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.SH
7.  Expressions
.LP
The precedence of expression operators is the same
as the order of the major
subsections of this section (highest precedence first).
Thus the expressions referred to as the operands of \fG+\fR
(\(sc7.4)
are those expressions defined in \(sc\(sc7.1-7.3.
Within each subsection, the operators have the same
precedence.
Left- or right-associativity is specified
in each subsection for the operators
discussed therein.
The precedence and associativity of all the expression
operators is summarized in the
collected grammar.
.PP
Otherwise the order of evaluation of expressions
is undefined.  In particular the compiler
considers itself free to
compute subexpressions in the order it believes
most efficient,
even if the subexpressions
involve side effects.
Expressions involving a commutative and associative
operator may be rearranged arbitrarily, even in the presence
of parentheses;
to force a particular order of evaluation
an explicit temporary must be used.
.SH
7.1  Primary expressions
.LP
Primary expressions
involving \fG.\fR\|, \fG\(mi>\fR, subscripting, and function calls
group left to right.
.SY
	primary-expression:
		\fIidentifier\fR
		\fIconstant\fR
		\fIstring\fR
		\fG(\fI expression \fG)\fR
		\fIprimary-expression\fG [\fI expression \fG]\fR
		\fIprimary-expression \fG( \fIexpression-list\*(op \fG)
		\fIprimary-lvalue \fG.\fI identifier\fR
		\fIprimary-expression \fG\(mi>\fI identifier\fR
.ES
.SY
	expression-list:
		expression
		expression-list \fG,\fI expression
.ES
An identifier is a primary expression, provided it has been
suitably declared as discussed below.
Its type is specified by its declaration.
However, if the type of the identifier is `array of .\|.\|.',
then the value of the identifier-expression
is a pointer
to the first object in the array, and the
type of the expression is
`pointer to .\|.\|.'.
Moreover, an array identifier is not an lvalue
expression.
Likewise, an identifier which is declared
`function returning .\|.\|.',
when used except in the function-name position
of a call, is converted to `pointer to function returning .\|.\|.'.
.PP
A decimal, octal, character, or floating
constant is a primary expression.
Its type may be
.Bd int,
.Bd long,
or
.Bd double
depending on its form.
.PP
A string is a primary expression.
Its type is originally `array of \fGchar\fR'; but following
the same rule given above for identifiers,
this is modified to `pointer to \fGchar\fR' and the
result is a pointer to the first character
in the string.
(There is an exception in certain initializers;
see \(sc8.6.)
.PP
A parenthesized expression is a primary expression
whose type and value are identical
to those of the unadorned expression.
The presence of parentheses does
not affect whether the expression is an
lvalue.
.PP
A primary expression followed by an expression in square
brackets is a primary expression.
The intuitive meaning is that of a subscript.
Usually, the primary expression has type `pointer to .\|.\|.',
the subscript expression is \fGint\fR,
and the type of the result is `\|.\|.\|.\|'.
The expression `E1[E2]' is
identical (by definition) to `\**\|(\|\|(\|E1\|)\|+\|(\|E2\|)\|\|)\|'.
All the clues
needed to understand
this notation are contained in this section together
with the discussions
in \(sc\(sc 7.1, 7.2, and 7.4 on identifiers,
\fG\**\fR, and \fG+\fR respectively;
\(sc14.3 below summarizes the implications.
.PP
A function call is a primary expression followed by parentheses
containing a possibly
empty, comma-separated list of expressions
which constitute the actual arguments to the
function.
The primary expression must be of type `function returning .\|.\|.',
and the result of the function call is of type `\|.\|.\|.\|'.
As indicated
below, a hitherto unseen identifier followed
immediately by a left parenthesis
is contextually declared
to represent a function returning
an integer;
thus in the most common case, integer-valued functions
need not be declared.
