Symbol table structure

. Assign variables to storage classes that prescribe scope, visibility, and lifetime

- scope rules prescribe the symbol table structure

- scope: unit of static program structure with one or more variable declarations

- scope may be nested

. Pascal: procedures are scoping units

. C: blocks, functions, files are scoping units

. Visibility, lifetimes, global variables

. Common (in Fortran)

. Automatic or stack storage

. Static variables

storage class : A storage class is an extra keyword at the beginning of a declaration which modifies the declaration in some way. Generally, the storage class (if any) is the first word in the declaration, preceding the type name. Ex. static, extern etc.

Scope: The scope of a variable is simply the part of the program where it may be accessed or written. It is the part of the program where the variable's name may be used. If a variable is declared within a function, it is local to that function. Variables of the same name may be declared and used within other functions without any conflicts. For instance,

int fun1()
{
    int a;
    int b;
    ....
}

int fun2()
{
    int a;
    int c;
    ....
}
Visibility: The visibility of a variable determines how much of the rest of the program can access that variable. You can arrange that a variable is visible only within one part of one function, or in one function, or in one source file, or anywhere in the program

Local and Global variables: A variable declared within the braces {} of a function is visible only within that function; variables declared within functions are called local variables. On the other hand, a variable declared outside of any function is a global variable , and it is potentially visible anywhere within the program

Automatic Vs Static duration: How long do variables last? By default, local variables (those declared within a function) have automatic duration : they spring into existence when the function is called, and they (and their values) disappear when the function returns. Global variables, on the other hand, have static duration : they last, and the values stored in them persist, for as long as the program does. (Of course, the values can in general still be overwritten, so they don't necessarily persist forever.) By default, local variables have automatic duration. To give them static duration (so that, instead of coming and going as the function is called, they persist for as long as the function does), you precede their declaration with the static keyword: static int i; By default, a declaration of a global variable (especially if it specifies an initial value) is the defining instance. To make it an external declaration, of a variable which is defined somewhere else, you precede it with the keyword extern: extern int j; Finally, to arrange that a global variable is visible only within its containing source file, you precede it with the static keyword: static int k; Notice that the static keyword can do two different things: it adjusts the duration of a local variable from automatic to static, or it adjusts the visibility of a global variable from truly global to private-to-the-file.