C – Design your own free( ) function

Today, I appeared for an interview and the interviewer asked me this,

Tell me the steps how will you design your own free( ) function for
deallocate the allocated memory.

How can it be more efficient than C’s default free() function ? What can you conclude ?

I was confused, couldn’t think of the way to design.

What do you think guys ?

EDIT : Since we need to know about how malloc() works, can you tell me the steps to write our own malloc() function

That’s actually a pretty vague question, and that’s probably why you got confused. Does he mean, given an existing malloc implementation, how would you go about trying to develop a more efficient way to free the underlying memory? Or was he expecting you to start discussing different kinds of malloc implementations and their benefits and problems? Did he expect you to know how virtual memory functions on the x86 architecture?

Also, by more efficient, does he mean more space efficient or more time efficient? Does free() have to be deterministic? Does it have to return as much memory to the OS as possible because it’s in a low-memory, multi-tasking environment? What’s our criteria here?

It’s hard to say where to start with a vague question like that, other than to start asking your own questions to get clarification. After all, in order to design your own free function, you first have to know how malloc is implemented. So chances are, the question was really about whether or not you knew anything about how malloc can be implemented.

If you’re not familiar with the internals of memory management, the easiest way to get started with understanding how malloc is implemented is to first write your own.

Check out this IBM DeveloperWorks article called “Inside Memory Management” for starters.

But before you can write your own malloc/free, you first need memory to allocate/free. Unfortunately, in a protected mode OS, you can’t directly address the memory on the machine. So how do you get it?

You ask the OS for it. With the virtual memory features of the x86, any piece of RAM or swap memory can be mapped to a memory address by the OS. What your program sees as memory could be physically fragmented throughout the entire system, but thanks to the kernel’s virtual memory manager, it all looks the same.

The kernel usually provides system calls that allow you to map in additional memory for your process. On older UNIX OS’s this was usually brk/sbrk to grow heap memory onto the edge of your process or shrink it off, but a lot of systems also provide mmap/munmap to simply map a large block of heap memory in. It’s only once you have access to a large, contiguous looking block of memory that you need malloc/free to manage it.

Once your process has some heap memory available to it, it’s all about splitting it into chunks, with each chunk containing its own meta information about its size and position and whether or not it’s allocated, and then managing those chunks. A simple list of structs, each containing some fields for meta information and a large array of bytes, could work, in which case malloc has to run through the list until if finds a large enough unallocated chunk (or chunks it can combine), and then map in more memory if it can’t find a big enough chunk. Once you find a chunk, you just return a pointer to the data. free() can then use that pointer to reverse back a few bytes to the member fields that exist in the structure, which it can then modify (i.e. marking chunk.allocated = false;). If there’s enough unallocated chunks at the end of your list, you can even remove them from the list and unmap or shrink that memory off your process’s heap.

That’s a real simple method of implementing malloc though. As you can imagine, there’s a lot of possible ways of splitting your memory into chunks and then managing those chunks. There’s as many ways as there are data structures and algorithms. They’re all designed for different purposes too, like limiting fragmentation due to small, allocated chunks mixed with small, unallocated chunks, or ensuring that malloc and free run fast (or sometimes even more slowly, but predictably slowly). There’s dlmalloc, ptmalloc, jemalloc, Hoard’s malloc, and many more out there, and many of them are quite small and succinct, so don’t be afraid to read them. If I remember correctly, “The C Programming Language” by Kernighan and Ritchie even uses a simple malloc implementation as one of their examples.

You can’t blindly design free() without knowing how malloc() works under the hood because your implementation of free() would need to know how to manipulate the bookkeeping data and that’s impossible without knowing how malloc() is implemented.

So an unswerable question could be how you would design malloc() and free() instead which is not a trivial question but you could answer it partially for example by proposing some very simple implementation of a memory pool that would not be equivalent to malloc() of course but would indicate your presence of knowledge.

One common approach when you only have access to user space (generally known as memory pool) is to get a large chunk of memory from the OS on application start-up. Your malloc needs to check which areas of the right size of that pool are still free (through some data structure) and hand out pointers to that memory. Your free needs to mark the memory as free again in the data structure and possibly needs to check for fragmentation of the pool.

The benefits are that you can do allocation in nearly constant time, the drawback is that your application consumes more memory than actually is needed.

Tell me the steps how will you design your own free( ) function for deallocate the allocated memory.

#include <stdlib.h>
#undef free
#define free(X) my_free(X)

inline void my_free(void *ptr) { }

How can it be more efficient than C’s default free() function ?

It is extremely fast, requiring zero machine cycles. It also makes use-after-free bugs go away. It’s a very useful free function for use in programs which are instantiated as short-lived batch processes; it can usefully be deployed in some production situations.

What can you conclude ?

I really want this job, but in another company.

Memory usage patterns could be a factor. A default implementation of free can’t assume anything about how often you allocate/deallocate and what sizes you allocate when you do.

For example, if you frequently allocate and deallocate objects that are of similar size, you could gain speed, memory efficiency, and reduced fragmentation by using a memory pool.

EDIT: as sharptooth noted, only makes sense to design free and malloc together. So the first thing would be to figure out how malloc is implemented.

malloc and free only have a meaning if your app is to work on top of an OS. If you would like to write your own memory management functions you would have to know how to request the memory from that specific OS or you could reserve the heap memory right away using existing malloc and then use your own functions to distribute/redistribute the allocated memory through out your app

There is an architecture that malloc and free are supposed to adhere to — essentially a class architecture permitting different strategies to coexist. Then the version of free that is executed corresponds to the version of malloc used.

However, I’m not sure how often this architecture is observed.

The knowledge of working of malloc() is necessary to implement free(). You can find a implementation of malloc() and free() using the sbrk() system call in K&R The C Programming Language Chapter 8, Section 8.7 “Example–A Storage Allocator” pp.185-189.

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C – Design your own free( ) function