15 213 Recitation 7 3 5 01 Outline Control Flow Memory Allocation Lab 3 Details Reminders Lab 3 Conservative Garbage Collector Checkpoint Due 3 13 Lab Due 3 21 At least there s nothing over Spring Break I think Shaheen Gandhi e mail sgandhi andrew cmu edu Office Hours Wednesday 1 30 2 30 Wean 3108 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 Signals siglongjmp 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 anyway Dynamic 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 collection Dynamic Memory Allocation How it s done First a picture kernel virtual memory stack Memory mapped region for shared libraries the brk ptr run time heap via malloc uninitialized data bss initialized data data program text text 0 Dynamic 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 concerns Conservative 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 find Conservative Garbage Collection An Example typedef 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 Read the Lab handout Checkpoint next Wednesday You need a working collector by then Now read it again Don t write any code until you know what you want to do Review Pointer Arithmetic
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