Memory Layout of C Programs

In C programs, the memory layout is organized into several regions, each with a specific purpose. Here’s an overview of the memory layout of a typical C program:

1. Text Segment (Code Segment)

• Purpose: This region stores the compiled code of the program. It contains the machine instructions that are executed by the CPU.
• Characteristics:
  • It is typically read-only to prevent modification of the code during runtime.
  • It is usually the smallest segment in the program.

2. Data Segment

• Purpose: The data segment contains global variables, static variables, and constants that are initialized by the programmer.
• Subsections:
  • Initialized Data Segment: Contains variables that are explicitly initialized by the programmer.
  • Uninitialized Data Segment (BSS): Contains global and static variables that are declared but not explicitly initialized. They are initialized to zero by default when the program starts.

3. Heap

• Purpose: The heap is used for dynamic memory allocation. Memory is allocated here at runtime using functions like malloc(), calloc(), realloc(), and freed using free().
• Characteristics:
  • It grows towards higher memory addresses.
  • Memory in the heap is managed manually by the programmer.

4. Stack

• Purpose: The stack is used for local variables, function parameters, return addresses, and control flow. It supports function calls and function returns.
• Characteristics:
  • It grows towards lower memory addresses.
  • Memory is managed automatically (i.e., it is automatically freed when the function that created the local variables returns).
  • The stack size is usually limited, and stack overflows can occur if too much memory is used.

5. Registers

• Purpose: Registers are used by the CPU for performing operations. These are very fast, but the number of registers is limited.
• Characteristics:
  • They store values such as temporary data or intermediate calculations.
  • They are not part of the memory layout visible to the programmer, but they play a key role in program execution.

Typical Memory Layout from Low to High Memory:

-----------------------------------------
|             Stack                     |
-----------------------------------------
|            Unused Space               |
-----------------------------------------
|             Heap                      |
-----------------------------------------
|             BSS (Uninitialized Data)  |
-----------------------------------------
|             Data (Initialized Data)   |
-----------------------------------------
|             Text (Code)               |
-----------------------------------------

Key Points:

• The stack and heap grow towards each other in memory, but the stack typically grows downward (toward lower memory), and the heap grows upward (toward higher memory).
• The text segment is typically read-only, and data segments store global and static variables.
• The heap provides dynamic memory allocation but needs careful management to avoid memory leaks.
• The stack is used for function calls and local variables, and is automatically managed, though it can overflow if too much memory is allocated.