• Due: 4:00pm Friday, December 18
  • Starter code: fork wellesleycs240 / cs240-malloc and add bpw as admin
  • Submissions: Commit and push your final version.
  • Relevant Reading:
  • Collaboration:
    • This assignment follows the group collaboration policy. You may optionally work with a partner to produce and submit one solution, sharing a single Bitbucket repository. You may discuss problems and high-level strategies with other groups, but your group’s solutions must be due to your group members’ work.



In this assignment you will implement a dynamic memory allocator for C programs, i.e., your own version of malloc and free. This task requires careful bit-level manipulation of memory contents using many of C’s unsafe pointer casting features. The code for your allocator will not be very large, but it will be subtle.

Plan carefully.

The code for your allocator will not be very large, but it will be subtle. Before coding:

  1. Review the workings of explicit free-list allocators.
  2. Read and understand the provided code and heap layouts.
  3. Write all the steps of your malloc and free algorithms in pseudocode and pictures before writing a single line of C.


In the past, students who took the time to read and understand material about allocators in the textbook went on to implement this allocator quickly and efficiently with relatively little debugging. Students who did not understand the preliminaries before starting struggled much more while coding and debugging. Here are two ways to prepare:

In lab

In lab, you will explore heap layout rules for this allocator, as well as provided code that establishes rules for manipulating the heap. As you go, you will annotate an under-documented version of the provided code. Be sure you understand the material examined in the lab well before writing any of your own code. Other general tips:

  • Take notes on anything else you learn during the lab that it does not explicitly require.
  • Read these instructions and comments in provided code carefully.
  • Draw diagrams of the heap and execute operations on the visual heap by hand to become familiar with the details.
  • Every line of provided code does something meaningful and necessary.

On your own

Read CSAPP section 9.9 to become familiar with the type of thinking and coding you will do to build the allocator. The book discusses an implicit free list allocator (we build an explicit freelist), but the material is still very relevant to your task. The following practice problems on memory allocation may be helpful in preparing for this assignment. They will not be graded. Solutions not in the textbook are available by visiting office hours.

Note “word” means 4 bytes for these problems.

  • Practice Problem 9.6
  • Practice Problem 9.7
  • Homework Problem 9.15
  • Homework Problem 9.16


The starter code includes several files. You will modify mm.c to implement an allocator interface as declared in mm.h.

int   mm_init();               // Provided
void* mm_malloc(size_t size);  // Implemented by you.
void  mm_free(void* ptr);      // Implemented by you.

The provided mm.c file partially implements an allocator based on an explicit free list. It provides several helper functions beyond the interface above. Your job is to complete mm_malloc() and mm_free(). The three main memory management functions should work as follows:

  • int mm_init() (provided): Initialize the heap. The return value is -1 if there was a problem in performing the initialization and 0 otherwise. The application program (in this case, the driver we provide) calls mm_init exactly once to initalize the system before calling mm_malloc() or mm_free().

  • void* mm_malloc(size_t size): Return a pointer to an allocated block payload of at least size bytes within the heap or NULL if an error occurred. All payloads must be aligned to 8 bytes. The entire allocated block should lie within the heap region and should not overlap with any other allocated block. (size_t is a type for describing sizes; it’s an unsigned integer large enough to represent any size within the address space.)

  • void mm_free(void* ptr): Free the allocated block referenced by the pointer ptr. Assume that ptr was returned by an earlier call to mm_malloc() and has not yet been freed. These semantics match the semantics of malloc and free in the standard C library. Type man malloc in the shell for complete documentation.

Provided Code and Block Layout

We provide code in mm.c to help maintain an explicit free list using a specific block layout, documented with the code. You are encouraged to use this approach and code, but you may implement mm.c in any way you wish. Be sure to understand the block layout and the preconditions and postconditions of the provided code before using it.

Memory Order vs. List Order

Explicit free-list allocators must distinguish memory order from list order. Memory order refers to the order of blocks as arranged in the memory address space. We use the terms adjacent, preceding, and following to refer to memory order. List order refers to the order of blocks in the explicit free list. We use the terms next and previous to refer to list order. Confusing these orders will lead to tricky bugs.

Memory System

The memlib.c package simulates the memory system for your dynamic memory allocator. You will not need to call these functions directly, but understanding their effects is necessary for understanding the provided helper functions above.

  • void* mem_sbrk(int incr): Expand the heap by incr bytes, where incr is a positive non-zero integer and returns a 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. (Run man sbrk if you want to learn more about what this does in Unix.) This function and its approach to managing heap bounds date from before virtual memory. We use it for simplicity (rather than more flexible ways of managing available pages).

  • void* mem_heap_lo(): Return a pointer to the first byte in the heap

  • void* mem_heap_hi(): Return a pointer to the last byte in the heap.

