Overview

• Dynamic means "at runtime" as opposed to "at compile time"
• In Java:
  • Memory is allocated dynamically every time an obect is created. (Heap memory is allocated.)
  • If no references point to an object, the object may be garbage collected
  • Garbage collection "GC" is done periodically by the JVM
  • It frees up memory for reuse and reorganizing memory if it becomes framented
  • Java does not give the programmer the power to garbage collect. It is all handled by the JVM.
  • Garbage collection was a huge developmental leap in programming languages.
  • Advantages: memory safety!; programmer has less control
  • Disadvantages: it takes time (may be slower); programmer has less control
• In C:
  • There are no objects and there is no garbage collection
  • The programmer requests memory with malloc(), which means "memory allocation"
  • It is the programmer's responsibility to explicity free up the memory if it is no longer needed by using the free()
  • This sounds easy but the failure to free unused memory is the crux of many "memory leaks"!
  • Variants of malloc() (which allocates unitialized memory) are calloc() (which initializes the memory to 0's) and realloc()

Files to Download

• All files needed are in memory.zip

Exercises

1. Study/compile/run mallocplay.c.
2. Study/compile/run play.c. Then uncomment each of the three commented lines in turn the scenarios described below. After uncommenting a line and seeing its effect, comment it out again before uncommenting the next one.
   • What happens if you try to assign a value to data before data is set to point to memory allocated by malloc()?
   • What happens if you try to free a pointer twice?
   • What happens if you try to acces *data after the memory that data was pointing to is freed?
   IMPORTANT NOTE: Don't generalize about the behaviors you notice. On a different system, or with a different program on the same system, the behavior may differ.
3. Study/compile/run printTwice.c. It should print 0 thru 4 twice. Figure out why it doesn't. Fix it by writing code that uses pointer notation (with the * operator, not the [] operator).
4. Study/compile/run debugit.c. It should be able to run "forever" without crashing. However in its current version it crashes after several iterations of the loop (the exact number is system dependent). Find out why the memory allocation error occurs and fix it so that the program will run "forever" (in which case the user needs to type CTRL C to abort it).

   number of malloc()'s: 
   1
   2
   3
   4
   5
   6
   7
   memory allocation error: data
   

Note

• If the SIZE of the space requested is 0, the behavior is implementation-dependent.
• The value returned will be either a null pointer or a non-null pointer.
• Non-null pointer can occur if:
  1. Some version of malloc do not do error checking  - assumes programmer knows best
  2. In some version of malloc If size is 0 then pretends that size is 1.
• Take any file and change it such that you ask a user to enter the SIZE. Input 0 as size and see what happens.