Unlike the stack, there are generally no restrictions on the size of the heap (or the variables it creates), other than the physical size of memory in the machine. Failure to free the memory when you are finished with it will result in what is known as a memory leak – memory that is still “being used”, and not available to other processes. This is memory that is not automatically managed – you have to explicitly allocate (using functions such as malloc), and deallocate (e.g. The heap is a large pool of memory that can be used dynamically – it is also known as the “free store”. The heap is the diametrical opposite of the stack. stack variables only exist whilst the function that created them exists.the stack grows and shrinks as variables are created and destroyed.the stack it not limitless – most have an upper bound.variables are allocated and freed automatically.the stack is managed by the CPU, there is no ability to modify it.Stack overflow also occurs in situations where recursion is incorrectly used. Stack overflow happens when all the memory in the stack has been allocated, and further allocations begin overflowing into other sections of memory. If a program tries to put too much information on the stack, stack overflow will occur. Note that there is generally a limit on the size of the stack – which can vary with the operating system (for example OSX currently has a default stack size of 8MB). Stack memory is divided into successive frames where each time a function is called, it allocates itself a fresh stack frame. The stack is a special region of memory, and automatically managed by the CPU – so you don’t have to allocate or deallocate memory. This leads to the “local” scope of function variables. Then when a function finishes running, all the variables associated with that function on the stack are deleted, and the memory they use is freed up. Every time a function declares a new variable it is “pushed” onto the stack. It’s a LIFO, “ L ast- I n,- F irst- O ut”, structure. The stack is used to store variables used on the inside of a function (including the main() function). For example, the same variable created inside a function using the static clause would allow it to be stored in static memory. It is also possible to force a variable to be static using the static clause. Inside a function the variable is allocated on the stack. Global variables are static, and there is only one copy for the entire program. If a variable is declared outside of a function, it is considered global, meaning it is accessible anywhere in the program. This memory can come from one of two places. On many systems this variable uses 4 bytes of memory. Static memory persists throughout the entire life of the program, and is usually used to store things like global variables, or variables created with the static clause. – heap: dynamic storage (large pool of memory, not allocated in contiguous order). – stack: local variable storage (automatic, continuous memory). – static: global variable storage, permanent for the entire run of the program. The great thing about C is that it is so intertwined with memory – and by that I mean that the programmer has quite a good understanding of “ what goes where“. C has three different pools of memory. Recursion – The Knight’s Tour (N.Wirth).
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