(Node **h){Node *n = *h;*h = (*h)->next;free(*h);}Node *head = malloc(sizeof(Node));head->next = malloc(sizeof(Node));taste(head);srees(&head);What’s wrong?Lab 3 Tips• Start Early– Checkpoint next Wednesday– You need a working collector by then• Read the Lab handout– Now read it again• Don’t write any code until you know what you want to do• Review Pointer
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