.PP
Any actual arguments of type \fGfloat\fR are
converted to \fGdouble\fR before the call;
any of type \fGchar\fR or \fGshort\fR are converted to \fGint\fR.
.PP
In preparing for the call to a function,
a copy is made of each actual parameter;
thus, all argument-passing in C is strictly by value.
A function may
change the values of its formal parameters, but
these changes cannot affect the values
of the actual parameters.
On the other hand, it is possible
to pass a pointer on the understanding
that the function may change the value
of the object to which the pointer points.
The order of evaluation of arguments is undefined by the language;
take note that the various compilers differ.
.PP
Recursive calls to any
function are permitted.
.PP
A primary expression followed by a dot followed by an identifier
is an expression.
The first expression must be an lvalue naming a structure or union, and the identifier
must name a member of the structure or union.
The result is an lvalue referring
to the named member of the structure or union.
.PP
A primary expression followed by an arrow (built from
a `\(mi' and a `>')
followed by an identifier
is an expression.
The first expression must be a pointer to a structure or a union
and the identifier must name a member of that structure or union.
The result is an lvalue referring to the named member
of the structure or union
to which the pointer expression points.
.PP
Thus the expression `E1\(mi>MOS' is the same as
`(\**E1)\fB.\fRMOS'.
Structures and unions are discussed in
\(sc8.5.
The rules given here for the use of
structures and unions
are not enforced strictly,
in order to allow an escape from the typing mechanism.
See \(sc14.1.
.SH
7.2  Unary operators
.LP
Expressions with unary operators
group right-to-left.
.SY
	unary-expression:
		\fG\**\fI expression\fR
		\fG&\fI lvalue\fR
		\fG\(mi\fI expression\fR
		\fG!\fI expression\fR
		\fG\*~\fI expression\fR
		++ \fIlvalue\fR
		\fR\(mi\(mi\fI lvalue\fR
		\fIlvalue ++
		\fIlvalue \(mi\(mi
		\fG( \fItype-name \fG) \fIexpression\fG
		\fGsizeof \fIexpression
		\fGsizeof ( \fItype-name \fG)
.ES
The unary \fG\**\fR operator
means
.It indirection:
the expression must be a pointer, and the result
is an lvalue referring to the object to
which the expression points.
If the type of the expression is `pointer to .\|.\|.',
the type of the result is `\|.\|.\|.\|'.
.PP
The result of the unary \fG&\fR operator is a pointer
to the object referred to by the
lvalue.
If the type of the lvalue is `\|.\|.\|.\|',
the type of the result is `pointer to .\|.\|.'.
.PP
The result
of the unary
.Bd \-
operator
is the negative of its operand.
The usual arithmetic conversions are performed.
The negative of an unsigned quantity is computed by
subtracting its value from
.MC
$2 sup wordsize$.
.mc
.PP
The result of the logical negation operator \fG!\fR
is 1 if the value of its operand is 0, 0 if the value of its
operand is non-zero.
The type of the result is \fGint\fR.
It is applicable to any arithmetic type
or to pointers.
.PP
The \*~ operator yields the one's complement of its operand.
The usual arithmetic conversions are performed.
The type of the operand must be integral.
.PP
The object referred to by the lvalue operand of prefix `++'
is incremented.
The value is the new value of the operand,
but is not an lvalue.
The expression `++a' is equivalent to
`(a@+=@1)'.
See the discussions of addition (\(sc7.4) and assignment
operators (\(sc7.14) for information on conversions.
.PP
The lvalue operand of prefix `\(mi\(mi' is decremented
analogously to the ++ operator.
.PP
When postfix `++' is applied to an lvalue
the result is the value of the object referred to by the lvalue.
After the result is noted, the object
is incremented in the same
manner as for the prefix ++ operator.
The type of the result is the same as the type of the lvalue expression.
.PP
When postfix `\(mi\(mi' is applied to an lvalue
the result is the value of the object referred to by the lvalue.
