When writing in C, how can I tell how much stack space is available in memory when I launch a program? How about heap space?
How can I tell how much memory is being used during the execution of my program?
Stack and Heap Space in Modern Computers – C Guide
c++heap-memorymemory-managementstack-memory
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The stack is the memory set aside as scratch space for a thread of execution. When a function is called, a block is reserved on the top of the stack for local variables and some bookkeeping data. When that function returns, the block becomes unused and can be used the next time a function is called. 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. 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 heap is memory set aside for dynamic 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. 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.
Each 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).
To answer your questions directly:
To what extent are they controlled by the OS or language runtime?
The OS allocates the stack for each system-level thread when the thread is created. Typically the OS is called by the language runtime to allocate the heap for the application.
What is their scope?
The stack is attached to a thread, so when the thread exits the stack is reclaimed. The heap is typically allocated at application startup by the runtime, and is reclaimed when the application (technically process) exits.
What determines the size of each of them?
The size of the stack is set when a thread is created. 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).
What makes one faster?
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. Also, each byte in the stack tends to be reused very frequently which means it tends to be mapped to the processor's cache, making it very fast. Another performance hit for the heap is that the heap, being mostly a global resource, typically has to be multi-threading safe, i.e. each allocation and deallocation needs to be - typically - synchronized with "all" other heap accesses in the program.
A clear demonstration:
Image source: vikashazrati.wordpress.com
I'm not entirely sure what you're asking, but I'll try my best to answer.
The following declares a variable i on the stack:
int i;
When I ask for an address using &i I get the actual location on the stack.
When I allocate something dynamically using malloc, there are actually TWO pieces of data being stored. The dynamic memory is allocated on the heap, and the pointer itself is allocated on the stack. So in this code:
int* j = malloc(sizeof(int));
This is allocating space on the heap for an integer. It's also allocating space on the stack for a pointer (j). The variable j's value is set to the address returned by malloc.
Best Answer
This is all Win32-specific (not really C-specific, all just OS API):
When a thread is created, it gets 1MB stack space by default, by that can be modified in whatever CreateThread API you use.
You can peek into the thread information block to find the actual stack info, but even though this is documented, this technique isn't officially supported, see http://en.wikipedia.org/wiki/Win32_Thread_Information_Block .
Also, for a 32-bit application, you can only address up to 2GB, so for an app that by design uses lots of memory, then the thing to watch out for is the total size of the process' virtual address space (committed + reserved), which includes all heap allocations. You can programmatically access the process' virtual memory with the GlobalMemoryStatusEx API, look at the ullTotalVirtual param for virtual address space. Once your process gets close to 1.8 or 1.9GB of VAS, then heap allocations and VirtualAlloc calls begin to fail. For "normal" apps, you don't have to worry about running out of VAS, but it's always good to check for fail allocs. Also, you shouldn't get a stack overflow, unless you have a bug, or a bad design.