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Homework 11 Implementing Dynamic Memory Allocation

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CS 2110 Homework 11
Implementing Dynamic Memory Allocation

Contents
1 Assignment 2
1.1 The Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Block Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 The Freelist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Simple Linked List: Allocating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Simple Linked List: Deallocating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.6 my malloc() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.7 my free() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.8 my realloc() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.9 my calloc() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.10 Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.11 Using the Makefile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.12 Deliverables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.13 Suggested Helper Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.14 Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2 Frequently Asked Questions 13
3 Rules and Regulations 14
3.1 General Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2 Submission Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3 Submission Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4 Syllabus Excerpt on Academic Misconduct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.5 Is collaboration allowed? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1
1 Assignment
In this assignment, you will be writing the dynamic memory allocation and deallocation functions of malloc,
free, realloc, and calloc. These functions are confusing to write, so we have provided an in-depth guide
below. Please read through this entire pdf before beginning. The specifics for each function are located in
malloc.c as well as subsections 1.6 – 1.9 below.
1.1 The Basics
It is the job of the memory allocator to process and satisfy the memory requests of the user. But where does
the allocator get its memory? Let us recall the structure of a program’s memory footprint.
+——————-+ (low memory)
| CODE |
+——————-+
| DATA |
+——————-+ <– Break
| |
| |
+——————-+
| STACK |
+——————-+ (high memory)
When a program is loaded into memory there are various “segments” created for different purposes: code,
stack, data, etc. In order to create some dynamic memory space, otherwise known as the heap, it is possible
to move the “break”, which is the first address after the end of the process’s uninitialized data segment.
A function called brk() is provided to set this address to a different value. There is also a function called
sbrk() which moves the break by some amount specified as a parameter.
For simplicity, a wrapper for the system call sbrk() has been provided for you as a function called my sbrk
located in suites/malloc suite.c. Make sure to use this call rather than a real call to sbrk, as
doing this can potentially cause a lot of problems. Note that any problems introduced by calling the
real sbrk will not be regraded, so make sure that everything is correct before turning in.
If you glance at the code for my sbrk(), you will quickly notice that upon the first call it always allocates 8
KiB. For the purposes of your program, you should treat the returned amount as whatever you requested.
For instance, the first time I call my sbrk() it will be done like this:
my_sbrk(SBRK_SIZE); /* SBRK_SIZE == 2 KB */
—————————————–
| 8 KB |
—————————————–
^
|
\______ The pointer returned to me by my_sbrk
Even though you have a full 8 KiB, you should treat it as if you were only returned SBRK SIZE bytes. Now
when you run out of memory and need more heap space you will need to call my sbrk() again. Once again,
the call is simply:
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my_sbrk(SBRK_SIZE);
—————————————–
| 2 KB | 6 KB |
—————————————–
^
|
\____ The pointer returned to me by my_sbrk
Notice how it returned a pointer to the address after the end of the 2 KB I had requested the first time.
my sbrk() remembers the end of the data segment you request each time and is able to return that value
to you as the beginning of the new data segment on a following call. Keep this in mind as you write the
assignment!
We’ve written my sbrk to be able to only hand out a certain amount of memory before returning -1 to
indicate that its done. This limit gives us the ability to test the behavior of the code when my sbrk can’t
get more memory.
1.2 Block Allocation
Trying to use sbrk() (or brk()) exclusively to provide dynamic memory allocation to your program would
be very difficult and inefficient. Calling sbrk involves a certain amount of system overhead, and we would
prefer not to have to call it every single time a small amount of memory is required. In addition, deallocation
would be a problem. Say we allocated several 100 byte chunks of memory and then decided we were done
with the first. Where would the break be? There’s no handy function to move the break back, so how could
we reuse that first 100 byte chunk?
What we need are a set of functions that manage a pool of memory allowing us to allocate and deallocate
efficiently. Typically, such schemes start out with no free memory at all. The first time the user requests
memory, the allocator will call sbrk() as discussed above to obtain a relatively large chunk of memory. The
user will be given a block with as much free space as they requested, and if there is any memory left over it
will be managed by placing information about it in a data structure where information about all such free
blocks is kept. This is called the freelist and we will return to this later.
In order to keep track of allocated blocks we will create a structure to store the information we need to know
about a block. Where should we put this structure? Can we simply call malloc() to allocate space for the
information?
