C Scope

Storage Class Specifiers
<no keyword>	Visible to everything else in the regular scope. As a global, possibly other files; external definition - visible to other files - defines storage
static	Visible no other files.
extern	External reference - visible to other files - declares reference to an external defiintion somewhere
auto	Exists.

An external definition

int a[10]; // external definition

An external reference; a reference to variable a from file1.c:

extern int a[10]; // external reference

If variable a was static, the extern in file2.c would fail.

	Outside a function definition	Inside a function definition
None	scope: external definition storage: static address	scope: within the function storage: on the stack) (same as auto)
auto	N/A	scope: within the function storage: on the stack (same as None)
static	scope: within the file only storage: static address	scope: external reference storage: static address; location determined by file containing the external definition (can’t have intializer either)
register	N/A	scope: within the function storage: register or stack (hint to compiler; use of & operator not allowed; seldom used)
Most important takeaways:

static has two meanings
- inside a function: static changes the storage location to static memory (see C Memory Layout) instead of on the stack
- outside a function: static changes the scope to be only visible with the file (the storage stays in static memory)
extern
- Compiler does not allocate sotrage
- For type check of the idenitifer name only
- another C file must allocate storage by defining that varible or function
- Typical way to link global variables between C files
volatile
- tells the compiler not to optimize away the variable
- use this for device registers
- prevents compiler from optimizing (removing) dead code

Can appear as part of the base type or within a declarator
Not mutally exclusive with a storage class
const - the value of this varibale is immutable after initialization
volatile - the compiler may not optimize reference to this variable (e.g. it’s a device register that may change value asynchronously)
- e.g. REG_DISPCTL for GBA
restrict - for the lifetime of a pointer, only the pointer itsel or a value directly derived from it may be used to access the object to which it points. This allows better optimizzation but isn’t required.

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