Princeton University COS 217: Introduction to Programming Systems C Variable Declarations and Definitions Variable declaration is a statement that informs the compiler of the name, type, scope, linkage, and duration of the variable. A variable definition is a declaration that causes the compiler to allocate memory. Scope (compile-time concept) File: The variable is accessible within the file in which it is declared, from the point of declaration to the end of the file. Block: The variable is accessible within the block in which it is declared, from the point of declaration to the end of the block. Linkage (link-time concept) External: The variable is accessible from multiple files. Internal: The variable is accessible from only the file in which it is declared. Duration (run-time concept) Temporary: The variable exists only during the execution of the function or block in which it is declared. Physically, the variable’s value is stored in the runtime Stack. Process: The variable exists throughout the entire process. Physically, the variable’s value is stored in the Data Section (if the programmer specifies an initial value) or the BSS Section (if the programmer does not specify an initial value). The variable’s value is initialized at program startup. If in the BSS section, its initial value is 0. C Code Decl/Def Scope Linkage Duration Location int a = 5; definition file external process Data int b; definition* file external process BSS extern int c = 5; definition file external process Data extern int d; declaration file external process ??? static int e = 5; definition file internal process Data static int f; definition file internal process BSS void fun(int g) { definition block internal temporary Stack int h = 5; definition block internal temporary Stack int i; definition block internal temporary Stack extern int j = 5; ILLEGAL extern int k; declaration block ??? process ??? static int l = 5; definition block internal process Data static int m; definition block internal process BSS ... } * Special rule: If a definition of b appears another .c file, then this becomes a declaration. Page 1 of 2Examples of Global Variable Declarations and Definitions Suppose a program consists of file1.c and file2.c (only). Consider these combinations of global variable declarations and definitions: file1.c file2.c Result Reasonable combinations: 1 static int i = 5; static int i = 5; static def / static def => OK 2 static int i = 5; static int i; static def / static def => OK 3 static int i; static int i; static def / static def => OK 4 int i = 5; extern int i; def / decl => OK 5 int i; extern int i; def / decl => OK Less reasonable combinations: 6 int i = 5; int i; def / decl => OK (by special rule) 7 int i; int i; def / decl => OK (by special rule) 8 int i = 5; static int i = 5; def / static def => OK 9 int i = 5; static int i; def / static def => OK 10 int i; static int i = 5; def / static def => OK 11 int i; static int i; def / static def => OK Erroneous combinations: 12 int i = 5; int i = 5; def / def => error 13 extern int i; extern int i; decl / decl => error 14 extern int i; static int i = 5; decl / static def => error 15 extern int i; static int i; decl / static def => error Copyright © 2010 by Robert M. Dondero, Jr. Page 2 of
View Full Document
Unlocking...