No we can’t! We’re writing malloc(); we can’t use it or we’d end up with infinite recursion. However,
there’s an easier way that will keep our bookkeeping structure right with the data we’re allocating for easy
access.
In order to keep track of allocated blocks, we will create a structure to store the information we need to
know about a block. We will store this information about the block, called metadata, inside the block
itself! A crucial part of the metadata is the canary. Canaries are integers that we generate via information
about the block itself. They buffer the user data, so if the canary is incorrect, the user data has been
altered. For more information about canaries see https://en.wikipedia.org/wiki/Buffer_overflow_
protection#Canaries, but note that the canary we implement will be one for memory allocated by malloc,
not static arrays.
Metadata (contains beg. canary) User Data End Canary
Figure 1. The beginning and end canaries buffer the area for user data, creating a ’block’
Whenever you malloc, you will set both of the beginning and end canaries. Since the canaries are psuedorandom numbers used for verification purposes, we will calculate them by xor’ing the address of the block
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with CANARY MAGIC NUMBER and adding 1 for fun.
unsigned long canary = ((uintptr_t)block ^ CANARY_MAGIC_NUMBER) + 1;
We will need to take into consideration the leading metadata and end canary whenever we allocate blocks.
To let the user have as much space as they requested, when they request a block of size n bytes we will
allocate a block of size sizeof(the metadata) + n + sizeof(tail canary). Along with the beginning
canary, this size will be stored in the metadata. As well, the metadata will contain four pieces of information
that is critical for the freelist discussed in the next section. As depicted in my malloc.h, this is the struct
definition for the metadata:
typedef struct metadata {
struct metadata *prev_addr;
struct metadata *next_addr;
struct metadata *prev_size;
struct metadata *next_size;
unsigned long size;
unsigned long canary;
} metadata_t;
The size portion of the metadata struct contains the size that the user requested and the TOTAL METADATA SIZE,
a macro holding the size in bytes of the metadata and end canary which will be described in detail in the
next section. For ease of reading, this macro will be represented as TMS in all of our block representations.
The user does not care about the metadata for the block, they just want the size they requested. Therefore,
when you return a block to the user, you will need to use pointer arithmetic to ’step over’ the metadata and
return the address of the data. What this looks like:
Metadata (contains beg. canary) User Data End Canary
Pointer returned to the user
Figure 2. When a block is returned to the user, the pointer returned points to the address of the area used
by the user
1.3 The Freelist
When we split up memory, we give one piece/block to the user. The remaining pieces/blocks are placed in a
linked list, called the freelist, to be used at a later time. For this semester, we are representing our freelist as
two separate doubly linked lists, one organized by address, the other organized by size. Both of these linked
lists should be defined as global file variables and to help you out, we have already defined them for you.
metadata_t *address_list;
metadata_t *size_list;
This may be a source of some confusion, as we have two linked lists for the representation of our freelist.
In reality, each block will be placed two linked lists, one ascending in address:
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
block in use
by the user
Block B
Meta Size: TMS
Usable Size: 30
Total:30+TMS
block in use
by the user
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
The other ascending by size:
4
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
block in use
by the user
Block B
Meta Size: TMS
Usable Size: 30
Total:30+TMS
block in use
by the user
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
Note: If two blocks are the same size, place the newer block before the older block in the size list.
For the remainder of the pdf, we will represent the freelist without spaces for the blocks currently in use by
the user like so:
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
Block B
Meta Size: TMS
Usable Size: 30
Total:30+TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
A Quick Note: The node representations in our freelists should be read as the following:
1. First Line: The name of the block (”Block B”)
2. Second Line: Meta Size → The size of the metadata for that block
3. Third Line: Usable Size → The size of the space available to the user
4. Fourth Line: Total → The total size of the memory taken up by this block
Both the address list and size list are doubly linked, which is why each block has both a previous and next
pointer for both of the lists. Whenever you remove or add to the lists, make sure to update all four of the
pointers present in the metadata.
1.4 Simple Linked List: Allocating
When we first allocate space for the heap, it is in our best interest not to just request what we need
immediately but rather to get a sizable amount of space, use a piece of it now, and keep the rest around
in the freelist until we need it. This reduces the amount of times we need to call sbrk(), the real version
of which, as we discussed earlier, involves significant system overhead. So how do we know how much to
allocate, how much to give to the user, and how much to keep?
