Stack & Heap - 栈与堆

因为从事的一直都不是底层代码开发工作,所以一直都搞不清楚堆栈的区别,忍无可忍之下,今天查了下stackoverflow。

简单来说,栈(stack),就是那些由编译器在需要的时候分配,在不需要的时候自动清除的变量存储区。里面的变量通常是局部变量、函数参数等。堆(heap),就是那些由new分配的内存块,他们的释放编译器不去管,由我们的应用程序去控制(java中则会自动由GC回收),一般一个new就要对应一个delete。如果程序员没有释放掉,那么在程序结束后,操作系统会自动回收。

栈(stack)的进出关系为:后进先出 LIFO (Last-In/First-Out) 。堆无所谓进出关系,都可以。

摘自stackoverflow

stackoverflow >> What and where are the stack and heap?

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 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 free. 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.

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参阅自另一个comment:stackoverflow >> What and where are the stack and heap?

Stack:

  • Stored in computer RAM just like the heap.
  • Variables created on the stack will go out of scope and automatically deallocate.
  • Much faster to allocate in comparison to variables on the heap.
  • Implemented with an actual stack data structure.
  • Stores local data, return addresses, used for parameter passing
  • Can have a stack overflow when too much of the stack is used. (mostly from inifinite (or too much) recursion, very large allocations)
  • Data created on the stack can be used without pointers.
  • You would use the stack if you know exactly how much data you need to allocate before compile time and it is not too big.
  • Usually has a maximum size already determined when your program starts

Heap:

  • Stored in computer RAM just like the stack.
  • Variables on the heap must be destroyed manually and never fall out of scope. The data is freed with delete, delete[] or free
  • Slower to allocate in comparison to variables on the stack.
  • Used on demand to allocate a block of data for use by the program.
  • Can have fragmentation when there are a lot of allocations and deallocations
  • In C++ data created on the heap will be pointed to by pointers and allocated with new or malloc
  • Can have allocation failures if too big of a buffer is requested to be allocated.
  • You would use the heap if you don't know exactly how much data you will need at runtime or if you need to allocate a lot of data.
  • Responsible for memory leaks