Memory

Linux devices often show behavior that looks odd at first: a large file is much faster to read the second time; write() returns quickly after writing data; free shows less available memory even though the system is not really leaking memory.

The same mechanism is often behind all of these observations: Page Cache.

Page Cache is the kernel layer that caches file contents in memory. It prevents file reads and writes from touching much slower storage on every operation. On reads, the kernel can check the cache first. On writes, the kernel can update cached pages first and write them back to storage later.

When a program crashes, logs may mention stack overflow, segmentation fault, out of memory, or heap corruption. These are all memory-related, but they are not the same kind of failure.

Stack, heap, globals, text, and mmap regions are not merely “different memory blocks.” They serve different lifetimes, access patterns, and runtime constraints.

A useful first model is: text stores instructions, data stores global state, the stack stores function calls and local execution state, the heap stores dynamically allocated objects, and mmap regions store file mappings, shared memory, large anonymous mappings, and dynamic libraries.

When a program prints a pointer, it looks like it has obtained “a memory address.” Many wrong assumptions start there.

On systems such as Linux, the address seen by a user program is usually not a physical memory address. It is a virtual address. The same 0x400000 in two different processes can point to completely different physical pages. A pointer from one process is not directly meaningful in another process.