For this assignment we will request blocks of size 2048 bytes from my sbrk(). We don’t want to waste
space, though, so we want to give to the user the smallest size block in which their request would fit. For
example, the user may request 256 bytes of space. It is tempting to give them a block that is 256 bytes, but
remember we are also storing the metadata inside the block. If our metadata and canaries takes up sizeof
(metadata t) + sizeof (int) = 20 bytes for example, we need at least a
256 + 20 = 276
byte block.
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Note that the size of your metadata will vary based on your computer’s architecture and platform. Use
sizeof() to avoid depending on the platform, and the macro TOTAL METADATA SIZE that sums the beginning
metadata and end canary so you don’t have to worry about it.
How do we get from one big free block of size 2048 bytes to the block of size 276 bytes we want to give to
the user? In this simple implementation, you will traverse the size list to find the best block to satisfy
the user’s request, which should be equal or greater than the size requested, and “split” off however much
you need from the front or the back. For this assignment, you must split off from the back.
Say we have the following situation:
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
Block B
Meta Size: TMS
Usable Size: 30
Total:30+TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
When we malloc for a certain size, we first want to use a block of that exact size, remove it from both the
address list and size list and return it to the user.
Ex: malloc(3) would leave the freelist as so:
Block B
Meta Size: TMS
Usable Size: 30
Total:30+TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
If we do not have a perfectly sized block, then find the next block that is big enough to split. i.e. A block
that is big enough for the size of the malloc call + TMS with room for another block, MIN BLOCK SIZE. (In
our case, MIN BLOCK SIZE is defined to be 1 byte + TMS)
Ex: my malloc(25) would split block B into two blocks B(size 25) and B’(size 5). Remember to split your
block from the back, in which the left portion of the block will remain in the freelist.
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
Block B
Meta Size: TMS
Usable Size:
TMS-25
Total: 25
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
Block B’
Meta Size: TMS
Usable Size: 5
Total:5+TMS
address list
size list
Once Block B is returned to the user, this call will leave the freelist as such:
6
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
Block B’
Meta Size: TMS
Usable Size: 5
Total:5+TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
Don’t forget to set both canaries and move the pointer to the beginning of the space the user uses after the
end of the metadata before returning the block to the user.
1.5 Simple Linked List: Deallocating
When we deallocate memory, we simply check the block’s canaries and return the block to the address list
and size list in the appropriate position. When the user calls the free function with a block body pointer,
we do some pointer arithmetic to find the starting point of the entire block (i.e. the metadata). Notice we
don’t clear out all the data. That really just takes too long when we’re not supposed to care about what’s
in memory after we free it anyway. For all of you who were wondering why sometimes you can still access
data in a dynamically allocated block even after you call free on its pointer, this is why! We like the freelists
to contain fairly large blocks so that large requests can be allocated quickly, so if the block on either side of
the block we’re freeing is also free, we can coalesce them, or join them into the bigger block like they were
before we split them.
How do we know what blocks we can join with? The left side one will have its address + its size = your
block’s address, and the right one will be your block’s size + it’s address.
To deallocate blocks, we would first iterate through the address list for the correct location of the block.
If the block could be merged with a curr block to the right or left, we would remove the curr block from
the size list, combine the blocks and re-enter it into the size list. If the block could not be merged,
we would insert it in the appropriate positions in both the size list and address list. The following
examples demonstrate a few of the possibilities with deallocation.
Let’s start with this situation:
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
Block B
Meta Size: TMS
Usable Size: 5
Total:5+TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
If we deallocated a block of size 25, we would first iterate through the address list for the correct location
of the block and check to see if the block needs to be merged either to the right of left. In this example,
the block to be entered is not directly next to any other blocks, address wise, so we would just insert it into
the address list. Finally, we would insert the block in the correct position in the size list leaving the
freelist as seen below (assuming Block D’s address places it in between Blocks B and A)
7
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
Block D
Meta Size: TMS
Usable Size: 25
Total:25+TMS
Block B
Meta Size: TMS
Usable Size: 5
Total:5+TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
If Block B and D were right next to eachother in memory (i.e. the address at end of block B is equal to the
address at the beginning of block D), then we would need to perform a merge. To perform this left merge,
pop block B from the size list , add block D to it, reset the size and canaries, and find the new block its’
proper home in the size list (Note: there is a way to perform a left merge without removing and inserting
blocks from the address list).
