Project 3a: Malloc and Free


There are three objectives to this part of the assignment:

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


At some point you will decide to use a header per each allocated block. The maximum size of such a header is 32 bytes.


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 must use a best fit algorithm to manage the free list.

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, int coalesce) 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 flag should be ignored. 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.)

    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 Best Fit (BF) policy to decide which chunk of free space to hand out; BF simply looks through your free list and finds the firts free space that is smallest in size (but still can hold the requested amount) and returns the requested size (the first part of the chunk) 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 figure out whether you return 8-byte aligned pointers, you could print the pointer this way printf("%p", ptr) . The last digit should be a multiple of 8 (i.e. 0 or 8).
  • int Mem_Free(void *ptr, int coalesce): 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. The coalesce flag determines whether the free routine should coalesce the freed space back into the bigger pool; see below for next details.

    Coalescing: When coalesce is non-zero, Mem_Free() should make sure to coalesce free space when possible; specifically, coalesce the entire list! Coalescing rejoins neighboring freed blocks into one bigger free chunk, thus ensuring that big chunks remain free for subsequent calls to Mem_Alloc(). If the coalesce flag is zero, though, no such coalescing should be done, thus making Mem_Free() faster but leaving small fragments in the free space.
  • 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 -c -fpic mem.c -Wall -Werror
gcc -shared -o mem.o

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 -lmem -L. -o myprogram mymain.c -Wall -Werror

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:


Note that setenv is what you use in tcsh; if you are using bash, you'll have to figure out the alternative command to set the environment.

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, so try to be efficient!