Difference between static memory allocation and dynamic memory allocation

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I would like to know what is the difference between static memory allocation and dynamic memory allocation   Could you explain this with any example

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Difference between STATIC MEMORY ALLOCATION  amp  DYNAMIC MEMORY ALLOCATION   Memory is allocated before the execution of the program begins  During Compilation   Memory is allocated during the execution of the program    No memory allocation or deallocation actions are performed during Execution  Memory Bindings are established and destroyed during the Execution    Variables remain permanently allocated  Allocated only when program unit is active    Implemented using stacks and heaps  Implemented using data segments    Pointer is needed to accessing variables  No need of Dynamically allocated pointers    Faster execution than Dynamic  Slower execution than static    More memory Space required  Less Memory space required

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This is a standard interview question   Dynamic memory allocation  Is memory allocated at runtime using calloc    malloc   and friends  It is sometimes also referred to as  heap  memory  although it has nothing to do with the heap data-structure ref   int   a   malloc sizeof int      Heap memory is persistent until free   is called  In other words  you control the lifetime of the variable   Automatic memory allocation  This is what is commonly known as  stack  memory  and is allocated when you enter a new scope  usually when a new function is pushed on the call stack   Once you move out of the scope  the values of automatic memory addresses are undefined  and it is an error to access them    int a   43    Note that scope does not necessarily mean function  Scopes can nest within a function  and the variable will be in-scope only within the block in which it was declared  Note also that where this memory is allocated is not specified   On a sane system it will be on the stack  or registers for optimisation   Static memory allocation  Is allocated at compile time   and the lifetime of a variable in static memory is the lifetime of the program    In C  static memory can be allocated using the static keyword  The scope is the compilation unit only    Things get more interesting when the extern keyword is considered  When an extern variable is defined the compiler allocates memory for it  When an extern variable is declared  the compiler requires that the variable be defined elsewhere  Failure to declare define extern variables will cause linking problems  while failure to declare define static variables will cause compilation problems   in file scope  the static keyword is optional  outside of a function    int a   32    But not in function scope  inside of a function    static int a   32    Technically  extern and static are two separate classes of variables in C   extern int a     Declaration    int a     Definition       Notes on static memory allocation  It s somewhat confusing to say that static memory is allocated at compile time  especially if we start considering that the compilation machine and the host machine might not be the same or might not even be on the same architecture    It may be better to think that the allocation of static memory is handled by the compiler rather than allocated at compile time    For example the compiler may create a large data section in the compiled binary and when the program is loaded in memory  the address within the data segment of the program will be used as the location of the allocated memory  This has the marked disadvantage of making the compiled binary very large if uses a lot of static memory  It s possible to write a multi-gigabytes binary generated from less than half a dozen lines of code  Another option is for the compiler to inject initialisation code that will allocate memory in some other way before the program is executed  This code will vary according to the target platform and OS  In practice  modern compilers use heuristics to decide which of these options to use  You can try this out yourself by writing a small C program that allocates a large static array of either 10k  1m  10m  100m  1G or 10G items  For many compilers  the binary size will keep growing linearly with the size of the array  and past a certain point  it will shrink again as the compiler uses another allocation strategy   Register Memory  The last memory class are  register  variables  As expected  register variables should be allocated on a CPU s register  but the decision is actually left to the compiler  You may not turn a register variable into a reference by using address-of    register int meaning   42  printf   p n   amp meaning      this is wrong and will fail at compile time       Most modern compilers are smarter than you at picking which variables should be put in registers     References    The libc manual K amp R s The C programming language  Appendix A  Section 4 1   Storage Class    PDF  C11 standard  section 5 1 2  6 2 2 3 Wikipedia also has good pages on Static Memory allocation  Dynamic Memory Allocation and Automatic memory allocation The C Dynamic Memory Allocation page on Wikipedia This Memory Management Reference has more details on the underlying implementations for dynamic allocators

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Static memory allocation  The compiler allocates the required memory space for a declared variable By using the address of operator the reserved address is obtained and this address may be assigned to a pointer variable Since most of the declared variable have static memory this way of assigning pointer value to a pointer variable is known as static memory allocation  memory is assigned during compilation time   Dynamic memory allocation  It uses functions such as malloc    or calloc    to get memory dynamically If these functions are used to get memory dynamically and the values returned by these functions are assingned to pointer variables  such assignments are known as dynamic memory allocation memory is assined during run time

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Static memory allocation is allocated memory before execution pf program during compile time  Dynamic memory alocation is alocated memory during execution of program at run time

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There are three types of allocation     static  automatic  and dynamic   Static Allocation means  that the memory for your variables is allocated when the program starts   The size is fixed when the program is created   It applies to global variables  file scope variables  and variables qualified with static defined inside functions   Automatic memory allocation occurs for  non-static  variables defined inside functions  and is usually stored on the stack  though the C standard doesn t mandate that a stack is used   You do not have to reserve extra memory using them  but on the other hand  have also limited control over the lifetime of this memory  E g  automatic variables in a function are only there until the function finishes   void func         int i      i  only exists during  func         Dynamic memory allocation is a bit different  You now control the exact size and the lifetime of these memory locations  If you don t free it  you ll run into memory leaks  which may cause your application to crash  since at some point of time  system cannot allocate more memory   int  func         int  mem   malloc 1024       return mem     int  mem   func       still accessible      In the upper example  the allocated memory is still valid and accessible  even though the function terminated  When you are done with the memory  you have to free it   free mem

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Static Memory Allocation    Variables get allocated permanently Allocation is done before program execution It uses the data structure called stack for implementing static allocation Less efficient There is no memory reusability   Dynamic Memory Allocation    Variables get allocated only if the program unit gets active Allocation is done during program execution It uses the data structure called heap for implementing dynamic allocation More efficient There is memory reusability   Memory can be freed when not required

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Static memory allocation  Memory allocated will be in stack   int a 10     Dynamic memory allocation  Memory allocated will be in heap   int  a   malloc sizeof int    10     and the latter should be freed since there is no Garbage Collector GC  in C   free a

[c] Difference between static memory allocation and dynamic memory allocation

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