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
Block D
Meta Size: TMS
Usable Size: 25
Total:25+TMS
Block B
Meta Size: TMS
Usable Size: 5
Total:5+TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
Block BD
Meta Size: TMS
Usable Size: 30 +TMS
Total:30+ 2*TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
If Block D and A were right next to eachother in memory (i.e. the address at the end of block D is equal
to the address at the beginning of block A), then we would need to perform a merge. To perform this right
merge, pop block A from the size_list, add block D to it, move block A’s metadata to block D, reset the
size and canaries, and find the new block its’ proper home in the size list.
Block A
Meta Size: TMS
Usable Size: 3
Total:3+TMS
Block D
Meta Size: TMS
Usable Size: 25
Total:25+TMS
Block B
Meta Size: TMS
Usable Size: 5
Total:5+TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
8
Block DA
Meta Size: TMS
Usable Size: 28 +TMS
Total:5+ 2*TMS
Block B
Meta Size: TMS
Usable Size: 5
Total:5+TMS
Block C
Meta Size: TMS
Usable Size: 10
Total:10+TMS
address list
size list
Note: To compare pointers, cast them to uintptr t first
1.6 my malloc()
You are to write your own version of malloc that implements simple linked-list based allocation:
1. Figure out what size block you need to satisfy the user’s request by adding TOTAL METADATA SIZE to
the requested block body size to include the size of the metadata and the tail canary, that will be the real
block size we need. (Note: if this size in bytes is over SBRK SIZE, set the error SINGLE REQUEST TOO LARGE
and return NULL. If the request size is 0, then mark NO ERROR and return NULL).
2. Now that we have the size we care about, we need to iterate through our freelist to find a block that
best fits. Best fit is defined as a block that is exactly the same size, or the smallest block big enough
to split and house a new block (MIN BLOCK SIZE is defined for you). If the block is not big enough to
split, it is not a valid block and cannot be used.
(a) If the block is exactly the same size, you can simply remove it from the both the address list
and size list, set the canaries, and return a pointer to the body of the block.
(b) If the block is big enough to house a new block, we need to split off the portion we will use.
Remember: pointer arithmetic can be tricky, make sure you are casting to a uint8 t * before
adding the size (in bytes) to find the split pointer!
(c) If no suitable blocks are found at all, then call my sbrk() with SBRK SIZE to get more memory.
You must use this macro; failure to do so will result in a lower grade. After setting up its
metadata and merging it if possible (in this assignment, there must never be two different blocks
in the freelist who are directly adjacent in memory), go through steps (a)-(c). In the event that
my sbrk() returns failure (by returning NULL), you should set the error code OUT OF MEMORY and
return NULL.
Remember that you want the address you return to be at the start of the block body, not the metadata.
This is sizeof (metadata t) bytes away from the metadata pointer. Since pointer arithmetic is in
multiples of the sizeof the data type, you can just add 1 to a pointer of type metadata t* pointing to
the metadata to get a pointer to the body. If you have not specifically set the error code during this
operation, set the error code to NO ERROR before returning.
3. The first call to my malloc() should call my sbrk(). Note that malloc should call my sbrk() when it
doesn’t have a block to satisfy the user’s request anyway, so this isn’t a special case.
1.7 my free()
You are also to write your own version of free that implements deallocation. This means:
1. Calculate the proper address of the block to be freed, keeping in mind that the pointer passed to any
call of my free() is a pointer to the block body and not to the block’s metadata.
2. Check the canaries of the block, starting with the head canary (so that if it is wrong you don’t try to
use corrupted metadata to find the tail canary) to make sure they are still their original value. If the
canary has been corrupted, set the CANARY CORRUPTED error and return.
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3. Attempt to merge the block with blocks that are consecutive in address space with it if those blocks are
free. That is, try to merge with the block to its left and its right in memory if they are in the freelist.
Finally, place the resulting block in both the address list and size list by setting the respective
previous and next address pointers for both the address list and size list in each node.
Just like the free() in the C standard library, if the pointer is NULL, no operation should be performed.
