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Consider the following expression:
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~ expression |
The result is the bitwise negation (one's complement) of the evaluated
expression. Each 1-bit is converted into a 0-bit and vice versa. The
expression must have an integer type. The compiler performs the usual
arithmetic conversions (see Section 6.10.1).
6.4.6 The Cast Operator
The cast operator forces the conversion of its scalar operand to a specified scalar data type, or to void. The operator consists of a type-name, in parentheses, that precedes an expression, as follows:
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( type-name ) expression |
The value of the expression is converted to the named data type, as if the expression were assigned to a variable of that type. The expression's type and value are not themselves changed; the value is converted to the cast type for the duration of the cast operation. The type-name has the following syntax:
type-name:
|
In simple cases, type-specifier is the keyword for a data type, such as char or double, and abstract-declarator is empty. For example:
(int)x; |
The type-specifier can also be an enum specifier, or a typedef name. The type-specifier can be a structure or union only if the abstract-declarator is a pointer. That is, the type-name can be a pointer to a structure or union, but cannot be a structure or union because structures and unions are not scalar types. For example:
(struct abc *)x /* allowed */ (struct abc)x /* not allowed */ |
The abstract-declarator in a cast operation is a declarator without an identifier. Abstract declarators have the following syntax:
abstract-declarator:
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The abstract-declarator cannot be empty in the following form:
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(abstract-declarator) |
Abstract declarators can include the brackets and parentheses that indicate arrays and functions. However, cast operations cannot force the conversion of any expression to an array, function, structure, or union. The brackets and parentheses are used in operations such as the following example, which casts the identifier P1 to pointer to array of int:
(int (*)[10]) P1; |
This kind of cast operation does not change the contents of P1; it only causes the compiler to treat the value of P1 as a pointer to such an array. For example, casting pointers this way can change the scaling that occurs when an integer is added to a pointer:
int *ip; ((char *)ip) + 1; /* Increments by 1 not by 4 */ |
Cast operators can be used in the following conversions that involve pointers:
Consider the syntax of the following expressions:
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sizeof expression |
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sizeof ( type-name ) |
type-name cannot be an incomplete type, function type, or a bit field. The sizeof operator produces a compile-time integer constant value. expression is inspected only to deduce its type; it is not fully evaluated. For example, sizeof(i++) is equivalent to sizeof(i).
The result of the sizeof operation is the size, in bytes, of the operand. In the first case, the result of sizeof is the size determined by the type of the expression. In the second case, the result is the size of an object of the named type. The expression should be enclosed in parentheses if it contains operators, because the precedence of sizeof is higher than that of most operators.
The syntax of type-name is the same as that for the cast operator. For example:
int x; x = sizeof(char *); /* assigns the size of a character pointer to x */ |
The type of the sizeof operator's result,
size_t, is an unsigned integer type. In
DEC C, size_t is unsigned int.
6.5 Binary Operators
The binary operators are categorized as follows:
The following sections describe these binary operators.
6.5.1 Multiplicative Operators
The multiplicative operators are *, /, and %. Operands must have arithmetic type. Operands are converted, if necessary, according to the usual arithmetic conversion rules (see Section 6.10.1).
The * operator performs multiplication.
The / operator performs division. When integers are divided, truncation is toward zero. If either operand is negative, the result is truncated toward zero (the largest integer of lesser magnitude than the algebraic quotient).
The % operator divides the first operand by the second and yields the remainder. Both operands must be integral. When both operands are unsigned or positive, the result is positive. If either operand is negative, the sign of the result is the same as the sign of the left operand.
The following statement is true if b is not zero:
(a/b)*b + a%b == a; |
The DEC C compiler, with the check option enabled, issues warnings for these undefined behaviors:
The additive operators + and -- perform addition and subtraction. Operands are converted, if necessary, according to the usual arithmetic conversion rules (see Section 6.10.1).
When two enum constants or variables are added or subtracted, the type of the result is int.
When an integer is added to or subtracted from a pointer expression, the integer is scaled by the size of the object being pointed to. The result has the pointer's type. For example:
int arr[10]; int *p = arr; p = p + 1; /* Increments by sizeof(int) */ |
An array pointer can be decremented by subtracting an integral value from a pointer or address; the same conversions apply as for addition. Pointer arithmetic also applies one element beyond the end of the array. For example, the following code works because the pointer arithmetic is limited to the elements of the array and to only one element beyond:
int i = 0;
int x[5] = {0,1,2,3,4};
int y[5];
int *ptr = x;
while (&y[i] != (ptr + 5)) { /* ptr + 5 marks one beyond the end of the array */
y[i] = x[i];
i++;
}
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When two pointers to elements of the same array are subtracted, the
result (calculated by dividing the difference between the two addresses
by the length of one element) is of type ptrdiff_t, which in DEC C is int, and represents the number of elements
between the two addressed elements. If the two elements are not in the
same array, the result of this operation is undefined.
