🛋️ memory

what's

• memory: an array of bytes within RAM
• memory block: a single unit (byte) within memory, used to hold some value
• memory address: the address of where a memory block is allocated

memory layout in C

It's organized in the following fashion

• empty space -> high address | -> command-line arguments and environment variables
• stack segment
• heap segment
• uninitialized data segments (bbs)| -> initialized to 0 by execution
• initialized data segments | -> read from program file by execution
• text/code segment -> low address | -> read from program file by execution

text/code segment

• contains machine code of the compiled program
• shareable so that only a single copy needs to be in memory
• often read-only, to prevent programs from accidentally modifying its instructions

initialized data segment

• stores all initialized global and static variables

uninitialized data segment

• often called the bbs (block started by symbol)
• stores all uninitialized global and static variables
• data is initialized by the kernel to arithmetic 0 before the program starts executing
  • int,float,double => 0
  • pointer => null
  • char => the null character('0' or '\0' or simply NULL)

heap segment

• free memory available for programmers to use
• where dynamic memory allocation usually takes place
• managed by malloc, realloc, and free
• the implementation of the heap varies
• called dynamic memory
• using the heap is called dynamic memory allocation
• has nothing to do with the heap data structure unlike the stack segment which is an implementation of the stack abstract data type
• it's not an implementation of the heap data structure unlike the stack segment

stack segment

• holds
  • local(automatic) variables
  • temporary information
  • function parameters
  • return address
• grows downward
  • items deeper in the call chain are ate numerically lower addresses and toward the heap
• when the stack pointer meet the heap pointer, free memory is exhausted and results in a stack overflow error
• this is how the recursive functions in c works, each time a recursive function calls itself, a new stack frame is used, so one set of variables doesn't interfere with the variables from another instance of the function
• the size of the stack is will be decided by the operating system and the compiler

empty space

• reserved to command-line arguments and environment variables

example

s[] = "helloworld" // initialized data segment
char *s = "helloworld" // initialized data segment
static int i; // uninitialized data segment
static in i = 10; // initialized data segment
int i; // uninitialized data segment
int i = 10; // initialized data segment
a()->b()->a()->b() // stack segment
malloc() // heap segment

types of memory allocations

• static memory allocation
• dynamic memory allocation

static memory allocation

• memory allocated during compile time
• fixed and cannot be increased or decreased during run time
• it happens in the stack segment

dynamic memory allocation

• memory allocated at run time
• it takes place in the heap segment
• pointer play an important role in dynamic memory allocation
• allocated memory can only be accessed through pointers
• built in functions to allocate or deallocate memory in the heap
  • using c
    • malloc calloc realloc free
  • using c++
    • malloc calloc realloc free new delete

heap vs stack segments

• stack segment is where memory is allocated or deallocated in a defined order
• heap segment is where memory is allocated or deallocated randomly without any order

notes

• you can't control any segment of the memory layout except the heap segment