1.8 my realloc()
You are to write your own version of realloc that will use your my malloc() and my free() functions.
my realloc() should accept two parameters, void *ptr and size t size. If the block’s canaries are
valid, it will attempt to effectively change the size of the memory block pointed to by ptr to size bytes,
and return a pointer to the beginning of the new memory block. If the canaries are invalid, it returns NULL
and sets my malloc errno to CANARY CORRUPTED.
Do not directly change the freelist or blocks in my realloc() — leave that to my malloc() and my free().
This means you don’t need to worry about shrinking or extending blocks in place1
; if size is nonzero, just
always call my malloc() to attempt to allocate a new block of the new size. Make sure to copy as much
data as will fit in the new block from the old block to the new block. The rest of the data in the new block
(if any) should be uninitialized.
Your my realloc() implementation must have the same features as the realloc() function in the standard
library. Specifically:
1. If the pointer is null – make a call to malloc using the size argument (i.e. malloc(size))
2. If the canaries are corrupted – set the CANARY CORRUPTED error code and return null
3. If the size is equal to zero, and pointer is non-null – make a call to free using the ptr argument and
return to null (i.e. free(ptr))
4. Else, create a new block via my malloc and and copy the old block’s data to the new block up to
min(new block data size, old block data size)
1.9 my calloc()
You are to write your own version of calloc that will use your my malloc() function. my calloc() should
accept two parameters, size t nmemb and size t size. It will allocate a region of memory for nmemb
number of elements, each of size size, zero out the entire block, and return a pointer to that block.
If my malloc() returns NULL, do not set any error codes (as my malloc() will have taken care of that) and
just return NULL directly.
1.10 Error Codes
For this assignment, you will also need to handle cases where users of your malloc do improper things with
their code. For instance, if a user asks for 12 gigabytes of memory, this will clearly be too much for your 8
kilobyte heap. It is important to let the user know what they are doing wrong. This is where the enum in
the my malloc.h comes into play. You will see the four types of error codes for this assignment listed inside
of it. They are as follows:
• NO ERROR: set whenever my calloc(), my malloc(), my realloc(), and my free() complete successfully.
1Even though we don’t extend or shrink blocks in place in this homework, keep in mind that real-world implementations
(which are not written in a panic right before finals) very well could.
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• OUT OF MEMORY: set whenever the user’s request cannot be met because there’s not enough
heap space.
• SINGLE REQUEST TOO LARGE: set whenever the user’s requested size plus the total metadata
size is beyond SBRK SIZE.
• CANARY CORRUPTED: set whenever either canary is corrupted in a block passed to free() or
realloc().
Inside the .h file, you will see a variable of type enum my malloc err called my malloc errno. Whenever any
of the cases above occur, you are to set this variable to the appropriate type of error. You may be wondering
what happens if a single request is too large AND it causes malloc to run out of memory. In this case, we will
let the SINGLE REQUEST TOO LARGE take precedence over OUT OF MEMORY. So in the case of a request of 9kb,
which is clearly beyond our biggest block and total heap size, we set ERRNO to SINGLE REQUEST TOO LARGE.
1.11 Using the Makefile
If you are not on docker, before running the Makefile, you need to install Check, a C unit testing library
the provided tests use. The following command should install the packages you need for this homework (you
should already have them installed but here it is again):
sudo apt-get install pkg-config check gdb
You can run the provided tests with make run-tests and run gdb with make run-gdb.
1.12 Deliverables
Submit only my malloc.c to GRADESCOPE under “Homework 11.” Please don’t zip it.
Do NOT modify or submit the header file, my malloc.h. We will grade with the original copy. Any functions
or variables you add should be marked static so they do not conflict with the grader.
Also, please note that the tests are not weighted, so the grade you get in your terminal will NOT be the
grade you get on this assignment. You can submit to Gradescope to get a better idea of that, but we reserve
the right to add test cases later.
1.13 Suggested Helper Methods
Coding malloc can seem like quite a daunting challenge, but your debugging process can be helped along
tremendously if you do not write all of malloc in one method and instead split it up into helper methods!
Helper methods are incredibly useful for understanding what is going on and also results in cleaner code, so
it’s a win-win strategy. Below are some TA recommended helper methods to implement, and while they are
not required and will not be tested with the autograder, we advise that you use them.
