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UNIX v6源代码 这几乎是最经典的unix版本 unix操作系统设计和莱昂氏unix源代码分析都是用的该版

former is converted to \fGdouble\fR, and the result is \fGdouble\fR;
if both are \fGfloat\fR or \fGdouble\fR, the result is \fGdouble\fR.
No other combinations are allowed.
.ms
7.3.2  \fIexpression \fG/\fI expression\fR
.et
The binary \fG/\fR operator indicates division.
The same type considerations as for multiplication
apply.
.ms
7.3.3  \fIexpression \fG%\fI expression\fR
.et
The binary \fG%\fR operator yields the remainder
from the division of the first expression by the second.
Both operands must be \fGint\fR or \fGchar\fR, and the
result is \fGint\fR.
In the current implementation,
the remainder has the same sign as the dividend.
.ms
7.4  Additive operators
.et
The additive operators \fG+\fR and \fG\(mi\fR group left-to-right.
.ms
7.4.1  \fIexpression \fG+\fI expression\fR
.et
The result is the sum of the expressions.
If both operands are \fGint\fR or \fGchar\fR,
the result is \fGint\fR.
If both
are \fGfloat\fR or \fGdouble\fR, the result is \fGdouble\fR.
If one is \fGchar\fR or \fGint\fR and one is \fGfloat\fR
or \fGdouble\fR, the former is converted to \fGdouble\fR and the result is \fGdouble\fR.
If an \fGint\fR or \fGchar\fR is added to a pointer, the former
is converted by multiplying it
by the length of the object to which the
pointer points and the result is a pointer
of the same type as the original pointer.
Thus if P is a pointer
to an object, the expression ``P+1'' is a pointer
to another object of the same type as
the first and immediately following
it in storage.
.pg
No other type combinations are allowed.
.ms
7.4.2  \fIexpression \fG\(mi \fIexpression\fR
.et
The result is the difference of the operands.
If both operands are \fGint\fR, \fGchar\fR, \fGfloat\fR, or \fGdouble\fR,
the same type considerations
as for \fG+\fR apply.
If an \fGint\fR or \fGchar\fR is subtracted
from a pointer, the
former is converted in the same way as explained under
\fG+\fR above.
.pg
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.
.ms
7.5  Shift operators
.et
The shift operators \fG<<\fR and \fG>>\fR group left-to-right.
.ms
7.5.1  \fIexpression \fG<< \fIexpression\fR
.br
7.5.2  \fIexpression \fG>> \fIexpression\fR
.et
Both operands must be \fGint\fR or \fGchar\fR,
and the result is \fGint\fR.
The second operand should
be non-negative.
The value of ``E1<<E2'' is E1 (interpreted as a bit
pattern 16 bits long) left-shifted E2 bits;
vacated bits are 0-filled.
The value of ``E1>>E2'' is E1 (interpreted as a two's
complement, 16-bit quantity) arithmetically
right-shifted E2 bit positions.
Vacated bits are filled by a copy of the sign bit of E1.
[Note: the use
of arithmetic rather than logical shift does not
survive transportation between machines.]
.ms
7.6  Relational operators
.et
The relational operators group left-to-right, but
this fact is not very useful; ``a<b<c'' does
not mean what it seems to.
.ms
7.6.1  \fIexpression \fG<\fI expression\fR
.br
.ne 4
7.6.2  \fIexpression \fG>\fI expression\fR
.br
.ne 4
7.6.3  \fIexpression \fG<=\fI expression\fR
.br
.ne 4
7.6.4  \fIexpression \fG>=\fI expression\fR
.et
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.
Operand conversion is exactly the same as for the \fG+\fR
operator except that pointers of any kind may be compared;
the result in this case depends on the relative
locations in storage of the pointed-to objects.
It does not seem to be very mean$ing$ful
to compare pointers with integers
other than 0.
.ms
.ti 0
7.7  Equality operators
.et
.ne 4
.ti 0
7.7.1  \fIexpression \fG==\fI expression\fR
.br
.ne 4
7.7.2  \fIexpression \fG!=\fI expression\fR
.et
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).
.ms
7.8  \fIexpression \fG&\fI expression\fR
.et
The \fG&\fR operator groups left-to-right.
Both operands must be \fGint\fR or \fGchar\fR;
the result is an \fGint\fR which is the bit-wise
logical \fGand\fR function of the operands.
.ms
.tr ^^
7.9  \fIexpression \fG^ \fIexpression\fR
.et
The \fG^\fR operator groups left-to-right.
The operands must be \fGint\fR or \fGchar\fR; the result is an \fGint\fR
which is the bit-wise exclusive \fGor\fR function of
its operands.
.tr ^\|
.ms
7.10  \fIexpression ^\(or \fIexpression\fR
.et
The \(or operator groups left-to-right.
The operands must be \fGint\fR or \fGchar\fR; the result is an \fGint\fR
which is the bit-wise inclusive \fGor\fR of its operands.
.ms
7.11  \fIexpression \fG&&\fI expression
.et
The \fG&&\fR operator 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.
.pg
The operands need not have the same type, but each
must have one of the fundamental
types or be a pointer.
.ms
7.12  \fIexpression ^\(or\(or \fIexpression
.et
The ^\(or\(or operator 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.
.pg
The operands need not have the same type, but each
must
have one of the fundamental types
or be a pointer.
.ms
7.13  \fIexpression \fG? \fIexpression \fG:\fI expression\fR
.et
Conditional expressions group left-to-right.
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 the types of the second and third operand are the same, the
result has their common type;
otherwise the same conversion rules as for \fG+\fR
apply.
Only one of the second and third
expressions is evaluated.
.ms
7.14  Assignment operators
.et
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.
.ms
7.14.1  \fIlvalue \fG= \fIexpression\fR
.et
The value of the expression replaces that of the object
referred
to by the lvalue.
The operands need not have the same type, but
both must be
\fGint\fR, \fGchar\fR, \fGfloat\fR, \fGdouble\fR,
or pointer.
If neither operand is a pointer,
the assignment takes place as expected, possibly
preceded by conversion of the expression on the right.
.pg
When both operands are \fGint\fR
or pointers of any kind, no conversion
ever takes place;
the value of the expression is simply stored
into the object referred to by the lvalue.
Thus it is possible to
generate pointers which will cause addressing
exceptions when used.
.ms
.ta \w'0.00.00 'u
7.14.2	\fIlvalue \fG=+ \fIexpression\fR
.br
7.14.3	\fIlvalue \fG=\(mi \fIexpression\fR
.br
7.14.4	\fIlvalue \fG=\** \fIexpression\fR
.br
7.14.5	\fIlvalue \fG=/ \fIexpression\fR
.br
7.14.6	\fIlvalue \fG=% \fIexpression\fR
.br
7.14.7	\fIlvalue \fG=>> \fIexpression\fR
.br
7.14.8	\fIlvalue \fG=<< \fIexpression\fR
.br
7.14.9	\fIlvalue \fG=& \fIexpression\fR
.br
.tr ^^
7.14.10	\fIlvalue \fG=^ \fIexpression\fR
.br
.tr ^\|
7.14.11	\fIlvalue \fG=^\(or \fIexpression\fR
.et
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.
Moreover,
expressions like ``i@=+@p''
in which a pointer is added to an integer, are forbidden.
.ms
7.15  \fIexpression \fG,\fI expression\fR
.et
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.
It should be avoided in situations where comma is given
a special meaning, for example in actual arguments
to function calls (\(sc7.1.6) and lists of initializers (\(sc10.2).