Stack-based memory allocation

For other uses, see Stack (disambiguation).

A typical stack, storing local data and call information for nested procedure calls (not necessarily nested procedures). This stack grows downward from its origin. The stack pointer points to the current topmost datum on the stack. A push operation decrements the pointer and copies the data to the stack; a pop operation copies data from the stack and then increments the pointer. Each procedure called in the program stores procedure return information (in yellow) and local data (in other colors) by pushing them onto the stack.

Stacks in computing architectures are regions of memory where data is added or removed in a last-in-first-out (LIFO) manner.

In most modern computer systems, each thread has a reserved region of memory referred to as its stack. When a function executes, it may add some of its state data to the top of the stack; when the function exits it is responsible for removing that data from the stack. At a minimum, a thread's stack is used to store the location of function calls in order to allow return statements to return to the correct location, but programmers may further choose to explicitly use the stack. If a region of memory lies on the thread's stack, that memory is said to have been allocated on the stack, i.e. stack-based memory allocation.

Advantages and disadvantages

Because the data is added and removed in a last-in-first-out manner, stack-based memory allocation is very simple and typically faster than heap-based memory allocation (also known as dynamic memory allocation). Another feature is that memory on the stack is automatically, and very efficiently, reclaimed when the function exits, which can be convenient for the programmer if the data is no longer required. If however, the data needs to be kept in some form, then it must be copied from the stack before the function exits. Therefore, stack based allocation is suitable for temporary data or data which is no longer required after the creating function exits.

In addition, a thread's assigned stack size can be as small as only a few bytes on some small CPUs. Allocating more memory on the stack than is available can result in a crash due to stack overflow.

System interface

Many Unix-like systems as well as Microsoft Windows implement a function called alloca for dynamically allocating stack memory in a way similar to the heap-based malloc. A compiler typically translates it to inlined instructions manipulating the stack pointer. Although there is no need of freeing, there exists a risk of overflow. And since alloca is a ad hoc expansion seen in many systems but never in POSIX or the C standard, its behavior in case of a stack overflow is undefined.

A safer version of alloca called _malloca, which reports errors, exists on Microsoft Windows. It requires the use of _freea. gnulib provides an equivalent interface, albeit instead of throwing an SEH exception on overflow, it delegates to malloc when an overlarge size is detected. A similar feature can be emulated using manual accounting and size-checking, such as in the uses of alloca_account in glibc.

Some processor families, such as the x86, have special instructions for manipulating the stack of the currently executing thread. Other processor families, including PowerPC and MIPS, do not have explicit stack support, but instead rely on convention and delegate stack management to the operating system's application binary interface (ABI).

See also

• Automatic variable
• Static variable
• Call stack
• Dynamic memory allocation
• Stack buffer overflow
• Stack machine
• Stack overflow