All helper methods must be declared static:
• static metadata_t* find_right(metadata_t*)
• static metadata_t* find_left(metadata_t*)
• static void merge(metadata_t* left, metadata_t* right)
• static void double_merge(metadata_t* left, metadata_t* middle, metadata_t* right)
• static metadata_t* split_block(metadata_t* block, size_t size)
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• static void add_to_address_list(metadata_t* add_block)
• static void add_to_size_list(metadata_t* add_block)
• static void remove_from_address_list(metadata_t* remove_block)
• static void remove_from_size_list(metadata_t* remove_block)
• static void set_canary(metadata_t* block)
Remember, this is not an exhaustive list of operations that can performed with helper methods. Feel free to
implement helper methods for any aspect of malloc that works for you.
Note: We are declaring these functions to be static because we want them to be private to
my malloc.c. DO NOT put any function prototypes in my malloc.h
1.14 Debugging
Yes, we assigned malloc
which makes us pretty cruel.
But here are some debugging tips
because we are actually kind of cool
When you run the tests, you will see a pretty hefty output in your terminal. Each line of the output provides
critical information depicting which tests you are failing/passing. The general format of:
suite filename.c:420:fun test case:test description
states a test named test description is failing/passing in an individual test case named fun test case,
located in that specific test suite suite filename.c at line 420. That is, test suites contain test cases which
contain tests. For example,
malloc suite.c:37:Malloc Perf Block1:test malloc perf block1 lists
tells us whether the address list and size list is correct when we malloc for a perfectly sized block. More
information about the test is written in malloc suite.c, and the assertion that failed is on line 37.
To run an individual test case, run
make run-tests TEST=Malloc_Perf_Block1
Every time you run the tests – a function will print out your address list and size list to the test.txt file
located in the actual folder. To see the difference between your address/size list and the expected address/size
list, look at the text.txt file located in the diff directory. Below is a sample diff:
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From the image above, please note that the expected address and size list is on the left hand side, while
the address and size list generated by your code is located on the right hand side. As well, the ’<’ symbol
denotes that the expected lists contained something that the actual lists did not. Furthermore, the ’>’
symbol denotes that the actual list contained an extra block. Finally, the ’|’ symbol denotes an change in
the line from expected to actual lists.
To debug an individual test case with gdb, run
make run-gdb TEST=Malloc_Perf_Block1
When debugging with gdb, anytime you want to print out a metadata t pointer, call the pp pointer()
function to pretty-print your pointer. For example, if you wanted to see what the head of the address list
was:
(gdb) p/x pp_pointer(address_list)
2 Frequently Asked Questions
1. I have a segfault, will you debug it for me?
No, debug it yourself with gdb. Here is the gdb video one of the TA’s created:

Here are some other gdb tutorials:
• https://www.cs.cmu.edu/~gilpin/tutorial/
• http://www.cs.yale.edu/homes/aspnes/pinewiki/C%282f%29Debugging.html
• http://heather.cs.ucdavis.edu/~matloff/UnixAndC/CLanguage/Debug.html
• http://heather.cs.ucdavis.edu/~matloff/debug.html
• http://www.delorie.com/gnu/docs/gdb/gdb_toc.html
2. Can we build our freelists with list heads/dummy nodes?
No. No dummy nodes. The autograder checks the state of the freelist and if you have dummy nodes
it will throw it off.
3. Should we first initialize the freelist to NULL?
No, it is static and is therefore already initialized to NULL by the compiler.
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4. The assignment says to just call my sbrk() again. But won’t this mean we then have 2
heaps?
Not exactly, it will expand the heap by another 2KB. You don’t get two heaps. Once it has been
expanded to 8KB, calls to my sbrk() will return NULL.
5. Are the provided tests comprehensive?
Yes. We reserve the right to change our mind on this, but if you get a 100 on the tester, you should
expect 100 on the homework. Just keep in mind that the tests may be weighted differently when
grading than in the provided student tester.
6. Can I use the malloc() from the C standard library?
No. Absolutely not.
3 Rules and Regulations
3.1 General Rules
1. Starting with the assembly homeworks, any code you write must be meaningfully commented. You
should comment your code in terms of the algorithm you are implementing; we all know what each
line of code does.
2. Although you may ask TAs for clarification, you are ultimately responsible for what you submit. This
means that (in the case of demos) you should come prepared to explain to the TA how any piece of
code you submitted works, even if you copied it from the book or read about it on the internet.
