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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

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9.  Statements
.et
Except as indicated, statements are executed in sequence.
.ms
9.1  Expression statement
.et
Most statements are expression statements, which have
the form
.dp 1
	expression \fG;
.ed
Usually expression statements are assignments or function
calls.
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9.2  Compound statement
.et
So that several statements can be used where one is expected,
the compound statement is provided:
.dp 2
	compound-statement:
		{ statement-list }

	statement-list:
		statement
		statement statement-list
.ed
9.3  Conditional statement
.et
The two forms of the conditional statement are
.dp 2
	\fGif ( \fIexpression \fG) \fIstatement
	\fGif ( \fIexpression \fG) \fIstatement \fGelse \fIstatement
.ed
In both cases the expression is evaluated
and if it is non-zero, the first substatement
is executed.
In the second case the second substatement is executed
if the expression is 0.
As usual the ``else'' ambiguity is resolved by connecting
an \fGelse\fR with the last encountered elseless \fGif\fR.
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9.4  While statement
.et
The \fGwhile\fR statement has the form
.dp 1
	\fGwhile ( \fIexpression \fG) \fIstatement
.ed
The substatement is executed repeatedly
so long as the value of the 
expression remains non-zero.
The test takes place before each execution of the
statement.
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9.5  Do statement
.et
The \fGdo\fR statement has the form
.dp 1
	\fGdo \fIstatement \fGwhile ( \fIexpression \fG) ;
.ed
The substatement is executed repeatedly until
the value of the expression becomes zero.
The test takes place after each execution of the
statement.
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9.6  For statement
.et
The \fGfor\fR statement has the form
.dp 1
.ft G
	for ( \fIexpression-1\*(op \fG; \fIexpression-2\*(op \fG ; \c
\fIexpression-3\*(op \fG) \fIstatement
.ed
This statement is equivalent to
.dp 5
	expression-1;
	\fGwhile (\fI^expression-2\fG^) {
		\fIstatement
		expression-3^\fG;
	}
.ed
Thus the first expression specifies initialization
for the loop; the second specifies
a test, made before each iteration, such
that the loop is exited when the expression becomes
0;
the third expression typically specifies an incrementation
which is performed after each iteration.
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Any or all of the expressions may be dropped.
A missing
.ft I
expression-2
.ft R
makes the
implied \fGwhile\fR clause equivalent to ``while(^1^)'';
other missing expressions are simply
dropped from the expansion above.
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9.7  Switch statement
.et
The \fGswitch\fR statement causes control to be transferred
to one of several statements depending on
the value of an expression.
It has the form
.dp 1
	\fGswitch ( \fIexpression \fG) \fIstatement
.ed
The expression must be \fGint\fR or \fGchar\fR.
The statement is typically compound.
Each statement within the statement
may be labelled with case prefixes
as follows:
.dp 1
	\fGcase \fIconstant-expression \fG:
.ed
where the constant
expression
must be \fGint\fR or \fGchar\fR.
No two of the case constants in a switch
may have the same value.
Constant expressions are precisely defined in \(sc15.
.pg
There may also be at most one statement prefix of the
form
.dp 1
	\fGdefault :
.ed
When the \fGswitch\fR statement is executed, its expression
is evaluated and compared with each case constant in
an undefined order.
If one of the case constants is
equal to the value of the expression,
control is passed to the statement
following the matched case prefix.
If no case constant matches the expression,
and if there is a \fGdefault\fR prefix, control
passes to the prefixed
statement.
In the absence of a \fGdefault\fR prefix
none of the statements in the
switch is executed.
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Case or default prefixes in themselves
do not alter the flow of control.
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9.8  Break statement
.et
The statement
.dp 1
	\fGbreak ;
.ed
causes termination of the smallest enclosing \fGwhile\fR, \fGdo\fR,
\fGfor\fR, or \fGswitch\fR statement;
control passes to the
statement following the terminated statement.
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9.9  Continue statement
.et
The statement
.dp 1
	\fGcontinue ;
.ed
causes control to pass to the loop-continuation portion of the
smallest enclosing \fGwhile\fR,
\fGdo\fR, or \fGfor\fR
statement; that is to the end of the loop.
More precisely, in each of the statements
.dp 4
.bG
.ta .5i 2.5i 4.5i
	while (^^.^.^.^^) {	do {	for (^^.^.^.^^) {
	  ^.^.^.^	  ^.^.^.^	  ^.^.^.^
	contin:^;	contin:^;	contin:^;
	}	} while (^^.^.^.^^)^;	}
.ed
.eG
.ta .5i 1i 1.5i 2i 2.5i 3i
a \fGcontinue\fR is equivalent to ``goto contin''.
.ms
9.10  Return statement
.et
A function returns to its caller by means of
the \fGreturn\fR statement, which has one of the
forms
.dp 2
.ft G
	return ;
	return ( \fIexpression \fG) ;
.ed
In the first case no value is returned.
In the second case, the value of the expression
is returned to the caller
of the function.
If required, the expression is converted,
as if by assignment, to the type of the
function in which it appears.
Flowing off the end of a function is
equivalent to a return with no returned value.
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9.11  Goto statement
.et
Control may be transferred unconditionally by means of
the statement
.dp 1
.ft G
	goto \fIexpression \fG;
.ed
The expression should be a label (\(sc\(sc9.12, 14.4)
or an expression of type ``pointer to \fGint\fR''
which evaluates to a label.
It is illegal to transfer to a label
not located in the current function
unless some extra-language provision
has been made to adjust
the stack correctly.
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9.12  Labelled statement
.et
Any statement may be preceded by
label prefixes of the form
.dp 1
	identifier \fG:
.ed
which serve to declare the identifier
as a label.
More details on the semantics of labels are
given in \(sc14.4 below.
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9.13  Null statement
.et
The null statement has the form
.dp 1
	\fG;
.ed
A null statement is useful to carry a label just before the ``}''
of a compound statement or to supply a null
body to a looping statement such as \fGwhile\fR.