6.10 Data-Type Conversions

C performs data-type conversions in the following four situations:

The following sections describe how operands and function arguments are converted.

6.10.1 Usual Arithmetic Conversions

The following rules-referred to as the usual arithmetic conversions- govern the conversion of all operands in arithmetic expressions. The effect is to bring operands to a common type, which is also the type of the result. The rules govern in the following order:

  1. If either operand is not of arithmetic type, no conversion is performed.

  2. If either operand has type long double , the other operand is converted to long double .

  3. Otherwise, if either operand has type double , the other operand is converted to double .

  4. Otherwise, if either operand has type float , the other operand is converted to float .

  5. Otherwise, the integral promotions are performed on both operands, and the following rules apply:

    1. If either operand has type unsigned long int , the other operand is converted to unsigned long int .

    2. Otherwise, if one operand has type long int and the other has type unsigned int , and if a long int can represent all values of an unsigned int , the operand of type unsigned int is converted to long int . If a long int cannot represent all the values of an unsigned int , both operands are converted to unsigned long int .

    3. Otherwise, if either operand has type long int , the other operand is converted to long int .

    4. Otherwise, if either operand has type unsigned int , the other operand is converted to unsigned int .

    5. Otherwise, both operands have type int .

The following sections elaborate on the usual arithmetic conversion rules.

6.10.1.1 Characters and Integers

A char , short int , or int bit field, either signed or unsigned, or an object that has enumeration type, can be used in an expression wherever an int or unsigned int is permitted. 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 . These conversion rules are called the integral promotions.

This implementation of integral promotion is called value preserving, as opposed to unsigned preserving in which unsigned char and unsigned short widen to unsigned int . DEC C uses value- preserving promotions, as required by the ANSI C standard, unless the common C mode is specified.

To help locate arithmetic conversions that depend on unsigned preserving rules, DEC C, with the check option enabled, flags any integral promotions of unsigned char and unsigned short to int that could be affected by the value-preserving approach for integral promotions.

All other arithmetic types are unchanged by the integral promotions.

In DEC C, variables of type char are bytes treated as signed integers. When a longer integer is converted to a shorter integer or to char , it is truncated on the left; excess bits are discarded. For example:

int i;
char c;

i = 0xFFFFFF41;
c = i;

This code assigns hex 41 ('A' ) to c . The compiler converts shorter signed integers to longer ones by sign extension.

6.10.1.2 Signed and Unsigned Integers

Conversions also take place between the various kinds of integers.

When a value with an integral type is converted to another integral type (such as int converted to long int ) and the value can be represented by the new type, the value is unchanged.

When a signed integer is converted to an unsigned integer of equal or greater size, and the signed integer value is nonnegative, its value is unchanged. If the signed integer value is negative, then:

When an integer value is demoted to an unsigned integer of smaller size, the result is the nonnegative remainder of the value divided by the number one greater than the largest representable unsigned value for the new integral type.

When an integer value is demoted to a signed integer of smaller size, or an unsigned integer is converted to its corresponding signed integer, the value is unchanged if it is small enough to be represented by the new type. Otherwise, the result is truncated; excess high-order bits are discarded and precision is lost.

Conversion between integral types of the same size, whether signed or unsigned, results in no machine-level representation change.

6.10.1.3 Floating and Integral

When a floating-type operand is converted to an integer, the fractional part is discarded.

When a floating-type value is to be converted at compile time to an integer or another floating type, and the result cannot be represented, the compiler reports a warning in the following instances:

When a value of integral type is converted to floating type, and the value is in the range of values that can be represented, but not exactly, the result of the conversion is either the next higher or next lower value, whichever is the natural result of the conversion on the hardware. See your DEC C documentation for the conversion result on your platform.

6.10.1.4 Floating Types

If an operand of type float appears in an expression, it is treated as a single-precision object unless the expression also involves an object of type double or long double , in which case the usual arithmetic conversion applies.

When a float is promoted to double or long double , or a double is promoted to long double , its value is unchanged.

The behavior is undefined when a double is demoted to float , or a long double to double or float , if the value being converted is outside the range of values that can be represented.

If the value being converted is inside the range of values that can be represented, but not exactly, the result is rounded to either the next higher or next lower representable float value.

6.10.2 Pointer Conversions

Although two types (for example, int and long ) can have the same representation, they are still different types. This means that a pointer to int cannot be assigned to a pointer to long without using a cast. Nor can a pointer to a function of one type be assigned to a pointer to a function of a different type without using a cast. In addition, pointers to functions that have different parameter- type information, including the old-style absence of parameter- type information, are different types. In these instances, if a cast is not used, the compiler issues an error. Because there are alignment restrictions on some target processors, access through an unaligned pointer can result in a much slower access time or a machine exception.

A pointer to void can be converted to or from a pointer to any incomplete or object type. If a pointer to any incomplete or object type is converted to a pointer to void and back, the result compares equal to the original pointer.

An integral constant expression equal to 0, or such an expression cast to the void * type, is called a null pointer constant. If a null pointer constant is assigned to or compared for equality with a pointer, the constant is converted to a pointer of that type. Such a pointer is called a null pointer, and is guaranteed to compare unequal to a pointer to any object or function.

An array designator is automatically converted to a pointer to the array type, and the pointer points to the first element of the array.

6.10.3 Function Argument Conversions

The data types of function arguments are assumed to match the types of the formal parameters unless a function prototype declaration is present. In the presence of a function prototype, all arguments in the function invocation are compared for assignment compatibility to all parameters declared in the function prototype declaration. If the type of the argument does not match the type of the parameter but is assignment compatible, C converts the argument to the type of the parameter (see Section 6.10.1). If an argument in the function invocation is not assignment compatible to a parameter declared in the function prototype declaration, an error message is generated.

If a function prototype is not present, all arguments of type float are converted to double , all arguments of type char or short are converted to type int , all arguments of type unsigned char and unsigned short are converted to unsigned int , and an array or function name is converted to the address of the named array or function. The compiler performs no other conversions automatically, and any mismatches after these conversions are programming errors.

A function designator is an expression that has function type. Except when it is the operand of the sizeof operator or the unary & operator, a function designator with type "function returning type" is converted to an expression that has type "pointer to function returning type."


Previous Page | Next Page | Table of Contents | Index