c5

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

.ul
10. External definitions
.et
A C program consists of a sequence of external definitions.
External definitions may be given for functions, for
simple variables, and for arrays.
They are used both to declare and to reserve storage for
objects.
An external definition declares an identifier to
have storage class \fGextern\fR and
a specified type.
The type-specifier (\(sc8.2) may be empty, in which
case the type is taken to be \fGint\fR.
.ms
10.1  External function definitions
.et
Function definitions have the form
.dp 2
.ta .5i 1i 1.5i 2i 2.5i
	function-definition:
		type-specifier\*(op function-declarator function-body
.ed
A function declarator is similar to a declarator
for a ``function returning ...'' except that
it lists the formal parameters of
the function being defined.
.dp 2
	function-declarator:
		declarator \fG( \fIparameter-list\*(op \fG)

.ft I
	parameter-list:
		identifier
		identifier \fG,\fI parameter-list
.ed
The function-body
has the form
.dp 1
	function-body:
		type-decl-list function-statement
.ed
The purpose of the
type-decl-list is to give the types of the formal parameters.
No other identifiers should be declared in this list, and formal
parameters should be declared only here.
.pg
The function-statement
is just a compound
statement which may have
declarations at the start.
.dp 2
	function-statement:
		{ declaration-list\*(op statement-list }
.ed
A simple example of a complete function definition is
.sp .7
.ne 7
.nf
.bG
	int max^(^a, b, c)
	int a, b, c;
	{
		int m;
		m = ^(^a^>^b^)? a^:^b^;
		return^(^m^>^c? m^:^c^)^;
	}
.sp .7
.eG
.fi
Here ``int'' is the type-specifier;
``max(a,@b,@c)'' is the function-declarator;
``int@a,@b,@c;'' is the type-decl-list for
the formal
parameters;
``{@.^.^.@}'' is the function-statement.
.pg
C converts all
\fGfloat\fR actual parameters
to \fGdouble\fR, so formal parameters declared \fGfloat\fR
have their declaration adjusted to read \fGdouble\fR.
Also, since a reference to an array in any context
(in particular as an actual parameter)
is taken to mean
a pointer to the first element of the array,
declarations of formal parameters declared ``array of ...''
are adjusted to read ``pointer to ...''.
Finally, because neither structures
nor functions can be passed
to a function, it is useless to declare
a formal parameter to be a structure or
function (pointers to structures or
functions are of course permitted).
.pg
A free \fGreturn\fR statement is supplied at the
end of each function definition, so
running off the end
causes
control, but no value, to be returned to
the caller.
.ms
10.2  External data definitions
.et
An external data definition has the form
.dp 2
	data-definition:
		\fGextern\*(op \fItype-specifier\*(op init-declarator-list\*(op \fG;
.ed
The optional
.ft G
extern
.ft R
specifier is discussed in \(sc 11.2.
If given, the init-declarator-list is a
comma-separated list of declarators each of which may be
followed by an initializer for the declarator.
.dp 2
	init-declarator-list:
		init-declarator
		init-declarator \fG, \fIinit-declarator-list
.ed
.dp 2
	init-declarator:
		declarator initializer\*(op
.ed
Each initializer represents the initial value for the
corresponding object
being defined (and declared).
.dp 5
	initializer:
		constant
		{ constant-expression-list }
.ed
.dp 3
	constant-expression-list:
		constant-expression
		constant-expression \fG,\fI constant-expression-list
.ed
Thus an initializer consists of
a constant-valued expression, or
comma-separated list of expressions,
inside braces.
The braces may be dropped
when the expression is just a plain
constant.
The exact meaning of a constant expression is discussed
in \(sc15.
The expression list is used to initialize arrays;
see below.
.pg
The type of the identifier being
defined
should be compatible
with the type of the initializer:
a \fGdouble\fR constant
may initialize a \fGfloat\fR or \fGdouble\fR identifier;
a non-floating-point expression
may
initialize an \fGint\fR, \fGchar\fR, or pointer.
.pg
An initializer for an array may contain
a comma-separated list of compile-time expressions.
The length of the array is taken to be the
maximum of the number of expressions in the list
and the square-bracketed constant in the
array's declarator.
This constant may be missing,
in which case 1 is used.
The expressions initialize successive members of the array
starting at the origin (subscript 0)
of the array.
The acceptable expressions for an array of type
``array of ...'' are the same as
those for type ``...''.
As a special case, a single
string may be given as the initializer
for an array of \fGchar\fRs; in this case,
the characters in the string are taken
as the initializing values.
.pg
Structures can be initialized, but this operation
is incompletely implemented and machine-dependent.
Basically the structure is regarded as a sequence
of words and the initializers are placed into those
words.
Structure initialization,
using a comma-separated list in braces,
is safe if all the members of the structure
are integers or pointers but is otherwise ill-advised.
.pg
The initial value of any externally-defined
object not explicitly initialized is guaranteed
to be 0.
.ul
11.  Scope rules
.et
A complete C program need not all
be compiled at the same time: the source text of the
program
may be kept in several files, and precompiled
routines may be loaded from
libraries.
Communication among the functions of a program
may be carried out both through explicit calls
and through manipulation of external data.
.pg
Therefore, there are two kinds of scope to consider:
first, what may be called the \fIlexical scope\fR
of an identifier, which is essentially the
region of a program during which it may
be used without drawing ``undefined identifier''
diagnostics;
and second, the scope
associated with external identifiers,
which is characterized by the rule
that references to the same external
identifier are references to the same object.
.ms
11.1  Lexical scope
.et
C is not a block-structured language;
this may fairly be considered a defect.
The lexical scope of names declared in external definitions
extends from their definition through
the end of the file
in which they appear.
The lexical scope of names declared at the head of functions
(either as formal parameters
or in the declarations heading the
statements constituting the function itself)
is the body of the function.
.pg
It is an error to redeclare identifiers already
declared in the current context,
unless the new declaration specifies the same type
and storage class as already possessed by the identifiers.
.ms
11.2  Scope of externals
.et
If a function declares an identifier to be
\fGextern\fR,
then somewhere among the files or libraries
constituting the complete program
there must be an external definition
for the identifier.
All functions in a given program which refer to the same
external identifier refer to the same object,
so care must be taken that the type and extent
specified in the definition
are compatible with those specified
by each function which references the data.
.pg
In \s8PDP\s10-11 C,
it is explicitly permitted for (compatible)
external definitions of the same identifier
to be present in several of the
separately-compiled pieces of a complete program,
or even twice within the same program file,
with the important limitation that the identifier
may be initialized in at most one of the
definitions.
In other operating systems, however, the compiler must know
in just which file the storage
for the identifier is allocated, and in which file
the identifier is merely being referred to.
In the implementations of C for such systems,
the appearance of the
.ft G
extern
.ft R
keyword before an external definition
indicates that storage for the identifiers
being declared
will be allocated in another file.
Thus in a multi-file program,
an external data definition without
the
.ft G
extern
.ft R
specifier must appear in exactly one of the files.
Any other files which wish to give an external definition
for the identifier must
include the
.ft G
extern
.ft R
in the definition.
The identifier can be initialized only in the file
where storage is allocated.
.pg
In \s8PDP\s10-11 C none of this nonsense is necessary
and the
.ft G
extern
.ft R