Scope Rules and Storage Types Professor Hugh C. Lauer CS-2303, System Programming Concepts (Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie, Absolute C++, by Walter Savitch, The C++ Programming Language, Special Edition, by Bjarne Stroustrup, and from C: How to Program, 5th and 6th editions, by Deitel and Deitel) CS-2303, A-Term 2012 Scope Rules and Storage Types

Scope Rules and Storage Types Data Storage in Memory Variables may be automatic or static Automatic variables are only declared within functions and compound statements (blocks) Storage allocated when function or block is entered Storage is released when function returns or block exits Parameters and result are (somewhat) like automatic variables Storage is allocated and initialized by caller of function Storage is released after function returns to caller. Definitions CS-2303, A-Term 2012 Scope Rules and Storage Types

Scope Rules and Storage Types Definitions Static data — data whose memory is allocated at compile time Actual memory location is set by Linker and Loader Values are retained across function calls Automatic data — data whose memory is allocated at run time on “The Stack” Each time function is called or block is entered Deleted when function or block exits! Values are not preserved from one call to next More about “The Stack” later CS-2303, A-Term 2012 Scope Rules and Storage Types

Scope of Automatic Variables Identifiers declared within a function or compound statement are visible only from the point of declaration to the end of that function or compound statement. Like Java CS-2303, A-Term 2012 Scope Rules and Storage Types

Scope Rules and Storage Types Example int fcn (float a, int b) { int i; double g; for (i = 0; i < b; i++) { double h = i*g; loop body – may access a, b, i, g, h } // for(i…) fcn body – may access a, b, i, g } // int fcn( … ) i is visible from this point to end of fcn g is visible from this point to end of fcn h is only visible from this point to end of loop! None of these are visible outside the function CS-2303, A-Term 2012 Scope Rules and Storage Types

Scope Rules and Storage Types Idiosyncrasies In traditional C (& Visual Studio) All variables must be declared at beginning of function or compound statement (i.e., before first statement); visible from that point on In gcc Variables may be declared anywhere in function or compound statement; visible from that point on In C99 & C++ Loop variables may be declared in for statement; visible only within loop body, but not beyond CS-2303, A-Term 2012 Scope Rules and Storage Types

Static Data – Very different Variables may be declared within or outside of functions Storage allocated when program is initialized Storage is released when program exits Variables outside of functions are usually visible to linker Unless you specify otherwise Compiler sets aside storage for all static variables at compiler or link time Values retained across function calls Initializations must evaluate to compile-time constants CS-2303, A-Term 2012 Scope Rules and Storage Types

Confusion in terminology Reserved word static in C & C++ programs does two things:– It makes the data static According to previous definition Even if it already was static It hides it from the linker! Variables declared outside of any function are called global They are static variables without the word “static”! CS-2303, A-Term 2012 Scope Rules and Storage Types

Static Variable Examples int j; //global, visible to linker & all functions in this program static float f; // not visible to linker, visible to // to all functions in this program int fcn (float a, int b) { // nothing inside of {} is visible to linker int i = b; //automatic double g; //automatic static double h; //static, not visible to // linker; value retained from call to call body – may access j, f, a, b, i, g, h } // int fcn( … ) CS-2303, A-Term 2012 Scope Rules and Storage Types

Static Variable Examples int j; //global, visible to linker & all functions in this program static float f; // not visible to linker, visible to // to all functions in this program int fcn (float a, int b) { // nothing inside of {} is visible to linker int i = b; //automatic double g; //automatic static double h; //static, not visible to // linker; value retained from call to call body – may access j, f, a, b, i, g, h } // int fcn( … ) Note: value of h is retained from one call to next Value of h is also retained across recursions CS-2303, A-Term 2012 Scope Rules and Storage Types

Scope Rules and Storage Types Extern Variables int j; //static, visible to linker & all functs static float f; // not visible to linker, visible to // to all functions in this program extern float p; // global, defined in another program and visible to linker int fcn (float a, int b) { // nothing inside of {} is visible to linker int i = b; //automatic double g; //automatic static double h; //static, not visible to // linker; value retained from call to call body – may access j, f, a, b, i, g, h , p } // int fcn( … ) extern :– connected by linker to global of another program CS-2303, A-Term 2012 Scope Rules and Storage Types

Extern Variables (continued) Examples:– stdin, stdout, stderr are extern variables that point to standard input, output, and error streams. extern variables:– Frequently occur in .h files. Each must be actually declared outside any function in exactly one .c file CS-2303, A-Term 2012 Scope Rules and Storage Types

Scope Rules and Storage Types Questions? Next Topic CS-2303, A-Term 2012 Scope Rules and Storage Types