Project 3: Malloc and Free

Important Dates

Questions about the project? Send them to .

Final Deadline: Monday 02/28 @ 9pm.


You should set m_error not merror (corrected below).

NEW: When debugging is on, Mem_Alloc should:

  • check that the free chunk to be used for allocation is filled with the correct pattern (DEADBEEF)
  • add padding of 64 bytes before and after the chunk of memory that is returned to the user
It is not required to check free or allocated chunks that are not affected by the current allocation.


There are three objectives to this assignment:

  • To understand the nuances of building a memory allocator.
  • To do so in a performance-efficient manner.
  • To create a shared library.


In this project, you will be implementing a memory allocator for the heap of a user-level process. Your functions will be to build your own malloc() and free().

Memory allocators have two distinct tasks. First, the memory allocator asks the operating system to expand the heap portion of the process's address space by calling either sbrk or mmap. Second, the memory allocator doles out this memory to the calling process. This involves managing a free list of memory and finding a contiguous chunk of memory that is large enough for the user's request; when the user later frees memory, it is added back to this list.

This memory allocator is usually provided as part of a standard library and is not part of the OS. To be clear, the memory allocator operates entirely within the virtual address space of a single process and knows nothing about which physical pages have been allocated to this process or the mapping from logical addresses to physical addresses.

When implementing this basic functionality in your project, we have a few guidelines. First, when requesting memory from the OS, you must use mmap() (which is easier to use than sbrk()). Second, although a real memory allocator requests more memory from the OS whenever it can't satisfy a request from the user, your memory allocator must call mmap() only one time (when it is first initialized). Third, you are free to use any data structures you want to manage the free list as well as any policy for choosing a chunk of memory. Use worst fit! The competition for <“>best allocator<”> is a performance competition; speed matters, if you want to win, that is.

Classic malloc() and free() are defined as follows:

  • void *malloc(size_t size): malloc() allocates size bytes and returns a pointer to the allocated memory. The memory is not cleared.
  • void free(void *ptr): free() frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc() (or calloc() or realloc()). Otherwise, or if free(ptr) has already been called before, undefined behaviour occurs. If ptr is NULL, no operation is performed.

For simplicity, your implementations of Mem_Alloc(int size) and Mem_Free(void *ptr) should basically follow what malloc() and free() do; see below for details.

You will also provide a supporting function, Mem_Dump(), described below; this routine simply prints which regions are currently free and should be used by you for debugging purposes.

Program Specifications

For this project, you will be implementing several different routines as part of a shared library. Note that you will not be writing a main() routine for the code that you handin (but you should implement one for your own testing). We have provided the prototypes for these functions in the file mem.h (which is available at ~cs537-1/public/mem.h); you should include this header file in your code to ensure that you are adhering to the specification exactly. You should not change mem.h in any way! We now define each of these routines more precisely.

  • int Mem_Init(int sizeOfRegion, int debug): Mem_Init is called one time by a process using your routines. sizeOfRegion is the number of bytes that you should request from the OS using mmap, and debug tells you whether to turn on debugging or not (described below).

    Note that you may need to round up this amount so that you request memory in units of the page size (see the man pages for getpagesize()). Note also that you need to use this allocated memory for your own data structures as well; that is, your infrastructure for tracking the mapping from addresses to memory objects has to be placed in this region as well. You are not allowed to malloc(), or any other related function, in any of your routines! Similarly, you should not allocate global arrays! However, you may allocate a few global variables (e.g., a pointer to the head of your free list.)

    When debugging is on, your library will help users find problems with their buggy code. It will do so by doing the following things: filling all free'd memory with a well-known pattern (e.g., 0xDEADBEEF), and putting some padding around each allocated space (specifically, 64 bytes on each side, filled with the pattern 0xABCDDCBA). How this will be used is described further below.

    Return 0 on a success (when call to mmap is successful). Otherwise, return -1 and set m_error to E_BAD_ARGS. Cases where Mem_Init should return a failure: Mem_Init is called more than once; sizeOfRegion is less than or equal to 0.
  • void *Mem_Alloc(int size): Mem_Alloc() is similar to the library function malloc(). Mem_Alloc takes as input the size in bytes of the object to be allocated and returns a pointer to the start of that object. The function returns NULL if there is not enough contiguous free space within sizeOfRegion allocated by Mem_Init to satisfy this request (and sets m_error to E_NO_SPACE).

