Variable names in C are made up of letters (upper and lower case) and digits. The underscore character ("_") is also permitted. Names must not begin with a digit.
Examples of valid (but not very descriptive) C variable names:
No difference, except extern int fun(); is probably in another file
extern int fun(); declaration in C is to indicate the existence of a global function and it is defined externally to the current module or in another file.
int fun(); declaration in C is to indicate the existence of a function inside the current module or in the same file.
In computing, 'real number' often refers to non-complex floating-point numbers. It include both rational numbers, such as 42 and 3/4, and irrational numbers such as pi = 3.14159265...
When the accuracy of the floating point number is insufficient, we can use the double to define the number. The double is same as float but with longer precision and takes double space (8 bytes) than float.
To extend the precision further we can use long double which occupies 10 bytes of memory space.
Declaring is the way a programmer tells the compiler to expect a particular type, be it a variable, class/struct/union type, a function type (prototype) or a particular object instance. (ie. extern int i)
Declaration never reserves any space for the variable or instance in the program's memory; it simply a "hint" to the compiler that a use of the variable or instance is expected in the program. This hinting is technically called "forward reference".
A function prototype in C or C++ is a declaration of a function that omits the function body but does specify the function's name, argument types and return type.
While a function definition specifies what a function does, a function prototype can be thought of as specifying its interface.