After the result is noted, the object
is decremented in the manner as for the prefix \(mi\(mi operator.
The type of the result is the same as the type of the lvalue
expression.
.PP
An expression preceded by the parenthesized name of a data type
causes conversion of the value of the expression to the named type.
The construction of type names is described in \(sc8.7.
.PP
The \fGsizeof\fR operator yields the size,
in bytes, of its operand.
(A
.It byte
is undefined by the language
except in terms of the value of
.Bd sizeof.
However in all existing implementations
a byte is the space required to hold a
.Bd char. "" )
When applied to an array, the result is the total
number of bytes in the array.
The size is determined from
the declarations of
the objects in the expression.
This expression is semantically an integer constant and may
be used anywhere a constant is required.
Its major use is in communication with routines
like storage allocators and I/O systems.
.PP
The \fGsizeof\fR operator
may also be applied to a parenthesized type name.
In that case it yields the size, in bytes, of an object
of the indicated type.
.PP
The construction `sizeof(type)' is taken to be a unit,
so the expression `sizeof(type)\(mi2' is the same as
`(sizeof(type))\(mi2'.
.SH
7.3  Multiplicative operators
.LP
The multiplicative operators
\**,
\fG/\fR, and \fG%\fR
group left-to-right.
The usual arithmetic conversions are performed.
.SY
	multiplicative-expression:
		\fIexpression\fG \** \fIexpression\fR
		\fIexpression \fG/\fI expression\fR
		\fIexpression \fG%\fI expression\fR
.ES
The binary \fG\**\fR operator indicates multiplication.
The \** operator is associative
and expressions with several multiplications at the same
level may be rearranged.
.PP
The binary \fG/\fR operator indicates division.
When positive integers are divided truncation is toward 0,
but the form of truncation is machine-dependent
if either operand is negative.
In all cases it is true that $(a/b) \** b~+~ a%b ~ == ~ a$.
.MC
On the \*(pd and \*I,
the remainder has the same sign as the dividend.
.mc
.PP
The binary \fG%\fR operator yields the remainder
from the division of the first expression by the second.
.MC
The usual arithmetic conversions are performed.
.mc
The operands must not be
floating.
.SH
7.4  Additive operators
.LP
The additive operators \fG+\fR and \fG\(mi\fR group left-to-right.
The usual arithmetic conversions are performed.
There are some additional type possibilities for each operator.
.SY
	additive-expression:
		\fIexpression \fG+\fI expression\fR
		\fIexpression \fG\(mi \fIexpression\fR
.ES
The result of the `+' operator is the sum of the operands.
A pointer to an object in an array and
a value of any integral type
may be added.
The latter is in all cases converted to
an address offset
by multiplying it
by the length of the object to which the
pointer points.
The result is a pointer
of the same type as the original pointer,
and which points to another object in the same array,
appropriately offset from the original object.
Thus if P is a pointer
to an object in an array, the expression `P+1' is a pointer
to the next object in the array.
.PP
No further type combinations are allowed.
.PP
The + operator is associative
and expressions with several additions at the same level may
be rearranged.
.PP
The result of the `\(mi' operator is the difference of the operands.
The usual arithmetic conversions are performed.
Additionally,
a value of any integral type
may be subtracted from a pointer,
and then the same conversions as for addition apply.
.PP
If two pointers to objects of the same type are subtracted,
the result is converted
(by division by the length of the object)
to an \fGint\fR representing the number of
objects separating
the pointed-to objects.
This conversion will in general give unexpected
results unless the pointers point
to objects in the same array, since pointers, even
to objects of the same type, do not necessarily differ
by a multiple of the object-length.
.SH
7.5  Shift operators
.LP
The shift operators \fG<<\fR and \fG>>\fR group left-to-right.
Both perform the usual arithmetic conversions on their operands,
each of which must be integral.
Then the right operand is converted to
.Bd int;
the type of the result is that of the left operand.
