We are going to implement malloc(), calloc(), realloc() and free(). This is a newbie degree article, so I cannot spell out each detail. This memory allocator will not be quick and efficient, we is not going to regulate allotted memory to align to a web page boundary, but we will build a Memory Wave Workshop allocator that works. If you wish to have a look at the code in full, take a look at my github repo memalloc. Earlier than we get into constructing the memory allocator, you need to be accustomed to the memory structure of a program. A process runs inside its personal digital deal with house that’s distinct from the digital tackle areas of other processes. As you can see within the picture, the stack and Memory Wave the heap develop in the other instructions. That is, brk factors to the top of the heap. Now if we wish to allocate extra memory within the heap, we need to request the system to increment brk.

Equally, to release memory we need to request the system to decrement brk. Assuming we run Linux (or a Unix-like system), we can make use of sbrk() system name that lets us manipulate the program break. Calling sbrk(0) provides the current address of program break. Calling sbrk(x) with a optimistic value increments brk by x bytes, because of this allocating memory. Calling sbrk(-x) with a detrimental value decrements brk by x bytes, because of this releasing memory. To be honest, sbrk() is just not our greatest buddy in 2015. There are better alternatives like mmap() out there right this moment. It could can only develop or shrink in LIFO order. Nonetheless, the glibc implementation of malloc still makes use of sbrk() for allocating memory that’s not too big in size. So, we'll go forward with sbrk() for our easy memory allocator. The malloc(dimension) perform allocates measurement bytes of memory and returns a pointer to the allotted memory. In the above code, we call sbrk() with the given dimension.

On success, size bytes are allocated on the heap. That was straightforward. Wasn’t it? The tricky part is freeing this memory. The free(ptr) operate frees the memory block pointed to by ptr, which will need to have been returned by a previous call to malloc(), calloc() or realloc(). However to free a block of memory, the primary order of enterprise is to know the scale of the memory block to be freed. In the present scheme of things, this is not attainable as the dimensions info is not stored wherever. So, we will have to find a solution to store the scale of an allocated block someplace. Furthermore, we need to know that the heap memory the operating system has supplied is contiguous. So we can solely release memory which is at the top of the heap. We can’t release a block of memory in the middle to the OS. Imagine your heap to be something like an extended loaf of bread that you may stretch and shrink at one end, however you've gotten to maintain it in a single piece.

To address this problem of not being able to release memory that’s not at the tip of the heap, we'll make a distinction between freeing memory and releasing memory. From now on, freeing a block of memory does not essentially imply we release memory back to OS. It just means that we keep the block marked as free. This block marked as free could also be reused on a later malloc() name. Since memory not at the end of the heap can’t be launched, this is the one approach forward for us. 2. Whether or not a block is free or not-free? To store this information, we will add a header to every newly allocated memory block. The thought is straightforward. We use this memory house returned by sbrk() to slot in both the header and the precise memory block. The header is internally managed, and is kept utterly hidden from the calling program. We can’t be fully positive the blocks of memory allotted by our malloc is contiguous.

Think about the calling program has a foreign sbrk(), or there’s a piece of memory mmap()ed in between our memory blocks. We additionally need a option to traverse by means of our blocks for memory (why traverse? we are going to get to know when we look on the implementation of free()). So to keep track of the memory allotted by our malloc, we'll put them in a linked record. Now, let’s wrap your complete header struct in a union along with a stub variable of dimension 16 bytes. This makes the header end up on a memory address aligned to sixteen bytes. Recall that the size of a union is the bigger measurement of its members. So the union ensures that the tip of the header is memory aligned. The tip of the header is where the actual memory block begins and due to this fact the memory provided to the caller by the allocator can be aligned to 16 bytes.

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Pub: 09 Aug 2025 01:49 UTC

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