C language compilers

Conversions and expressions

Implicit conversions

Characters and integers

Any of the following may be used in an expression where an int or unsigned int may be used.

char.
short int.
A char, short, or int bit-field.
The signed or unsigned varieties of any of the above types.
An object or bit-field that has enumeration type.

If an int can represent all values of the original type, the value is converted to an int; otherwise it is converted to an unsigned int. This process is called ``integral promotion''.

NOTE: The promotion rules for ANSI C are different from previous releases. Use the -Xt mode to get the older behavior.

Compilation mode dependencies that affect unsigned types

Under compilation mode -Xt, unsigned char and unsigned short are promoted to unsigned int.
Under compilation modes -Xa and -Xc, unsigned char and unsigned short are promoted to int.

Signed and unsigned integers

When an integer is converted to another integral type, the value is unchanged if the value can be represented by the new type.
If a negative signed integer is converted to an unsigned integer with greater size, the signed integer is first promoted to the signed integer corresponding to the unsigned integer.

Integral and floating

When a floating type is converted to any integral type, any fractional part is discarded.

Float and double

A float is promoted to double or long double, or a double is promoted to long double without a change in value.

The actual rounding behavior that is used when a floating point value is converted to a smaller floating point value depends on the rounding mode in effect at the time of execution. The default rounding mode is ``round to nearest.'' See ``Floating point operations'' and the IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985) for a more complete discussion of rounding modes.

Usual arithmetic conversions

Some binary operators convert the types of their operands in order to yield a common type, which is also the type of the result. These are called the ``usual arithmetic conversions'':

If either operand is type long double, the other operand is converted to long double.
Otherwise, if either operand has type double, the other operand is converted to double.
Otherwise, if either operand has type float, the other operand is converted to float.
Otherwise, the integral promotions are performed on both operands, such that the smaller of the two is promoted to type of the larger, where both are of at least size int. The sizes of the types are considered as follows, from largest to smallest:
```
   long double > double > float > unsigned long long > long long> unsigned long > long > unsigned int > int
```
However, if one operand is an unsigned type and the other is a potentially larger signed type, and they are the same size, they are converted to the unsigned version of the signed type. For example, in the case of long + unsigned int, both operands are converted to unsigned long. In the case of unsigned long + long long, the unsigned long is converted to long long.