The result is undefined if the right operand is negative
or larger than the number of
bits in the object.
.SY
	shift-expression:
		\fIexpression \fG<< \fIexpression\fR
		\fIexpression \fG>> \fIexpression\fR
.ES
The value of `E1<<E2' is E1 (interpreted as a bit
pattern) left-shifted E2 bits;
vacated bits are 0-filled.
The value of `E1>>E2' is E1
right-shifted E2 bit positions.
The shift is guaranteed to be logical
(0-fill)
if E1 is
.Bd unsigned;
otherwise it may be
(and is, on the \*(pd)
arithmetic (fill by a copy of the sign bit).
.SH
7.6  Relational operators
.LP
The relational operators group left-to-right, but
this fact is not very useful; `a<b<c' does
not mean what it seems to.
.SY
	relational-expression:
		\fIexpression \fG<\fI expression\fR
		\fIexpression \fG>\fI expression\fR
		\fIexpression \fG<=\fI expression\fR
		\fIexpression \fG>=\fI expression\fR
.ES
The operators < (less than), > (greater than), <= (less than
or equal to) and >= (greater than or equal to)
all yield 0 if the specified relation is false
and 1 if it is true.
The type of the result is
.Bd int.
The usual arithmetic conversions are performed.
Two pointers may be compared,
and the result depends on the relative locations in the address space
of the pointed-to objects.
Pointer comparison is portable only when the pointers point to objects
in the same array.
.SH
7.7  Equality operators
.LP
.SY
	equality-expression:
		\fIexpression \fG==\fI expression\fR
		\fIexpression \fG!=\fI expression\fR
.ES
The \fG==\fR (equal to) and the \fG!=\fR (not equal to) operators
are exactly analogous to the relational
operators except for their lower
precedence.
(Thus `a<b\ ==\ c<d' is 1 whenever
a<b and c<d
have the same truth-value).
.PP
A pointer may be compared to an integer,
but the result is machine dependent unless the
integer is the constant 0.
A pointer to which 0 has been assigned is guaranteed
not to point to any object,
and will appear to be equal to 0;
in conventional usage, such a pointer is considered to be null.
.SH
7.8  Bitwise \fIand \fGoperator
.LP
.SY
	and-expression:
		\fIexpression \fG&\fI expression\fR
.ES
The \fG&\fR operator is associative
and expressions involving \fG&\fR may be rearranged.
The usual arithmetic conversions are performed;
the result is the bit-wise
`and' function of the operands.
The operator applies only to integral
operands.
.SH
7.9  Bitwise \fIexclusive \fIor \fGoperator
.LP
.SY
	exclusive-or-expression:
		\fIexpression \fG\*^ \fIexpression\fR
.ES
.LP
The \fG\*^\fR operator is associative
and expressions involving \*^ may be rearranged.
The usual arithmetic conversions are performed;
the result is
is the bit-wise `exclusive or' function of
the operands.
The operator applies only to integral
operands.
.SH
7.10  Bitwise \fIinclusive \fIor \fGoperator
.LP
.SY
	inclusive-or-expression:
		\fIexpression \|\(or \fIexpression\fR
.ES
The \(or operator is associative
and expressions with \(or may be rearranged.
The usual arithmetic conversions are performed;
the result is the bit-wise `inclusive or' function of its operands.
The operator applies only to integral
operands.
.SH
7.11  Logical \fIand \fGoperator
.LP
.SY
	logical-and-expression:
		expression \fG&&\fI expression
.ES
The \fG&&\fR operator groups left-to-right.
It returns 1 if both its operands
are non-zero, 0 otherwise.
Unlike \fG&\fR, \fG&&\fR guarantees left-to-right
evaluation; moreover the second operand is not evaluated
if the first operand is 0.
.PP
The operands need not have the same type, but each
must have one of the fundamental
types or be a pointer.
The result is always
.Bd int.
