Simple memory allocation examples

chp9/README.md

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+# Chapter 9 - Memory Management
+
+This document summarizes the key concepts from **Chapter 9: Memory Management** in *Linux System Programming, 2nd Edition* by Robert Love.
+
+---
+
+## Overview
+
+Memory management in Linux involves allocating, using, and freeing memory efficiently while avoiding leaks and fragmentation. This chapter explains how user-space programs interact with the kernel’s memory manager through system calls, library functions, and advanced allocation techniques.
+
+---
+
+## Key Topics
+
+### 1. Memory Layout of a Process
+
+A typical Linux process memory layout includes:
+
+- **Text Segment** – Executable code of the program (read-only).
+- **Data Segment** – Initialized global and static variables.
+- **BSS Segment** – Uninitialized global and static variables.
+- **Heap** – Dynamically allocated memory (via `malloc`, `calloc`, `realloc`).
+- **Stack** – Function call frames, local variables.
+- **Memory Mappings** – Shared libraries, `mmap` allocations, etc.
+
+---
+
+### 2. Dynamic Memory Allocation
+
+- **malloc(size_t size)** – Allocates memory but leaves it uninitialized.
+- **calloc(size_t nmemb, size_t size)** – Allocates and zeroes memory.
+- **realloc(void *ptr, size_t size)** – Resizes a previously allocated block.
+- **free(void *ptr)** – Frees allocated memory.
+
+#### Common pitfalls
+
+- **Memory leaks** – Forgetting to free memory.
+- **Double free** – Calling `free` twice on the same pointer.
+- **Use-after-free** – Accessing memory after it has been freed.
+
+---
+
+### 3. `brk` and `sbrk`
+
+- Low-level system calls to manage the program break (end of the data segment).
+- Rarely used directly; `malloc` and friends handle these internally.
+
+---
+
+### 4. Memory Mapping with `mmap`
+
+- **mmap()** maps files or anonymous memory into the process address space.
+- Advantages:
+  - Direct file access without extra copy operations.
+  - Efficient large memory allocations.
+- Common use cases:
+  - Loading large files.
+  - Shared memory between processes.
+- Paired with `munmap()` to release mappings.
+
+---
+
+### 5. Memory Locking
+
+- **mlock() / mlockall()** – Lock memory pages into RAM to avoid swapping.
+- Useful for:
+  - Real-time applications.
+  - Security-sensitive data (e.g., cryptographic keys).
+
+---
+
+### 6. Advanced Memory Techniques
+
+- **mprotect()** – Change protection (read, write, execute) of memory regions.
+- **shm_open() / shm_unlink()** – POSIX shared memory objects.
+- **posix_memalign()** – Allocate memory aligned to a specified boundary.
+
+---
+
+### 7. Debugging Memory Issues
+
+- Tools like **valgrind** and **AddressSanitizer** help detect:
+  - Memory leaks.
+  - Invalid reads/writes.
+  - Use-after-free errors.
+- Good practices:
+  - Always initialize pointers.
+  - Free resources in reverse order of allocation.
+
+---
+
+## Key Takeaways
+
+- Understand the process memory layout to avoid common pitfalls.
+- Use the right allocation function for your needs.
+- Prefer `mmap` for large or shared allocations.
+- Memory debugging tools are essential for writing reliable code.
+
+---
+
+## References
+
+- *Linux System Programming, 2nd Edition* – Robert Love, O’Reilly Media.
+- `man malloc`, `man mmap`, `man mprotect`, `man mlock`

chp9/malloc.c

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+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+void *xmalloc(size_t size);
+
+int main(void)
+{
+	char *str = xmalloc(1024);
+	char *src = "HelloWorld";
+	for (int i = 0; i < (int)strlen(src); i++)
+		str[i] = src[i];
+	printf("%s\n", str);
+
+	int *numbers = calloc(10, sizeof(int));
+	for (int i = 0; i < 10; i++) {
+		numbers[i] = i + 1;
+		printf("%d ", numbers[i]);
+	}
+
+	printf("\n");
+
+	int *p = realloc(numbers, 2 * sizeof(int));
+	if (!p) {
+		perror("realloc");
+		exit(1);
+	}
+
+	for (int i = 0; i < 20; i++) {
+		printf("%d ", p[i]);
+	}
+	return EXIT_SUCCESS;
+}
+
+void *xmalloc(size_t size)
+{
+	void *p;
+	p = malloc(size);
+	if (!p) {
+		perror("xmalloc");
+		exit(1);
+	}
+	return p;
+}