    Your allocator should use a Worst Fit (WF) policy to decide which chunk of free space to hand out; WF simply looks through your free list and finds the free space that is largest in size and returns the requested size to the user, keeping the rest of the chunk in its free list.

    For performance reasons, Mem_Alloc() should return 8-byte aligned chunks of memory. For example if a user allocates 1 byte of memory, your Mem_Alloc() implementation should return 8 bytes of memory so that the next free block will be 8-byte alligned too. To debug whether you return 8-byte aligned pointers, you could print the pointer this way printf("%08x", ptr) . The last digit should be a multiple of 8 (i.e. 0 or 8).

    Note: This part has been simplified, as described above. When debugging is on, Mem_Alloc() should do a little extra work. Specifically, it should check that the free space that will be used for the allocation is filled with the correct pattern (DEADBEEF). Specifically, when looking through the free list, it should check that each free space is filled with the correct pattern, and each padding is filled with the correct pattern. If not, Mem_Alloc() should fail (return NULL) and set m_error to E_CORRUPT_FREESPACE.
  • int Mem_Free(void *ptr): Mem_Free() frees the memory object that ptr points to. Just like with the standard free(), if ptr is NULL, then no operation is performed. The function returns 0 on success, and -1 otherwise.

    When debugging is on, Mem_Free() must do a little extra work. Specifically, it should check whether the padding at either end of the block has been overwritten; if so, it should return -1 and set the global variable m_error to E_PADDING_OVERWRITTEN. It should also make sure to set the freed memory (except perhaps for a header) to 0xDEADBEEF (freed memory has no padding). Further, if the pointer passed by the user to free doesn't seem to point to something that Mem_Alloc() handed out earlier, return -1 and set m_error to E_BAD_POINTER. To check for this, you might wish to use a magic number of some kind.

    Mem_Free() should make sure to coalesce free space when possible. Coalescing rejoins smaller freed blocks into one bigger chunk, thus ensuring that big chunks remain free for subsequent calls to Mem_Alloc().
  • void Mem_Dump(): This is just a debugging routine for your own use. Have it print the regions of free memory to the screen.

You must provide these routines in a shared library named "". Placing the routines in a shared library instead of a simple object file makes it easier for other programmers to link with your code. There are further advantages to shared (dynamic) libraries over static libraries. When you link with a static library, the code for the entire library is merged with your object code to create your executable; if you link to many static libraries, your executable will be enormous. However, when you link to a shared library, the library's code is not merged with your program's object code; instead, a small amount of stub code is inserted into your object code and the stub code finds and invokes the library code when you execute the program. Therefore, shared libraries have two advantages: they lead to smaller executables and they enable users to use the most recent version of the library at run-time. To create a shared library named, use the following commands (assuming your library code is in a single file "mem.c"):

gcc -m32 -c -fpic mem.c
gcc -m32 -shared -o mem.o

Note that you should compile with -m32 to make sure your binary is a 32-bit binary. To link with this library, you simply specify the base name of the library with "-lmem" and the path so that the linker can find the library "-L.".

gcc -m32 -lmem -L. -o myprogram mymain.c

Of course, these commands should be placed in a Makefile. Before you run "myprogram", you will need to set the environment variable, LD_LIBRARY_PATH, so that the system can find your library at run-time. Assuming you always run myprogram from this same directory, you can use the command:


If the setenv command returns an error "LD_LIBRARY_PATH: Undefined variable", do not panic. The error implies that your shell has not defined the environment variable. In this case, you simply need to run:


Unix Hints

In this project, you will use mmap to map zero'd pages (i.e., allocate new pages) into the address space of the calling process. Note there are a number of different ways that you can call mmap to achieve this same goal; we give one example here:

// open the /dev/zero device
int fd = open("/dev/zero", O_RDWR);

// sizeOfRegion (in bytes) needs to be evenly divisible by the page size
void *ptr = mmap(NULL, sizeOfRegion, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
if (ptr == MAP_FAILED) { perror("mmap"); exit(1); }

// close the device (don't worry, mapping should be unaffected)
return 0;


Your implementation will be graded on functionality. However, we will also be comparing the performance of each of your projects; the details for the performance challenge will be coming out soon.

Handing in your Code

Hand in your source code and a README file. The usual place: ~cs537-1/handin/NAME/p3/, where NAME is your login name.

You should copy all of your source files (*.c and *.h) and a Makefile to your p3 handin directory. Do not submit any .o files.