Memory Layout of C Programs

When a C program gets executed, it utilizes system memory to store variables, functions, and instructions. It's critical for a programmer to understand the different areas of memory that a program accesses as this significantly impacts performance and security. This tutorial chapter will focus on examining the memory layout of a C program to provide you with a deeper understanding of the inner workings of memory management in C.

The following are the primary components of the memory layout in a C program: Text Segment, Initialized Data Segment, Uninitialized Data Segment, Stack, and Heap.

Text Segment

Also known as the Code Segment, this is the area where the compiled code resides. The program instructions lie in the text segment.

#include <stdio.h>
int main() {
    printf("Hello, World!");
    return 0;
}

The above lines of code will reside in the text segment of the memory layout. The code segment is read-only and attempting to modify it typically causes a segmentation fault error.

Initialized Data Segment

This segment of memory is used for global, static, and constant data. It also includes external variables initialized by the user. The size of this segment is determined by the size of the values in the program's source code.

int max_count = 100; // This will go into the initialized data segment

In the above example, max_count is a global variable that is initialized to 100, so it is stored in the initialized data segment.

Uninitialized Data Segment

This segment, also known as the BSS (Block Started by Symbol), contains uninitialized global and static variables. These variables are automatically initialized to zero.

int count; // This will go into the uninitialized data segment

In the above example, count is a global variable that has not been initialized, resulting in its allocation into the uninitialized data segment.

Stack

The stack is used for non-static local variables and for information that will be used after the function returns. As functions are called, frames are added (pushed) to the top of the stack. When functions complete execution, their frames are removed (popped) from the stack.

void myFunction() {
    int x = 10;  // This will go into the stack
}

In the above example, x is a local variable in myFunction, and it is allocated on the stack.

Heap

Heap memory is used for dynamic memory allocation during the execution of a program, accomplished with functions like malloc() and calloc() in C. The heap is shared between threads. It's important to manage your heap usage carefully, as errors can lead to problems such as memory leaks or program crashes.

int *dynamicVar = malloc(sizeof(int));

dynamicVar is a pointer to an integer that is created dynamically using the malloc statement. The memory for this variable is allocated in the heap.

Understanding this memory layout helps us write efficient programs and also assists in debugging. By managing memory correctly, programmers can build faster, safer, and more robust applications.