8.4) Memory Layout of C programs

The memory layout of C programs refers to how a C program’s data and code are organized in a computer’s memory. Understanding this layout is essential for efficient memory management and optimization. A typical memory layout includes:

Text Segment (Code Segment)

This segment holds the executable code of the program. It’s usually read-only and contains the compiled instructions that the CPU executes. The text segment is usually shared among different processes that are executing the same program, saving memory.

Initialized Data Segment (Data Segment)

This segment stores global and static variables that are explicitly initialized by the programmer. These variables have fixed memory addresses and are stored in read-write memory. Global variables are available throughout the program, while static variables have local scope within their function or file.

Uninitialized Data Segment (BSS Segment)

This segment contains global and static variables that are not explicitly initialized by the programmer. These variables are initialized to zero or a null pointer, depending on their data type. Since these variables start with default values, they are not explicitly stored in the executable file, which helps save space.

Heap

The heap is a region of memory used for dynamic memory allocation. It’s managed by functions like malloc() and free(). Memory allocated in the heap must be explicitly deallocated to prevent memory leaks. The heap’s size can change during program execution, making it flexible for runtime memory requirements.

Stack

The stack is a region of memory used for managing function calls and local variables. Each function call creates a new stack frame, which includes space for function arguments, local variables, and the return address. When a function returns, its stack frame is removed. Stack memory is managed automatically by the compiler.

Environment Variables and Command Line Arguments

This segment holds the command line arguments passed to the program and the environment variables. Command line arguments provide input to the program, and environment variables store configuration data.

Here’s a simplified visual representation of the memory layout:

+-----------------------------+
|        Text Segment         |
|   (Executable Instructions) |
+-----------------------------+
|                             |
| Initialized Data Segment    |
|    (Global & Static Vars)   |
+-----------------------------+
|                             |
| Uninitialized Data Segment  |
|    (BSS Section)            |
+-----------------------------+
|                             |
|           Heap              |
|  (Dynamic Memory Allocation)|
+-----------------------------+
|                             |
|           Stack             |
| (Function Call & Local Vars)|
+-----------------------------+
|                             |
| Environment Variables &     |
|  Command Line Arguments     |
+-----------------------------+

This visual representation provides a more detailed view of each segment’s purpose and contents within the memory layout of a C program:

1. Text Segment:
   • Contains the compiled executable instructions of the program.
   • Often read-only to prevent modification.
   • Holds the actual code that the CPU fetches and executes.
2. Initialized Data Segment:
   • Stores initialized global and static variables.
   • Variables have fixed memory addresses.
   • Variables hold initial values set by the programmer.
3. Uninitialized Data Segment (BSS Segment):
   • Stores uninitialized global and static variables.
   • Variables are initialized to default values (zero or null).
   • Helps save space by not storing default values in the executable.
4. Heap:
   • Used for dynamic memory allocation.
   • Managed by functions like malloc(), calloc(), and realloc().
   • Memory allocation and deallocation occur during program execution.
5. Stack:
   • Manages function calls and local variables.
   • Each function call creates a new stack frame.
   • Follows the Last-In-First-Out (LIFO) principle.
   • Automatic memory management by the compiler.
6. Environment Variables and Command Line Arguments:
   • Holds command line arguments and environment variables.
   • Command line arguments provide input to the program.
   • Environment variables store configuration settings.

• Constants and string literals are often stored in the text segment.
• Function code resides in the text segment and is read-only.
• The size of the heap can change dynamically during program execution.
• The stack typically has a fixed size per thread, determined by the system or compiler.
• Global and static variables can be accessed from multiple parts of the program, leading to potential issues if not managed carefully.

Understanding the memory layout is crucial for optimizing memory usage, avoiding memory-related errors, and ensuring proper memory management. It helps programmers make informed decisions about where to allocate memory, how to manage variables, and how to efficiently structure their programs.