Malloc Lab: Writing a Dynamic Storage Allocator solution


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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
You may work in a group of up to two people. Any clarifications and revisions to the assignment will be
posted on the course Web page.
3 Hand Out Instructions
Start by copying malloclab-handout.tar 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.)
Looking at the file mm.c you’ll notice a C structure team into which you should insert the requested
identifying information about the one or two individuals comprising your programming team. Do this right
away so you don’t forget.
When you have completed the lab, you will hand in only one file (mm.c), which contains your solution.
4 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 initializations, such as allocating the initial heap area. The return value should
be -1 if there was a problem in performing the initialization, 0 otherwise.
• mm malloc: The mm malloc routine returns a pointer to an allocated block payload of at least
size bytes. The entire allocated block should lie within the heap region and should not overlap with
any other allocated chunk.
We will comparing your implementation to the version of malloc supplied in the standard C library
(libc). Since the libc malloc always returns payload pointers that are aligned to 8 bytes, your
malloc implementation should do likewise and always return 8-byte aligned pointers.
• 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.
• 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 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
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.
These semantics match the the semantics of the corresponding libc malloc, realloc, and free routines.
Type man malloc to the shell for complete documentation.
5 Heap Consistency Checker
Dynamic memory allocators are notoriously tricky beasts to program correctly and efficiently. They are
difficult to program correctly because they involve a lot of untyped pointer manipulation. You will find it
very helpful to write a heap checker that scans the heap and checks it for consistency.
Some examples of what a heap checker might check are:
• Is every block in the free list marked as free?
• Are there any contiguous free blocks that somehow escaped coalescing?
• Is every free block actually in the free list?
• Do the pointers in the free list point to valid free blocks?
• Do any allocated blocks overlap?
• Do the pointers in a heap block point to valid heap addresses?
Your heap checker will consist of the function int mm check(void) in mm.c. It will check any invariants
or consistency conditions you consider prudent. It returns a nonzero value if and only if your heap is
consistent. You are not limited to the listed suggestions nor are you required to check all of them. You are
encouraged to print out error messages when mm check fails.
This consistency checker is for your own debugging during development. When you submit mm.c, make
sure to remove any calls to mm check as they will slow down your throughput. Style points will be given
for your mm check function. Make sure to put in comments and document what you are checking.
6 Support Routines
The memlib.c package simulates the memory system for your dynamic memory allocator. You can invoke
the following functions in memlib.c:
• void *mem sbrk(int incr): Expands the heap by incr bytes, where incr is a positive
non-zero integer and returns a generic pointer to the first byte of the newly allocated heap area. The
semantics are identical to the Unix sbrk function, except that mem sbrk accepts only a positive
non-zero integer argument.
• void *mem heap lo(void): Returns a generic pointer to the first byte in the heap.
• void *mem heap hi(void): Returns a generic pointer to the last byte in the heap.
• size t mem heapsize(void): Returns the current size of the heap in bytes.
• size t mem pagesize(void): Returns the system’s page size in bytes (4K on Linux systems).
7 The Trace-driven Driver Program
The driver program mdriver.c in the malloclab-handout.tar distribution tests your mm.c package
for correctness, space utilization, and throughput. The driver program is controlled by a set of trace files
that are included in the malloclab-handout.tar distribution. Each trace file contains a sequence of
allocate, reallocate, and free directions that instruct the driver to call your mm malloc, mm realloc, and
mm free routines in some sequence. The driver and the trace files are the same ones we will use when we
grade your handin mm.c file.
The driver mdriver.c accepts the following command line arguments:
• -t