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

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