Where C Variables are Stored in Memory

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Memory sections

  • stack - the portion of memory that stores variables that were declared inside functions (i.e. stores local variables and constants)
    • local variables are freed when the function returns (these variables cannot be accessed once the function ends)
    • Stack memory grows downward
  • heap - the portion of memory that stores things that were created by malloc()
    • Data on the heap persists across functions (unlike the local variables of stack memory)
    • Heap memory grows upward
  • static - the portion of memory that stores "pre-allocated" variables including static and global variables (variables declared outside functions), constants, and string literals 
    • Two sections of static memory: 
      1. read-only (stores variables that can only be read and not modified)
      2. read-write (stores variables that can be both read and written to (modified))
    • Static memory does not grow or shrink
  • code (text) - the portion of memory that stores the executable instructions and constants
    • Code memory is loaded when the program starts
    • Code memory does not grow or shrink 

Where data of a program can be stored in memory

  • Static variables: Static memory 
  • Local variables: Stack memory 
  • Global variables: Static memory 
    • Constants: Code, Static, and Stack memory 
    • Constants in Code: #define CONSTANT (~value~) 
    • Constants in Static memory: declare const (~variable~) = (~value~); outside of all functions 
    • Constants in Stack memory: declare const (~variable~) = (~variable~); inside a function 
  • Machine instructions: Code memory 
  • Result of malloc: Heap memory 
  • String literals/constants: Static memory

Where C strings are stored in memory

  • Pointer initialization of a string will be stored in static memory (a.k.a. string literals)
    • Example: char * s = "Hello"
      • "Hello" is a string stored in static memory
      • Pointer s points to that string stored in static memory
      • The section of memory that variable s is stored in depends on where/how it was declared (e.g. if s was declared in a function, s is in stack memory)
      • A string literal is immutable (the string is stored in a read-only block in static memory)
  • Array initialization of the string will be stored in stack memory
    • Example: char s[6] = "Hello"
      • Array s is put onto the stack, and the characters "h" "e" "l" "l" "o" are inserted into the location on the stack where the array is 
      • Both s and s[0] are in stack memory

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