  • size_t mem_heapsize(): Return the current size of the heap in bytes.

  • size_t mem_pagesize(): Return the system’s page size in bytes (4K on Linux systems).

Programming Rules

  • Do not change any of the function types in mm.c. You may add as many helper functions and macros as you wish.

  • Do not invoke any standard memory-management related library calls or system calls. Do not use malloc, calloc, free, realloc, sbrk, brk or any variants of these calls in your code. (You may use all the functions in memlib.c, of course.)

  • Do not define any global or static compound data structures such as arrays, structs, trees, or lists in your mm.c program. You may declare global scalar variables such as integers, floats, and pointers in mm.c, but avoid them if at all possible.

Compiling and Testing

The driver program mdriver.c tests your mm.c implementation for correctness, space utilization, and throughput. 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 described below).

The driver program is controlled by a set of trace files. Each trace file contains a sequence of allocate and free directions that instruct the driver to call your mm_malloc and mm_free functions in some sequence. The driver and the trace files are the same ones we will use when we grade your submitted implementation.

The mdriver executable accepts the following command line arguments:

  • -t <tracedir>: Look for the default trace files in directory tracedir instead of the default directory defined in config.h.
  • -f <tracefile>: Use one particular tracefile for testing instead of the default set of tracefiles.
  • -h: Print a summary of the command line arguments.
  • -l: Run and measure libc malloc in addition to the student’s malloc package.
  • -v: Verbose output. Print a performance breakdown for each tracefile in a compact table.
  • -V: More verbose output. Prints additional diagnostic information as each trace file is processed. Useful during debugging for determining which trace file is causing your malloc package to fail.
Debug Methodically

Debugging haphazardly is slow and frustrating. Debugging methodically is much more efficient. See the tips below to become a better debugger.

Design and Debugging Hints

Low-level unguarded memory manuiplation code like this allocator is prone to subtle and frustrating bugs. There’s a reason you often use higher-level languages with more protections! The following strategies can help you replace long hours of frustration and confusion with shorter effective debugging efforts.

There are three main pieces to effective debugging. The first steps toward effective debugging happen before you write any code to help avoid bugs in the first place and make them more obvious if they do occur.

Plan Carefully

Never write code until you have a good idea of what you are doing.

  • Understand the malloc implementation in the textbook. The textbook has a detailed example of a simple implicit free list allocator. Use this as a point of departure. Do not start working on your allocator until you understand everything about the simple implicit list allocator.

  • Read and understand the provided code before writing any of your own.

  • Write out your algorithms in pseudocode and execute them in pictures before writing a single line of C.

Code Defensively

Expect things to go wrong. Anywhere your code relies on an assumption, check it explicitly. Explain to your partner why your code should work. Write error-prone code once carefully and reuse it.

  • Use assertions. Anywhere you assume a property of data to hold, assert that property explicitly. Assertions document your assumptions. They are also checked explicitly as your program runs, making it easier to localize the source of an error if one occurs.

  • Encapsulate pointer arithmetic in C preprocessor macros. Pointer arithmetic in memory managers is confusing and error-prone because of all the casting that is necessary. We have supplied maros that do this: see PADD and PSUB.

Debug Methodically

  • Did your program hit an explicit error? A segfault?
    • Localize it. Exactly what line of code, what operation in this line, and what value used by this operation manifested the error and crashed?
      • Use gdb to run the code until it crashes, then find the line that crashed.
      • Determine what ill-formed pointer was dereferenced to cause a segmentation fault. Use print to inspect the value of a variable or other expression.
    • Trace backward through the dependences of this operation to the original logic error.
      • Did any of this operation’s arguments hold invalid values?
        • If so, what operations produced these invalid values?
        • Continue tracing backwards treating these producer operations as broken operations.
      • Was it invalid to apply this operation here given correct arguments (or regardless of the arguments)?
        • If so, what control flow decision led to execution of this operation?
        • Continue tracing backwards treating these producer operations as broken operations.
      • You may need to trace back beyond the function where the crash happened.
        • Get a backtrace to see where the current function was called. (Don’t just assume you know which function called this one. Get the truth.)
        • If you want to inspect local variables in functions that called this one, use up and down in gdb to step up and down the call stack, then print what you want to see.
      • Remember, mm_malloc and mm_free are called many times during the execution of the driver program. They depend on their arguments, but also the entire contents of the heap as it exists when they are called, so changes made by earlier calls to mm_malloc and mm_free affect data used by this one.
        • Use the examine_heap function to scan the heap.
        • Run it at arbitrary times in gdb with call examine_heap().
        • Or hard-code calls to it in your code so you see how the heap evolves with each mm_malloc or mm_free call.
  • valgrind may be less useful since you are doing very low-level work, implementing the memory allocator itself. Using valgrind “below” the memory allocator intended for analyzing programs that use the memory allocator.

