C has three categories of built-in data types: pointer types, integral types, and floating-point types. Pointer types are the most restrictive in terms of the operations that can be performed on them. They are limited to
- subtraction of two pointers, valid only when both pointers point to elements in the same array. The result is the same as subtracting the integer subscripts corresponding to the two pointers.
+ addition of a pointer and an integral type. The result is a pointer that points to the element which would be selected by that integer.
Floating-point types consist of the built-in types float, double, and long double. Integral types consist of char, unsigned char, short, unsigned short, int, unsigned int, long, and unsigned long. All of these types can have the following arithmetic operations performed on them:
+ Addition
- Subtraction
* Multiplication
/ Division
Integral types also can have those four operations performed on them, as well as the following operations: %Modulo or remainder of division
<< Shift left
>> Shift right
& Bitwise AND operation
| Bitwise OR operation
^ Bitwise exclusive OR operation
! Logical negative operation
~ Bitwise "one's complement" operation
Although C permits "mixed mode" expressions (an arithmetic expression involving different types), it actually converts the types to be the same type before performing the operations (except for the case of pointer arithmetic described previously). The process of automatic type conversion is called "operator promotion."

If an operation is specified with operands of two different types, they are converted to the smallest type that can hold both values. The result has the same type as the two operands wind up having. To interpret the rules, read the following table from the top down, and stop at the first rule that applies.

There are two situations in which to use a type cast. The first use is to change the type of an operand to an arithmetic operation so that the operation will be performed properly. The variable f1 is set to the result of dividing the integer i by the integer j. The result is 0, because integer division is used. The variable f2 is set to the result of dividing i by j as well. However, the (float) type cast causes i to be converted to a float. That in turn causes floating-point division to be used and gives the result 0.75.
#include
main()
{
int i = 3;
int j = 4;
float f1 = i / j;
float f2 = (float) i / j;
printf("3 / 4 == %g or %g depending on the type used.\n", f1, f2);
}
The second case is to cast pointer types to and from void * in order to interface with functions that expect or return void pointers. For example, the following line type casts the return value of the call to malloc() to be a pointer to a foo structure.
struct foo *p = (struct foo *) malloc(sizeof(struct foo));

A type cast should not be used to override a const or volatile declaration. Overriding these type modifiers can cause the program to fail to run correctly.
A type cast should not be used to turn a pointer to one type of structure or data type into another. In the rare events in which this action is beneficial, using a union to hold the values makes the programmer's intentions clearer.

A global variable that must be accessed from more than one file can and should be declared in a header file. In addition, such a variable must be defined in one source file. Variables should not be defined in header files, because the header file can be included in multiple source files, which would cause multiple definitions of the variable.
The ANSI C standard will allow multiple external definitions, provided that there is only one initialization. But because there's really no advantage to using this feature, it's probably best to avoid it and maintain a higher level of portability.
"Global" variables that do not have to be accessed from more than one file should be declared static and should not appear in a header file.