What are local variables and global variables?

Answer 1

Local variables:

These variables only exist inside the specific function that creates them. They are unknown to other functions and to the main program. As such, they are normally implemented using a stack. Local variables cease to exist once the function that created them is completed. They are recreated each time a function is executed or called.

Global variables:

These variables can be accessed (ie known) by any function comprising the program. They are implemented by associating memory locations with variable names. They do not get recreated if the function is recalled.

Answer 2

Global variables are allocated in the program's data segment and therefore exist in memory for the entire duration the program is running. Local variables are allocated on the call stack and therefore only exist in memory while the variable is in scope.

Global variables are frowned upon because there's no way to limit their scope. Any code can gain access to a global variable, but only local code can gain access to a local variable. Many programmers go out of their way to avoid globals, however if a concept is truly global then it makes sense to use a global. The problem is that far too many programmers use globals for concepts that are categorically non-global.

Answer 3

In programming, local variables are those that are only accessible within a particular (not global) scope, while global variables are those accessible from anywhere within the same program.

In the C Programming language, global variables are all those declared at a file (module) level but not constraint with the static keyword.

Variables declared at a file level but constraint by the static keyword are sometimes called file-global and sometimes called file-local variables.

In the C programming language, all other variable declarations occur either within a function argument list declaration, or within a pair of curly braces. The function, and the pair of curly braces, defines the scope of the variable definition.

As each variable must have a unique name within its scope, global variables' names must be unique within the entire program, while other variables' names must be unique within their declaration scope (the file, the function, the pair of curly braces).