How does malloc work internally

In my last blog , I mentioned I was asked to look at a malloc performance issue, but discussed the methods for measuring performance. In this blog, I'll talk about the malloc issue itself, and some measures I took to address it.

I'll also talk a bit about how malloc's internals work, and how that affects your performance. First off, a bit of terminology - throughout this blog, these terms are defined to mean as follows In an old-school single-threaded application, when you malloc memory, a chunk is chosen or created from the heap back then, there was only one , and returned to the application. If you're lucky, they don't step on each others' toes, but So, malloc uses a lock to make the threads "take turns" accessing malloc's internal structures.

Still, taking turns means one thread is doing nothing for a while, which can't be good for performance, and these locks themselves can take a significant amount of time as they often need to synchronize access across CPU caches and sometimes across physical CPUs.

In The GNU C Library's glibc's malloc, this is partly addressed by allowing applications to have more than one arena from which memory can be allocated.

The natural implementation of a reference is indeed a pointer. However, do not depend on this in your code. First at all malloc aligns the pointers to 16 byte boundaries. Furthermore they store at least one pointer or allocated length in the addresses preceding the returned value. Then they probably add a magic value or release counter to indicate that the linked list is not broken or that the memory block has not been released twice free ASSERTS for double frees.

I think you're looking at the wrong layer. The logic in vfprintf is responsible for formatting its arguments and writing them through the underlying stdio functions, usually into a buffer in the FILE object it's targetting. It seems like the administration kept for memory allocation has been corrupted. This typically consists of a header that hold information on the size of the cell as well as a pointer to the next heap cell.

This makes a heap effectively a linked list. When one starts a process, the heap contains a single cell that contains all the heap space assigned on startup. This cell exists on the heap's free list.

When one calls malloc , memory is taken from the large heap cell, which is returned by malloc. The rest is formed into a new heap cell that consists of all the rest of the memory. When one frees memory, the heap cell is added to the end of the heap's free list. Subsequent malloc 's walk the free list looking for a cell of suitable size. As can be expected the heap can get fragmented and the heap manager may from time to time, try to merge adjacent heap cells.

When there is no memory left on the free list for a desired allocation, malloc calls brk or sbrk which are the system calls requesting more memory pages from the operating system. It's also important to realize that simply moving the program break pointer around with brk and sbrk doesn't actually allocate the memory, it just sets up the address space.

On Linux, for example, the memory will be "backed" by actual physical pages when that address range is accessed, which will result in a page fault, and will eventually lead to the kernel calling into the page allocator to get a backing page. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow.

Learn more. Create a free Team What is Teams? Learn more. How does glibc malloc work? Ask Question. Asked 4 years, 7 months ago. Active 1 year, 8 months ago. Viewed 9k times. Improve this question. Sherlock Holmes. Sherlock 1, 1 1 gold badge 19 19 silver badges 34 34 bronze badges. Add a comment. Active Oldest Votes. Sizes of free chunks are stored both in the front of each chunk and at the end. This makes consolidating fragmented chunks into bigger chunks very fast.

The size fields also hold bits representing whether chunks are free or in use. When additional threads are spawned, each thread receives its own arena up to a configurable limit, after which arenas are reused for multiple threads , and the chunks in these arenas have the A bit set. This makes it easier to deal with alignments etc but can be very confusing when trying to extend or adapt this code. Because they are allocated one-by-one, each must contain its own trailing size field.

If the M bit is set, the other bits are ignored because mmapped chunks are neither in an arena, nor adjacent to a freed chunk.