1 Homework Turn in HW2 at start of next class. Starting Chapter 2 K&R. Read ahead. HW3 is on line. –Due: class 9, but a lot to do! –You may want to get a head start. Questions?

2 Call by Value vs Call by Reference Simple variables passed as function parameters in C are actually only copies on the stack. (Note: Stack pointer is a register) foo(i, j); void foo(int i, int j) { } Stack Pointer Before call and after return Unused Stack Pointer After call and before return Unusedij Return Data Stack Data Stack Data Copy of Value

3 This is known as Call by Value. Unlike some languages, which have call by reference You can't change arguments in original location within the function -- just change the stack copy To make changes, you must pass pointers to original variables. See next slide. You’re not responsible for pointers yet, but start getting used to them! Call by Value vs Call by Reference

4 Pointers as Arguments Pointer variables passed as function parameters in C are still only value on the stack but can be used to access original location indirectly foo(&i, &j); void foo(int *i, int *j) { } Stack Pointer Before call and after return Unused Stack Pointer After call and before return Unused &i&j Return Data Stack Data Stack Data Point to values

5 Pointers as Arguments The following doesn’t work!!! void exchgint (int a, int b) { int dummy; dummy = a; a = b; b = dummy; }

6 Pointers as Arguments Must be done with pointers!!! void exchgint (int *pa, int *pb) { int dummy; dummy = *pa; *pa = *pb; *pb = dummy; }

7 Pointers as Arguments An array name is automatically passed as a pointer You don't need to create a pointer yourself with the “address of” operator (&) int array1[256], array2[256]; foo(array1, array2); Unless you are passing a pointer to a specific array element (Remember printf in visitype?) foo(&array1[4*i], &array2[4*j]);

8 Scope of Variables – Local Automatic Each local variable of a function (inside { }) comes into existence only when it is called disappears when a return is performed. Local variables are said to be automatic. Memory is allocated on the stack after the calling sequence argument values Stack Pointer After local variables are allocated Stack Data Unusedij Return Data Local Variables

9 Scope of Variables – Local Static A static variable declared in a function is assigned a fixed location in memory that is NOT on the stack. Still local - can only be used inside function { } but its value is preserved between successive calls to the function E.g., the seed of a random number generator so it will have memory from one invocation to the next and not always give the same random number. int rand( ) { static int seed = 1; /* initialize to 1 and remember value between calls to rand */ }

10 Scope of Variables – Local Static Good for implementing a “Finite State Machine” –They allow a function to preserve its state value(s) But, be careful using local static variables!! –A function may behave differently when it is called with different values preserved in local static variables –Makes testing of a function more complex because you need to test its behavior with all possible values of any local static variables –Complex sequence of calls to function may be required to drive local static variables into desired test “state(s)”

11 Scope of Variables - External Alternative to local is known as external Look at maxline program on page 32 of K&R Variables declared ahead of/outside of all { }’s are available to all functions within the program These are sometimes called “global variables” Can be used to pass data between functions Values are preserved like static local variables Not a good idea – Why not?

12 Scope of Variables - External Global variables are not a good idea because: –They can be accessed from anywhere. If their value gets corrupted, it is NOT easy to figure out what code in which function caused the corruption. –They make the functions dependent on their external environment instead of being able to stand alone using arguments to get their input values and a return value to pass back an output value. Software architecture/design standards for most projects will prohibit use of “global variables” or severely restrict their use. Don’t use them!!

13 Scope of Variables – External Static Another alternative known as external static No good examples that I see in K&R Static variables declared ahead of/outside of all { }’s are available to all functions within this file only These are very similar to OOP “class variables” Can be used to pass data between functions in file only Values are preserved like static local variables These are more acceptable than global variables

14 Data Types Finished K&R Chapter 1 / Now starting Chapter 2 Variable Names Data Types: –char –int –short int –long int –float –double

15 Variable / Symbolic Constant Names Rules for generating names: –All letters and digits are allowed –Uppercase and lower case letters are different –First character must be a letter –Underscore _ counts as a letter (useful for improving readability) –But, don’t begin variable names with _ (reserved for library routines)

16 Variable / Symbolic Constant Names Rules for generating names: –Internal - at least 31 characters are significant –External - only count on 6 characters, single case –Avoid using C keywords (if, else, int, float, etc.) –Choose variable names that are related to the purpose or use of the variable

17 Data Types char (character) char has 8 bits (stored in one byte in memory) unsigned: 0 ≤ value ≤ ≤ value ≤ Overflow at 255( = 0) Underflow at 0(0 – 1 = 255) signed (if supported in the implementation): -2 7 ≤ value ≤ ≤ value ≤ Overflow at 127 ( = -128) Underflow at –128 (-128 – 1 = 127)

18 Data Types int (integer on our machines) int has 32 bits (stored in four sequential bytes in memory) unsigned: 0 ≤ value ≤ x ≤ value ≤ 0xffffffff Overflow at ( = 0) Underflow at 0(0 – 1 = ) signed: ≤ value ≤ x ≤ value ≤ 0x7fffffff Overflow at ( = – ) Underflow at – ( – 1 = )

19 Data Types short int (short integer on our machines) short int has 16 bits (stored in two sequential bytes in memory) unsigned: 0 ≤ value ≤ x0000 ≤ value ≤ 0xffff Overflow at 65535( = 0) Underflow at 0(0 – 1 = 65535) signed: ≤ value ≤ x8000 ≤ value ≤ 0x7fff Overflow at ( = –32768) Underflow at –32768 ( – 1 = 32767)

20 Data Types long int (long integer on our machines – same as int) long int has 32 bits (stored in four sequential bytes in memory) unsigned: 0 ≤ value ≤ x ≤ value ≤ 0xffffffff Overflow at ( = 0) Underflow at 0(0 – 1 = ) signed: ≤ value ≤ x ≤ value ≤ 0x7fffffff Overflow at ( = – ) Underflow at – ( – 1 = )

21 Data Types float, double –Both represent numbers with fractional parts –“double” is a “float” with more precision –Don't do much with float or double in this course In absence of a co-processor to do floating point computations, it takes a lot of compute time to do them in software. –Not often used in real time, embedded systems. –A cost versus performance tradeoff!