Data segment

The code segment, also known as text segment, contains executable code and is generally read-only and fixed size.

This shows the typical layout of a simple computer's program memory with the text, various data, and stack and heap sections.

The data segment contains initialized static variables, i.e. global variables and local static variables which have a defined value and can be modified. Examples in C include:

int i = 3;
char a[] = "Hello World";
static int b = 2023;    // Initialized static global variable
void foo (void) {
  static int c = 2023; // Initialized static local variable
}

The BSS segment contains uninitialized static data, both variables and constants, i.e. global variables and local static variables that are initialized to zero or do not have explicit initialization in source code. Examples in C include:

static int i;
static char a[12];

The heap segment contains dynamically allocated memory, commonly begins at the end of the BSS segment and grows to larger addresses from there. It is managed by malloc, calloc, realloc, and free, which may use the brk and sbrk system calls to adjust its size (note that the use of brk/sbrk and a single heap segment is not required to fulfill the contract of malloc/calloc/realloc/free; they may also be implemented using mmap/munmap to reserve/unreserve potentially non-contiguous regions of virtual memory into the process' virtual address space). The heap segment is shared by all threads, shared libraries, and dynamically loaded modules in a process.

The stack segment contains the call stack, a LIFO structure, typically located in the higher parts of memory. A "stack pointer" register tracks the top of the stack; it is adjusted each time a value is "pushed" onto the stack. The set of values pushed for one function call is termed a "stack frame". A stack frame consists at minimum of a return address. Automatic variables are also allocated on the stack.

The stack segment traditionally adjoined the heap segment and they grew towards each other; when the stack pointer met the heap pointer, free memory was exhausted. With large address spaces and virtual memory techniques they tend to be placed more freely, but they still typically grow in a converging direction. On the standard PC x86 architecture the stack grows toward address zero, meaning that more recent items, deeper in the call chain, are at numerically lower addresses and closer to the heap. On some other architectures it grows the opposite direction.