Memory in C – an example

So what actually happens when memory is used in a program? The executable file that a C compiler creates contains the following information (say the generic a.out):

  • a.out magic number
  • other a.out contents
  • size needed for BSS (Better Save Space) segment
  • data segment (initialized global and static variables)
  • text segment (executable instructions)

Local variables don’t go into a.out, they are created at runtime, and stored in the stack. When an executable, such as a.out, is run, the parts of the program are put into memory. Memory space for items such as local variables, parameter passing in function calls, etc. is created in a stack segment, often called the runtime stack, and heap space is allocated for dynamically allocated memory. The stack provides a storage area for local variables. The stack stores the “housekeeping” information associated with function calls, known as a stack frame, and  works as a scratch-pad area for temporary storage.

On many systems the stack grows automatically as more space is needed. When all the space in the stack has been used up, stack overflow occurs. Consider the follow examples of variables declared in a function:

char str[]="Do or do not, there is no try!";
char *s="I am the master!";
int x;

In the stack region of memory, the following data is stored:

Name   Type                       Value
str    array of char, size=[31]   Do or do not, there is no try!\0
s      pointer (to char)          00E9
x      int

In the heap region of memory, the following data is stored:

Type             Address   Value
string literal   00E9      I am the master!\0

In the case of str, it has a size implicitly set at 31 (for the 30 characters enclosed in double quotes, and the \0 terminating character). str has memory reserved for it in the stack. The pointer variable s is declared to hold the address (00E9) of the first character of the string. The string is stored in the heap region of memory, not the stack.


Memory in C – the stack, the heap, and static

The great thing about C is that it is so intertwined with memory – and by that I mean that the programmer has quite a good understanding of “what goes where“. C has three different pools of memory.

static: global variable storage, permanent for the entire run of the program.
stack: local variable storage (automatic, continuous memory).
heap: dynamic storage (large pool of memory, not allocated in contiguous order).


Static memory

Static memory persists throughout the entire life of the program, and is usually used to store things like global variables, or variables created with the static clause. For example:

int theforce;

On many systems this variable uses 4 bytes of memory. This memory can come from one of two places. If a variable is declared outside of a function, it is considered global, meaning it is accessible anywhere in the program. Global variables are static, and there is only one copy for the entire program. Inside a function the variable is allocated on the stack. It is also possible to force a variable to be static using the static clause. For example, the same variable created inside a function using the static clause would allow it to be stored in static memory.

static int theforce;

Stack memory

The stack is used to store variables used on the inside of a function (including the main() function). It’s a LIFO, “Last-In,-First-Out”, structure. Every time a function declares a new variable it is “pushed” onto the stack. Then when a function finishes running, all the variables associated with that function on the stack are deleted, and the memory they use is freed up. This leads to the “local” scope of function variables. The stack is a special region of memory, and automatically managed by the CPU – so you don’t have to allocate or deallocate memory. Stack memory is divided into successive frames where each time a function is called, it allocates itself a fresh stack frame.

Note that there is generally a limit on the size of the stack – which can vary with the operating system (for example OSX currently has a default stack size of 8MB). If a program tries to put too much information on the stack, stack overflow will occur. Stack overflow happens when all the memory in the stack has been allocated, and further allocations begin overflowing into other sections of memory. Stack overflow also occurs in situations where recursion is incorrectly used.

A summary of the stack:

  • the stack is managed by the CPU, there is no ability to modify it
  • variables are allocated and freed automatically
  • the stack it not limitless – most have an upper bound
  • the stack grows and shrinks as variables are created and destroyed
  • stack variables only exist whilst the function that created them exists

Heap memory

The heap is the diametrical opposite of the stack. The heap is a large pool of memory that can be used dynamically – it is also known as the “free store”. This is memory that is not automatically managed – you have to explicitly allocate (using functions such as malloc), and deallocate (e.g. free) the memory. Failure to free the memory when you are finished with it will result in what is known as a memory leak – memory that is still “being used”, and not available to other processes. Unlike the stack, there are generally no restrictions on the size of the heap (or the variables it creates), other than the physical size of memory in the machine. Variables created on the heap are accessible anywhere in the program.

Oh, and heap memory requires you to use pointers.

A summary of the heap:

  • the heap is managed by the programmer, the ability to modify it is somewhat boundless
  • in C, variables are allocated and freed using functions like malloc() and free()
  • the heap is large, and is usually limited by the physical memory available
  • the heap requires pointers to access it

An example of memory use

Consider the following example of a program containing all three forms of memory:

#include <stdio.h>
#include <stdlib.h>

int x;          

int main(void) 
    int y;   
    char *str; 

    y = 4;
    printf("stack memory: %d\n", y);

    str = malloc(100*sizeof(char)); 
    str[0] = 'm';
    printf("heap memory: %c\n", str[0]); 
    return 0;

The variable x is static storage, because of its global nature. Both y and str are dynamic stack storage which is deallocated when the program ends. The function malloc() is used to allocate 100 pieces of of dynamic heap storage, each the size of char, to str. Conversely, the function free(), deallocates the memory associated with str.