By default a real number is treated as a

extern int x; - is an external variable declaration.

double pow(double, double); - is a function prototype declaration.

Therefore, 1 and 3 are declarations. 2 is definition.

External Linkage-> means global, non-static variables and functions.
Internal Linkage-> means static variables and functions with file scope.
None Linkage-> means Local variables.

/* Example for ceil() and floor() functions: */

#include<stdio.h>
#include<math.h>

int main()
{
    printf("\n Result : %f" , ceil(1.44) );
    printf("\n Result : %f" , ceil(1.66) );
 
    printf("\n Result : %f" , floor(1.44) );    
    printf("\n Result : %f" , floor(1.66) );

    return 0;
}
// Output:
// Result : 2.000000
// Result : 2.000000
// Result : 1.000000
// Result : 1.000000

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".

fmod(x,y) - Calculates x modulo y, the remainder of x/y.
This function is the same as the modulus operator. But fmod() performs floating point divisions.

Example:

#include <stdio.h>
#include <math.h>

int main ()
{
  printf ("fmod of 3.14/2.1 is %lf\n", fmod (3.14,2.1) );
  return 0;
}

Output:
fmod of 3.14/2.1 is 1.040000

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:
=> foo
=> Bar
=> BAZ
=> foo_bar
=> _foo42
=> _
=> QuUx

Here, Multiplication will happen before the addition, but in which order the functions would be called is undefined. In an arithmetic expression the parenthesis tell the compiler which operands go with which operators but do not force the compiler to evaluate everything within the parenthesis first.

An operation with only one operand is called unary operation.
Unary operators:
! Logical NOT operator.
~ bitwise NOT operator.
sizeof Size-of operator.

&& Logical AND is a logical operator.

Therefore, 1, 2, 3 are unary operators.

C uses left associativity for evaluating expressions to break a tie between two operators having same precedence.

Simply called as BODMAS (Bracket of Division, Multiplication, Addition and Subtraction).

How Do I Remember ? BODMAS !

B - Brackets first

O - Orders (ie Powers and Square Roots, etc.)

DM - Division and Multiplication (left-to-right)

AS - Addition and Subtraction (left-to-right)