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Defining vs declaring a variable

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• Prelude: information required for a compiler to do its job
  • Recall from the assembler programming lectures where:

    To access an int type variable x: x: .4byte 777 we must use: ldr ... // access an int type variable To access an short type variable x: x: .2byte 777 we must use: ldrsh ... // access an short type variable To access an byte type variable x: x: byte 77 we must use: ldrsb ... // access an byte type variable

    Conclussion:

    The compiler must know the data type of a variable in order to generate the correct assembler instruction to access the variable !!!

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• What is exactly defining a variable ?
  • Defining a variable:

    Defining a variable consists of these actions by the C compiler: Recording the name of the variable Recording the data type of the variable Reserving memory space used to store the value of the variable

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• Intro to declaring a variable
  • Consider the following example:

    int main( int argc, char *argv[] ) { printf("x = %d\n", x); } int x = 4; /* Definition of global var x */

    When you compile this program, you will get this error:

    declare1.c: In function 'main': declare1.c:7:23: error: 'x' undeclared (first use in this function) printf("x = %d\n", x); ^

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/declare1.c

    How to run the program:

    To compile:   gcc declare1.c          To run: ./a.out

    Reason:

    The global variable x was define after it was used in main( ) !!!

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  • Note:

    This behavior is different than Java because: This similar Java program will compile correctly: public class declare1 { public static void main( String[] argv ) { System.out.printf("x = %d\n", x); } static int x = 4; /* Definition of global var x */ } because: The Java compiler is a 2-pass compiler The Java compiler will read the input Java program twice The first time is used to collect all the (variable) definitions The second pass is used to translate the statements using the definitions gathered in pass 1

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  • The C compiler is a 1-pass compiler (which works faster than a 2-pass compiler !!!)

    In a 1-pass compiler, at the time the compiler processes a variable name:

    The data type of the variable must be known to the compiler or else, the compiler will signal an error !!!

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• Declaring a variable
  • Declaring a variable:

    Declaring a variable consists of these actions by the C compiler: Recording the name of the variable Recording the data type of the variable In other words: A declaration of a variable will tell the compiler: the name of the variable the data type of the variable It will not create the variable (= reserver memory space for the variable) !!!

    Note:

    You can only declare global variables !!!

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  • Syntax to declare a variable

    extern dataType varName ;

    Example:

    extern int x;

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• How to use a declaration to fix the above example ?
  • Previously, I showed you a program that contains an error:

    int main( int argc, char *argv[] ) { printf("x = %d\n", x); } int x = 4; /* Definition of global var x */

    We can fix this error (without moving the definition of the variable x) using a declaration:

    extern int x; // Declare x int main( int argc, char *argv[] ) { printf("x = %d\n", x); } int x = 4; /* Definition of global var x */

    I'll show you that it fixes the problem in class by compiling the updated program

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• C programming rules concerning variables usage
  • Rules in C concerning usage of a variable:

    A variable must be defined or declared before the variable is used in a statement: // define x float x; // or declare x; extern float x; // Before you can use the variable x in statements x = x + y; sin(x); You can define a variable only once (Because you create the variable when you define it !!!)          You can declare a variable as often as you want (But you cannot have contradicting declarations !!!)

    Example:

    extern int x; // Declare x extern int x; // Declare x again extern int x; // Declare x and again int main( int argc, char *argv[] ) { printf("x = %d\n", x); // Use x in a statement } int x = 4; /* Definition of global var x */ // Cannot have 2 definitions of x !!!

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• Postlude: common use of variable declaration
  • I have introduced the concept of variable declaration by defining a variable at the end of a program.

    The variable declaration is typically used when:

    A variable is defined in some C program file And the variable is used in (a function in) another C program file

    We will postpone the discussion of multi-file C programs for a later time