3. Please read the assignment in its entirety before asking questions.
4. Please start assignments early, and ask for help early. Do not email us the night the assignment is due
with questions.
5. If you find any problems with the assignment it would be greatly appreciated if you reported them to
the author (which can be found at the top of the assignment). Announcements will be posted if the
assignment changes.
3.2 Submission Conventions
1. All files you submit for assignments in this course should have your name at the top of the file as
a comment for any source code file, and somewhere in the file, near the top, for other files unless
otherwise noted.
2. When preparing your submission you may either submit the files individually to Canvas/Gradescope
or you may submit an archive (zip or tar.gz only please) of the files. You can create an archive by right
clicking on files and selecting the appropriate compress option on your system. Both ways (uploading
raw files or an archive) are exactly equivalent, so choose whichever is most convenient for you.
3. Do not submit compiled files, that is .class files for Java code and .o files for C code. Only submit the
files we ask for in the assignment.
4. Do not submit links to files. The autograder does not understand it, and we will not manually grade
assignments submitted this way as it is easy to change the files after the submission period ends.
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3.3 Submission Guidelines
1. You are responsible for turning in assignments on time. This includes allowing for unforeseen circumstances. If you have an emergency let us know IN ADVANCE of the due time supplying documentation (i.e. note from the dean, doctor’s note, etc). Extensions will only be granted to those who contact
us in advance of the deadline and no extensions will be made after the due date.
2. You are also responsible for ensuring that what you turned in is what you meant to turn in. After
submitting you should be sure to download your submission into a brand new folder and test if it
works. No excuses if you submit the wrong files, what you turn in is what we grade. In addition, your
assignment must be turned in via Canvas/Gradescope. Under no circumstances whatsoever we will
accept any email submission of an assignment. Note: if you were granted an extension you will still
turn in the assignment over Canvas/Gradescope.
3. There is a 6-hour grace period added to all assignments. You may submit your assignment without
penalty up until 11:55PM, or with 25% penalty up until 5:55AM. So what you should take from this is
not to start assignments on the last day and plan to submit right at 11:54AM. You alone are responsible
for submitting your homework before the grace period begins or ends; neither Canvas/Gradescope, nor
your flaky internet are to blame if you are unable to submit because you banked on your computer
working up until 11:54PM. The penalty for submitting during the grace period (25%) or after (no
credit) is non-negotiable.
3.4 Syllabus Excerpt on Academic Misconduct
Academic misconduct is taken very seriously in this class. Quizzes, timed labs and the final examination are
individual work.
Homework assignments are collaborative, In addition many if not all homework assignments will be evaluated
via demo or code review. During this evaluation, you will be expected to be able to explain every aspect of
your submission. Homework assignments will also be examined using computer programs to find evidence
of unauthorized collaboration.
What is unauthorized collaboration? Each individual programming assignment should be coded by you.
You may work with others, but each student should be turning in their own version of the assignment.
Submissions that are essentially identical will receive a zero and will be sent to the Dean of Students’ Office
of Academic Integrity. Submissions that are copies that have been superficially modified to conceal that
they are copies are also considered unauthorized collaboration.
You are expressly forbidden to supply a copy of your homework to another student via electronic means. This includes simply e-mailing it to them so they can look at it. If you supply
an electronic copy of your homework to another student and they are charged with copying,
you will also be charged. This includes storing your code on any site which would allow other
parties to obtain your code such as but not limited to public repositories (Github), pastebin,
etc. If you would like to use version control, use github.gatech.edu
3.5 Is collaboration allowed?
Collaboration is allowed on a high level, meaning that you may discuss design points and concepts relevant
to the homework with your peers, share algorithms and pseudo-code, as well as help each other debug code.
What you shouldn’t be doing, however, is pair programming where you collaborate with each other on a
single instance of the code. Furthermore, sending an electronic copy of your homework to another student for
them to look at and figure out what is wrong with their code is not an acceptable way to help them, because
it is frequently the case that the recipient will simply modify the code and submit it as their own. Consider
instead using a screen-sharing collaboration app, such as http://webex.gatech.edu/, to help someone with
debugging if you’re not in the same room.
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Figure 1: Collaboration rules, explained colorfully
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