CSC 253 Lecture 2

Some differences between Java and C  Compiled C code is machine specific, whereas Java compiles for a virt. machine.  Virtual machines are much slower than natively executed code.  Compiled code can interact with specifics of that machine (e.g., devices).  Java does “automatic” memory management.  In C, you must free memory explicitly.  You could forget to do it … this causes a memory leak.  However, you can get by with less memory.  And, you don’t have to worry about garbage collection causing you to miss real-time deadlines.  Compiled C code is machine specific, whereas Java compiles for a virt. machine.  Virtual machines are much slower than natively executed code.  Compiled code can interact with specifics of that machine (e.g., devices).  Java does “automatic” memory management.  In C, you must free memory explicitly.  You could forget to do it … this causes a memory leak.  However, you can get by with less memory.  And, you don’t have to worry about garbage collection causing you to miss real-time deadlines.

C vs. Java  Why are OSs written in C?  The very bottom layer of sw. has to be native machine code.  Two steps in the Java compile-run sequence are “skipped” with C.  What two steps?  Interpretation—JVM xlates byte code to machine code.  What’s the advantage of skipping them?  Faster  What’s “bad” about skipping them?  Machine dependence.  No place for run-time bounds-checking.  Why are OSs written in C?  The very bottom layer of sw. has to be native machine code.  Two steps in the Java compile-run sequence are “skipped” with C.  What two steps?  Interpretation—JVM xlates byte code to machine code.  What’s the advantage of skipping them?  Faster  What’s “bad” about skipping them?  Machine dependence.  No place for run-time bounds-checking.

Pointers  Advantages?  You can perform arithmetic on pointers … this may save a few instructions.  You can get a pointer to anything in C.  Disadvantages?  “Dangling pointers”—pointers that don’t point to allocated memory anymore.  Forgetting to dereference a pointer.  Advantages?  You can perform arithmetic on pointers … this may save a few instructions.  You can get a pointer to anything in C.  Disadvantages?  “Dangling pointers”—pointers that don’t point to allocated memory anymore.  Forgetting to dereference a pointer.

Summary: Advantages of C  Efficiency  Close to the machine  Uses much less memory  You have better control of your program (talking directly to machine).  Efficiency  Close to the machine  Uses much less memory  You have better control of your program (talking directly to machine).

Summary: Advantages of Java  Portability  Safety  Less error prone  Portability  Safety  Less error prone

Variables used by > 1 function  Should they be …  Global?  …  How can such a value be passed into a function?  How can such a value be returned from the function?  Should they be …  Global?  …  How can such a value be passed into a function?  How can such a value be returned from the function?

Why not use lots of globals?  You can prevent accesses to variables from places they’re not supposed to be accessed from.  Name-space pollution.  Local variables are deallocated upon procedure returns, whereas global variables aren’t. (Uses more memory.)  You can prevent accesses to variables from places they’re not supposed to be accessed from.  Name-space pollution.  Local variables are deallocated upon procedure returns, whereas global variables aren’t. (Uses more memory.)

How can you avoid globals?  Pass pointers to structures.  You can return it from a function.  You can pass in & return pointers, and in the function, modify what’s pointed to.  Pass pointers to structures.  You can return it from a function.  You can pass in & return pointers, and in the function, modify what’s pointed to.

Why should you not do arithmetic on characters?

Let’s write a program to …  determine the value of INT_MAX ;  that is, show an integer i that prints as positive, whereas i +1 prints as negative.  determine the value of INT_MAX ;  that is, show an integer i that prints as positive, whereas i +1 prints as negative.

Let’s write a program to  look at the bits in INT_MAX and see how it’s represented in our computer.