.SH
7.12  Logical \fIor \fGoperator
.LP
.SY
	logical-or-expression:
		\fIexpression \|\(or\(or \fIexpression
.ES
The \(or\(or operator groups left-to-right.
It returns 1 if either of its operands
is non-zero, and 0 otherwise.
Unlike \|\(or\|,
\|\(or\(or guarantees left-to-right evaluation; moreover,
the second operand is not evaluated
if the value of the first operand is non-zero.
.PP
The operands need not have the same type, but each
must
have one of the fundamental types
or be a pointer.
The result is always
.Bd int.
.SH
7.13  Conditional operator
.LP
.SY
	conditional-expression:
		\fIexpression \fG? \fIexpression \fG:\fI expression\fR
.ES
Conditional expressions group right-to-left.
The first expression is evaluated
and if it is non-zero, the result is the value of the
second expression, otherwise that of third expression.
If possible, the usual arithmetic conversions are performed
to bring the second and third expressions to a common type;
otherwise, if both are pointers of the same type,
the result has the common type;
otherwise, one must be a pointer and the other the constant 0,
and the result has the type of the pointer.
Only one of the second and third
expressions is evaluated.
.SH
7.14  Assignment operators
.LP
There are a number of assignment operators,
all of which group right-to-left.
All require an lvalue as their left operand,
and the type of an assignment expression is that
of its left operand.
The value is the value stored in the
left operand after the assignment has taken place.
The two parts of a compound assignment operator are separate
tokens.
.SY
	assignment-expression:
		\fIlvalue \fG= \fIexpression\fR
		\fIlvalue \fG+ = \fIexpression\fR
		\fIlvalue \fG\(mi = \fIexpression\fR
		\fIlvalue \fG\** = \fIexpression\fR
		\fIlvalue \fG/ = \fIexpression\fR
		\fIlvalue \fG% = \fIexpression\fR
		\fIlvalue \fG>> = \fIexpression\fR
		\fIlvalue \fG<< = \fIexpression\fR
		\fIlvalue \fG& = \fIexpression\fR
		\fIlvalue \fG\*^ = \fIexpression\fR
		\fIlvalue \fG\(or = \fIexpression\fR
.ES
Notice that the representation of the compound assignment operators
has changed; formerly the `=' came first and the other operator
came second (without any space).
The compiler continues to accept the previous notation.
.PP
In the simple assignment with `=',
the value of the expression replaces that of the object
referred
to by the lvalue.
If both operands have arithmetic type,
the right operand is converted to the type of the left
preparatory to the assignment.
.PP
The behavior of an expression
of the form `E1\ op =\ E2' may be inferred by
taking it as equivalent to `E1\ =\ E1\ op\ E2';
however, E1 is evaluated only once.
In
.Bd +=
and
.Bd \(mi=,
the left operand may be a pointer, in which case the (integral) right
operand is converted as explained
in \(sc7.4;
all right operands and all non-pointer left operands must
have arithmetic type.
.PP
The compiler currently allows a pointer to be assigned to an integer,
an integer to a pointer, and a pointer to a pointer of another type.
The assignment is a pure copy operation, with no conversion.
This usage is nonportable, and may produce pointers which
cause addressing exceptions when used.
However, it is guaranteed that assignment of the constant 0
to a pointer will produce a null pointer
distinguishable from a pointer to any object.
.SH
7.15  Comma operator
.LP
.SY
	comma-expression:
		\fIexpression \fG,\fI expression\fR
.ES
A pair of expressions separated by a comma is evaluated
left-to-right and the value of the left expression is
discarded.
The type and value of the result are the
type and value of the right operand.
This operator groups left-to-right.
In contexts where comma is given a special meaning,
for example in a list of actual arguments
to functions (\(sc7.1) and lists of initializers (\(sc8.6),
the comma operator as described in this section
can only appear in parentheses; for example,
`f(a, (t = 3, t+2), c)'
has three arguments, the second of which has the value 5.