Where in Memory (Stack or Heap) Are C Global Variables Allocated?

Understanding Memory Allocation for Global Variables: Stack vs. Heap in C

In C , when a data structure is declared globally, the issue of memory allocation arises, whether it resides in the stack or the heap. To answer this question, it's important to understand the key differences between these two memory areas.

Stack vs. Heap Memory

• Stack: The stack is a structured memory region where variables and function parameters are allocated and deallocated dynamically. Stack memory is allocated on a last-in-first-out (LIFO) basis, meaning that the most recently allocated data is stored on top.
• Heap: The heap is an unordered memory region where objects are dynamically allocated using special operators like new and delete. Allocations on the heap are not subject to any specific order.

Determining the Memory Allocation for Global Variables

Whether a global data structure in C is allocated in the stack or heap depends on its properties:

• Static Global Variables: Global variables with static storage duration are stored in the initialized data segment, which is part of the data segment of the executable. This memory allocation is essentially fixed and immutable throughout the program's execution.
• Automatic Global Variables: Global variables with automatic storage duration are allocated on the stack. They are created when the program starts and are destroyed when the program exits. Unlike static global variables, automatic global variables can be reassigned during execution.

In the provided code example:

The array arr is declared as a global automatic variable. Since it is not static, it will be allocated on the stack, which typically has a fixed size limit. The size of the array is quite large (59,652,323 elements), so it is likely to exceed the available stack space and cause a runtime error.

Additional Considerations

It's important to note that global variables, regardless of their allocation location, have a longer lifetime than local variables within functions. They exist throughout the program's execution unless explicitly deallocated.

For memory management in C , it's critical to optimize resource utilization by considering the appropriate storage location for data based on its lifetime and access patterns.

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