CS 105 Malloc Lab Writing a Dynamic Storage Allocator See Web page for due date 1 Introduction In this lab you will be writing a dynamic storage allocator for C programs i e your own version of the malloc free and realloc routines You are encouraged to explore the design space creatively and implement an allocator that is correct efficient and fast 2 Logistics As usual you must work in pairs Any clarifications and revisions to the assignment will be e mailed to the class list or posted on the course Web page 3 Handout Instructions Start by downloading malloclab handout tar from the Web page to a protected directory in which you plan to do your work Then give the command tar xvf malloclab handout tar This will cause a number of files to be unpacked into the directory The only file you will be modifying and handing in is mm c The mdriver c program is a driver program that allows you to evaluate the performance of your solution Use the command make to generate the driver code and run it with the command mdriver V The V flag displays helpful summary information As with other labs mm c contains a C structure named team which you should fill in with information about your programming team Do this right away so you don t forget When you have completed the lab you will submit only one file mm c which contains your solution 4 Getting a Low Score The malloc lab is by far the most difficult lab in CS 105 The primary reason students have problems is that they follow poor programming practices Here are some good ways to make the lab a disaster 1 Don t read the hints at the end of this handout Just start work right away using the information on page 1 Don t put any effort into the heap checker Writing a thorough heap checker is difficult and requires careful thinking Never mind that the time invested pays off tenfold just go straight to the lab Be sloppy and take shortcuts Convince yourself that you ll fix your sloppiness later Take hasty approaches because you don t want to waste time writing a correct helper function Slap in temporary code of the Hey why don t we try adding 4 variety to see if it fixes your bug and then just leave it there while you get distracted by the next problem Only call the heap checker once It slows your code down besides it keeps crashing your program with an error message you don t understand Better to just turn it off Don t use paper to diagram your ideas It s easy to keep track of all those pointer manipulations in your head right Why kill trees drawing them out to make sure you understand what s going on On the other hand if you want to make this lab a breeze start with diagrams design a heap checker that can handle all of the conditions listed below and be maniacal about neatness 5 How to Work on the Lab Your dynamic storage allocator will consist of the following four functions which are declared in mm h and defined in mm c int mm init void void mm malloc size t size void mm free void ptr void mm realloc void ptr size t size The mm c file we have given you implements the simplest but still functionally correct malloc package that we could think of Using this as a starting place modify these functions and possibly define other private static functions so that they obey the following semantics mm init Before calling mm malloc mm realloc or mm free the application program i e the trace driven driver program that you will use to evaluate your implementation calls mm init to perform any necessary initialization such as allocating the initial heap area The return value should be 0 if all was OK and 1 if there was a problem during initialization Note mm init may be called more than once by the driver It should not remember any external state and should not assume that it is only called once Each call to mm init should reset the allocator forgetting everything that has gone before 2 mm malloc The mm malloc routine returns a pointer to an allocated block of at least size bytes The entire allocated block should lie within the heap region and should not overlap with any other allocated chunk Note that the returned value should point to the payload the area available for use by the caller rather than to whatever header you might choose to put on the block We will comparing your implementation to the version of malloc supplied in the standard C library libc The libc malloc always returns payload pointers that are aligned to 8 bytes which is excessive on Linux your malloc implementation can gain a few utilization points by aligning only to 4 bytes In the Linux specification it is legal to allocate 0 bytes your implementation should behave gracefully in this case It is up to you whether to return NULL or to return a non NULL pointer that can legally be passed to mm free mm free The mm free routine frees the block pointed to by ptr It returns nothing This routine is only guaranteed to work when the passed pointer ptr was returned by an earlier call to mm malloc or mm realloc and has not yet been freed In the Linux specification NULL can be passed to free You can write mm free to accept NULL pointers but the test driver will never exercise this case mm realloc The mm realloc routine returns a pointer to an allocated region of at least size bytes with the following constraints If ptr is NULL the call is equivalent to mm malloc size If size is equal to zero the call is equivalent to mm free ptr If ptr is not NULL it must have been returned by an earlier call to either mm malloc or mm realloc The call to mm realloc changes the size of the memory block pointed to by ptr the old block to size bytes and returns the address of the new block Notice that the address of the new block might be the same as the old block or it might be different depending on your implementation the amount of internal fragmentation in the old block and the size of the realloc request The contents of the new block are the same as those of the old ptr block up to the minimum of the old and new sizes Everything else is uninitialized For example if the old block is 8 bytes and the new block is 12 bytes then the first 8 bytes of the new block are identical to the first 8 bytes of the old block and the last 4 bytes are uninitialized Similarly if the old block is 8 bytes and the new block is 4 bytes then the contents of the new block are identical to the first 4 bytes of the old block …
View Full Document
Unlocking...