  • Use the mdriver -f option. During initial development, using tiny trace files will simplify debugging and testing. We have included two such trace files (short1-bal.rep and short2-bal.rep) that you can use for initial debugging.

  • Use the mdriver -v and -V options. The -v option will give you a detailed summary for each trace file. The -V will also indicate when each trace file is read, which will help you isolate errors.

  • Does your code seem to be running forever? That might be because it is no longer broken! Comment out those calls to examine_heap. Printing the full contents of the heap on every allocation event costs orders of magnitude more than the allocations or frees themselves.


Your grade will be calculated as follows:

  • Correctness (60%). Based on mdriver tests.
  • Style (25%).
    • Your code should use as few global variables as possible (ideally none!).
    • Your code should be as clear and concise as possible.
    • Each function should have a header comment that describes what it does, how it does it, and what it assumes. Keep this description high-level.
    • Since some of the unstructured pointer manipulation inherent to allocators can be confusing, short inline comments on steps of the allocation algorithms are also recommended. (These will also help us give you partial credit if you have a partially working implementation.)
  • Performance (15%). We are most concerned about the correctness of your implementation. For the most part, a correct implementation based on our provided code will yield reasonable performance. Two performance metrics will be used to evaluate your solution:
    • Space utilization: The peak ratio between the aggregate amount of memory used by the driver (i.e., allocated via mm_malloc but not yet freed via mm_free) and the size of the heap used by your allocator. The optimal ratio is 1. You should find good policies to minimize fragmentation in order to make this ratio as close as possible to the optimal.
    • Throughput: The average number of operations completed per second.

    The driver program summarizes the performance of your allocator by computing a performance index, which is a weighted sum of the space utilization and throughput:

    P = 0.6U + 0.4 min (1, T/Tlibc)

    where U is your space utilization, T is your throughput, and Tlibc is the estimated throughput of the standard C libary (libc) version of malloc on the default traces. The performance index favors space utilization over throughput. To receive a good performance score, you must achieve a balance between utilization and throughput. A performance index of 0.7-0.8 or above is good.

OPTIONAL Extensions

There are a number of interesting allocator features you could add if looking for extensions.

Heap Consistency Checker

This is an optional, but recommended, addition that will help you check to see if your allocator is doing what it should (or figure out what it’s doing wrong if not). 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. In addition to the usual debugging techniques, you may find it 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. Feel free to rename it, break it into several functions, and call it wherever you want. It should check any invariants or consistency conditions you consider prudent. It returns a nonzero value if and only if your heap is consistent. This is not required, but may prove useful. When you submit mm.c, make sure to remove any calls to mm_check as they will slow down your throughput.

Extended Error-Checking

Building from a heap consistency checker, consider how to augment the allocator to do additional error-checking on the fly to detect common errors when using a memory allocator. You could insert extra padding, use canary values, place recently freed blocks in a waiting area, or track additional information about earlier allocations and frees to detect errors. Come talk if you are curious.

Implement realloc

Implement a final memory allocation-related function: mm_realloc. The signature for this function, which you will find in your mm.h file, is:

extern void* mm_realloc(void* ptr, size_t size);

Similarly, you should find the following in your mm.c file:

void* mm_realloc(void* ptr, size_t size) {
  // ... implementation here ...

Follow the contract of the standard C library’s realloc exactly (pretending that malloc and free are mm_malloc and mm_free, etc.). The man page entry for realloc says:

   The realloc() function changes the size of the memory block pointed to by
   ptr to size bytes.  The contents will be unchanged in the range from the
   start of the region up to the minimum of the old and new sizes.  If the
   new size is larger than the old size, the added memory will not be
   initialized.  If ptr is NULL, then the call is equivalent to
   malloc(size), for all values of size; if size is equal to zero, and ptr
   is not NULL, then the call is equivalent to free(ptr).  Unless ptr is
   NULL, it must have been returned by an earlier call to malloc(), calloc()
   or realloc().  If the area pointed to was moved, a free(ptr) is done.

A good test would be to compare the behavior of your mm_realloc to that of realloc, checking each of the above cases. Your implementation of mm_realloc should also be performant. Avoid copying memory if possible, making use of nearby free blocks. Do not use memcpy to copy memory; instead, copy WORD_SIZE bytes at a time to the new destination while iterating over the existing data.

To run tracefiles that test mm_realloc, compile using make mdriver-realloc. Then, run mdriver-realloc with the -f flag to specify a tracefile, or first edit config.h to include additional realloc tracefiles in the default list.

Alternative Policies

Reimplement the searchFreeList function to implement next-fit or best-fit.


Convert our single-list allocator to a seglist allocator and measure performance impact. This will involve changing or replacing provided code at a fairly deep level. It will be more work than the assignment so far, but very interesting!