6.5.3 Shift Operators
The shift operators << and >> shift their left operand to the left or to the right, respectively, by the number of bits specified by the right operand. Both operands must be integral. The compiler performs integral promotions on each of the operands (see Section 6.10.1.1). The type of the result is the type of the promoted left operand. Consider the following expression:
E1 << E2 |
The result is the value of expression E1 shifted to the left by E2 bits. Bits shifted off the end are lost. Vacated bits are filled with zeros. The effect of shifting left is to multiply the left operand by 2 for each bit shifted. In the following example, the value of i is 100:
int n = 25; int m = 2; int i; i = n << m; |
Consider the following expression:
E1 >> E2 |
The result is the value of expression E1 shifted to the right by E2 bits. Bits shifted off the end are lost. If E1 is unsigned or if E1 has a signed type but nonnegative value, vacated bits are filled with zeros. If E1 has a signed type and negative value, vacated bits are filled with ones.
The result of the shift operation is undefined if the right operand is negative or if its value is greater than the number of bits in an int.
For a nonnegative left operand, the effect of shifting right is to divide the left operand by 2 for each bit shifted. In the following example, the value of i is 12:
int n = 100; int m = 3; int i; i = n >> m; |
The relational operators compare two operands and produce a result of type int. The result is 0 if the relation is false, and 1 if it is true. The operators are: less than (<), greater than (>), less than or equal (<=), and greater than or equal (>=). Both operands must have an arithmetic type or must be pointers to compatible types. The compiler performs the necessary arithmetic conversions before the comparison (see Section 6.10.1).
When two pointers are compared, the result depends on the relative locations of the two addressed objects. Pointers to objects at lower addresses are less than pointers to objects at higher addresses. If two addresses indicate elements in the same array, the address of an element with a lower subscript is less than the address of an element with a higher subscript.
The relational operators associate from left to right. Therefore, the following statement relates a to b, and if a is less than b, the result is 1 (true). If a is greater than or equal to b, the result is 0 (false). Then, 0 or 1 is compared with c for the expression result. This statement does not determine "if b is between a and c".
if ( a < b < c )
statement;
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To check if b is between a and c, use the following code:
if ( a < b && b < c )
statement;
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The equality operators, equal (:=,=) and not-equal (!=), produce a result of type int, so that the result of the following statement is 1 if both operands have the same value, and 0 if they do not:
a == b |
Operands must have one of the following type combinations:
Operands are converted, if necessary, according to the usual arithmetic conversion rules (see Section 6.10.1).
Two pointers or addresses are equal if they identify the same storage location.
Although different symbols are used for assignment (=) and equality (:=,=), C allows either operator in all contexts, so be careful not to confuse them. Consider the following example:
In this example, statement_1 always executes, because the result of the assignment x = 1 is equivalent to the value of x, which equals 1 (or true). |
The bitwise operators require integral operands. The usual arithmetic conversions are performed (see Section 6.10.1). The result of the expression is the bitwise AND (&), inclusive OR (|), or exclusive OR (^), of the two operands. The order of evaluation of their operands is not guaranteed.
The operands are evaluated bit by bit. The result of the & operator is 0 if one bit value is 0 and the other is 1, or if both bit values are 0. The result is 1 if both bit values are 1.
The result of the | operator is 0 if both bit values are 0. The result for each bit is 1 if either bit value is 1, or both bit values are 1.
The result of the ^ operator is 0 if both bit values are 0, or if both
bit values are 1. The result for each bit is 1 if either bit value is 1
and the other is 0.
6.5.7 Logical Operators
The logical operators are AND (&&) and OR (||). These operators guarantee left-to-right evaluation. The result of the expression (of type int) is either 0 (false) or 1 (true). The operands need not have the same type, but both types must be scalar. If the compiler can make an evaluation by examining only the left operand, the right operand is not evaluated. Consider the following expression:
E1 && E2 |
The result of this expression is 1 if both operands are nonzero, or 0 if one operand is 0. If expression E1 is 0, expression E2 is not evaluated because the result is the same regardless of E2's value.
Similarly, the following expression is 1 if either operand is nonzero, and 0 otherwise. If expression E1 is nonzero, expression E2 is not evaluated, because the result is the same regardless of E2's value.
E1 || E2 |
The conditional operator (?:) takes three operands. It tests the result of the first operand and then evaluates one of the other two operands based on the result of the first. Consider the following example:
E1 ? E2 : E3 |
If expression E1 is nonzero (true), then E2 is evaluated, and that is the value of the conditional expression. If E1 is 0 (false), E3 is evaluated, and that is the value of the conditional expression. Conditional expressions associate from right to left. In the following example, the conditional operator is used to get the minimum of x and y:
a = (x < y) ? x : y; /* a = min(x, y) */ |
There is a sequence point after the first expression (E1). The following example's result is predictable, and is not subject to unplanned side effects:
i++ > j ? y[i] : x[i]; |
The conditional operator does not produce an lvalue. Therefore, a statement such as a ? x : y = 10 is not valid.
The following restrictions apply:
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