The stack is faster because the access pattern makes it trivial to allocate and deallocate memory from it (a pointer/integer is simply incremented or decremented), while the heap has much more complex bookkeeping involved in an allocation or deallocation. The size of the heap is set on application startup, but can grow as space is needed (the allocator requests more memory from the operating system). The size of the stack is set when a thread is created. What determines the size of each of them? The heap is typically allocated at application startup by the runtime, and is reclaimed when the application (technically process) exits. The stack is attached to a thread, so when the thread exits the stack is reclaimed.
Typically the OS is called by the language runtime to allocate the heap for the application. The OS allocates the stack for each system-level thread when the thread is created. To what extent are they controlled by the OS or language runtime?
#Stack vs heap persistent free#
This makes it much more complex to keep track of which parts of the heap are allocated or free at any given time there are many custom heap allocators available to tune heap performance for different usage patterns.Įach thread gets a stack, while there's typically only one heap for the application (although it isn't uncommon to have multiple heaps for different types of allocation). Unlike the stack, there's no enforced pattern to the allocation and deallocation of blocks from the heap you can allocate a block at any time and free it at any time. The heap is memory set aside for dynamic allocation. This makes it really simple to keep track of the stack freeing a block from the stack is nothing more than adjusting one pointer. The stack is always reserved in a LIFO (last in first out) order the most recently reserved block is always the next block to be freed. When that function returns, the block becomes unused and can be used the next time a function is called. When a function is called, a block is reserved on the top of the stack for local variables and some bookkeeping data. The stack is the memory set aside as scratch space for a thread of execution.
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