birt code 128

birt code 128

birt code 128

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Notice that main() passes myVar to the function PassAlong(). PassAlong() doesn t actually make use of myVar. Instead, it just passes myVar along to the function PrintMyVar(). PrintMyVar() prints myVar and then returns. If myVar were a global, you could have avoided some parameter passing. main() and PrintMyVar() could have shared myVar without the use of parameters. When should you use parameters When should you use globals In a nutshell, you should avoid using globals. Whenever a global variable solves a problem, there s always a way to use parameters to solve the same problem. Global variables offer a shortcut that saves you from having to pass information up and down your chain of function calls. They do save time but at the cost of proper program design. As you move on to object programming languages like Objective-C, you ll find that you just don t need globals. So why learn about them They are part of the C language: the concept of global variables is important to understand, and you may find yourself having to maintain or fix someone else s code. If that legacy code uses global variables, you ll need to know how they work. Let s take a look at the proper way to add globals to your programs.

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Adding globals to your programs is easy. Just declare a variable at the beginning of your source code before the start of any of your functions. Here s the example I showed you earlier, using globals in place of parameters:

#include <stdio.h> void PassAlong( void ); void PrintMyVar( void );

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gMyVar;

Simplescalar simulation suite available from the Simplescalar website at http://www -simplescalarcom A companion website for the book contains additional support material for the instructor, including a complete set of lecture slides (wwwmhhecom/shen)

int main (int argc, const char * argv[]) { gMyVar = 10; PassAlong(); return 0; } void PassAlong( void ) { PrintMyVar(); } void PrintMyVar( void ) { printf( "gMyVar = %d", gMyVar ); }

Lab Exercise 502: Recognizing CPU Sockets 102 Lab Analysis Test 104 Key Term Quiz 104

This example starts with a variable declaration, right at the top of the program. Because gMyVar was declared at the top of the program, gMyVar becomes a global variable, accessible to each of the program s functions. Notice that none of the functions in this version use parameters. As a reminder, when a function is declared without parameters, use the keyword void in place of a parameter list.

Did you notice that letter g at the beginning of the global s name Many C programmers start each of their global variables with the letter g (for global). Doing this will distinguish your local variables from your global variables and will make your code much easier to read.

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Before we get to our source code examples, we have one more subject to cover. In addition to passing a parameter and using a global variable, there s one more way to share data between two functions. Every function returns a value to the function that called it. You can use this return value to pass data back from a called function. So far, all of our examples have ignored function return values. The return value only comes into play when you call a function in an expression, like this:

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#include <stdio.h> int AddTheseNumbers( int num1, int num2 );

int main (int argc, const char * argv[]) { int sum; sum = AddTheseNumbers( 5, 6 ); printf( "The sum is %d.", sum ); return 0; } int } AddTheseNumbers( int num1, int num2 ) { return( num1 + num2 );

Lab Exercise 601: Determining the Amount of RAM in Your PC 106 Lab Exercise 602: Identifying Types of RAM 111 Lab Analysis Test 115 Key Term Quiz 115