Stack Memory vs Heap Memory
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What and where are the stack and heap
I am programming in C++ and I am always wondering what exactly is stack memory vs heap memory. All I know is when I call new, I would get memory from heap. If if create local variables, I would get memory from stack. After some research on internet, the most common answer is stack memory is temporary and heap memory is permanent.
Is stack and heap memory model a concept of operating system or computer architecture? So some of it might not follow stack and heap memory model or all of them follow it?
Stack and heap memory is the abstraction over the memory model of the virtual memory ( which might swap memory between disk and RAM). So both stack and heap memory physically might be RAM or the disk? Then what is the reason where heap allocation seems to be slower than the stack counterpart?
Also, the main program would be run in the stack or a heap?
Also, what would happen if a process run out of the stack memory or heap memory allocated?
Thanks
Stack memory is specifically the range of memory that is accessible via the Stack register of the CPU. The Stack was used as a way to implement the "Jump-Subroutine"-"Return" code pattern in assembly language, and also as a means to implement hardware-level interrupt handling. For instance, during an interrupt, the Stack was used to store various CPU registers, including Status (which indicates the results of an operation) and Program Counter (where was the CPU in the program when the interrupt occurred).
Stack memory is very much the consequence of usual CPU design. The speed of its allocation/deallocation is fast because it is strictly a last-in/first-out design. It is a simple matter of a move operation and a decrement/increment operation on the Stack register.
Heap memory was simply the memory that was left over after the program was loaded and the Stack memory was allocated. It may (or may not) include global variable space (it's a matter of convention).
Modern pre-emptive multitasking OS's with virtual memory and memory-mapped devices make the actual situation more complicated, but that's Stack vs Heap in a nutshell.
In C++ the stack memory is where local variables get stored/constructed. The stack is also used to hold parameters passed to functions.
The stack is like a very like the std::stack class, you push parameters onto it and then call a function. The function then knows the parameters it expects can be found on the end of the stack. Likewise the function can push locals onto the stack and pop them off it before returning from the function. (caveat- compiler optimizations and calling conventions all mean things aren't this simple)
The stack is really best understood from a low level and I'd recommend this link Art of Assembly - Passing Parameters on the Stack. Rarely if ever would you consider any sort of manual stack manipulation from C++.
Generally speaking the stack is preferred as it is usually in the CPU cache, so operations involving objects stored on it tend to be faster. However the stack is a limited resource, and shouldn't be used for anything large. Running out of stack memory is called a Stack buffer overflow. It's a serious thing to encounter, but you really shouldn't come across one unless you have a crazy recursive function or something similar.
Heap memory is much as rskar says. Generally speaking in C++ objects allocated with new, or blocks of memory allocated with the likes of malloc ends up on the heap. Heap memory almost always must be manually freed, though you should reallly use a smart pointer class or similar to avoid needing to remember to do so. Running out of heap memory can (will?) result in a std::bad_alloc.
It's a language abstraction - some languages have both, some one, some neither.
In the case of C++, the code is not run in either the stack or the heap. You can test what happens if you run out of heap memory by repeatingly calling new to allocate memory in a loop without calling delete to free it it. But make a system backup before doing this .
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