V6/usr/doc/c/c5
.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
specifier is ignored in external definitions.
.ul
12. Compiler control lines
.et
When a line of a C program begins
with the character \fG#\fR, it is interpreted not by
the compiler itself, but by a preprocessor which is capable
of replacing instances of given identifiers
with arbitrary token-strings and of inserting
named files into the source program.
In order to cause this preprocessor to be invoked, it
is necessary that the very first line of the program
begin with \fG#\fR.
Since null lines are ignored by the preprocessor,
this line need contain no other information.
.ms
12.1 Token replacement
.et
A compiler-control line of the form
.dp 1
\fG# define \fIidentifier token-string
.ed
(note: no trailing semicolon)
causes the preprocessor to replace subsequent instances
of the identifier with the given string of tokens
(except within compiler control lines).
The replacement token-string has comments removed from
it, and it is surrounded with blanks.
No rescanning of the replacement string is attempted.
This facility is most valuable for definition of ``manifest constants'',
as in
.sp .7
.nf
.bG
# define tabsize 100
.^.^.
int table[tabsize];
.sp .7
.eG
.fi
.ms
12.2 File inclusion
.et
Large C programs
often contain many external data definitions.
Since the lexical scope of external definitions
extends to the end of the program file,
it is good practice
to put all the external definitions for
data at the start of the
program file, so that the functions defined within
the file need not repeat tedious and error-prone
declarations for each external identifier they use.
It is also useful to put a heavily used structure definition
at the start and use its structure tag to declare
the \fGauto\fR pointers to the structure
used within functions.
To further exploit
this technique when a large C program
consists of several files, a compiler control line of
the form
.dp 1
\fG# include "\fIfilename^\fG"
.ed
results in the replacement of that
line by the entire contents of the file
\fIfilename\fR.
.pg
.ul
13. Implicit declarations
.et
It is not always necessary to specify
both the storage class and the type
of identifiers in a declaration.
Sometimes the storage class is supplied by
the context: in external definitions,
and in declarations of formal parameters
and structure members.
In a declaration inside a function,
if a storage class but no type
is given, the identifier is assumed
to be \fGint\fR;
if a type but no storage class is indicated,
the identifier is assumed to
be \fGauto\fR.
An exception to the latter rule is made for
functions, since \fGauto\fR functions are mean$ing$less
(C being incapable of compiling code into the stack).
If the type of an identifier is ``function returning ...'',
it is implicitly declared to be \fGextern\fR.
.pg
In an expression, an identifier
followed by \fG(\fR and not currently declared
is contextually
declared to be ``function returning \fGint\fR''.
.pg
Undefined identifiers not followed by \fG(\fR
are assumed to be labels which
will be defined later in the function.
(Since a label is not an lvalue, this accounts
for the ``Lvalue required'' error
message
sometimes noticed when
an undeclared identifier is used.)
Naturally, appearance of
an identifier as a label declares it as such.
.pg
For some purposes it is best to consider formal
parameters as belonging to their own storage class.
In practice, C treats parameters as if they
were automatic (except that, as mentioned
above, formal parameter arrays and \fGfloat\fRs
are treated specially).
.ul
14. Types revisited
.et
This section summarizes the operations
which can be performed on objects of certain types.
.ms
14.1 Structures
.et
There are only two things that can be done with
a structure:
pick out one of its members (by means of the
\fG^.^\fR or \fG\(mi>\fR operators); or take its address (by unary \fG&\fR).
Other operations, such as assigning from or to
it or passing it as a parameter, draw an
error message.
In the future, it is expected that
these operations, but not necessarily
others, will be allowed.
.ms
14.2 Functions
.et
There are only two things that
can be done with a function:
call it, or take its address.
If the name of a function appears in an
expression not in the function-name position of a call,
a pointer to the function is generated.
Thus, to pass one function to another, one
might say
.bG
.sp .7
.nf
int f(^^);
...
g(^f^);
.sp .7
.eG
.ft R
Then the definition of \fIg \fRmight read
.sp .7
.bG
g^(^funcp^)
int (\**funcp)^(^^);
{
.^.^.
(\**funcp)^(^^);
.^.^.
}
.sp .7
.eG
.fi
Notice that \fIf\fR was declared
explicitly in the calling routine since its first appearance
was not followed by \fG(\fR^.
.ms
14.3 Arrays, pointers, and subscripting
.et
Every time an identifier of array type appears
in an expression, it is converted into a pointer
to the first member of the array.
Because of this conversion, arrays are not
lvalues.
By definition, the subscript operator
\fG[^]\fR is interpreted
in such a way that
``E1[E2]'' is identical to
``\**(^(^E1)^+^(E2^)^)''.
Because of the conversion rules
which apply to \fG+\fR, if E1 is an array and E2 an integer,
then E1[E2] refers to the E2-th member of E1.
Therefore,
despite its asymmetric
appearance, subscripting is a commutative operation.
.pg
A consistent rule is followed in the case of
multi-dimensional arrays.
If E is an \fIn^\fR-dimensional
array
of rank
.ft I
i^\(mu^j^\(mu^.^.^.^\(muk,
.ft R
then E appearing in an expression is converted to
a pointer to an (\fIn\fR\(mi1)-dimensional
array with rank
.ft I
j^\(mu^.^.^.^\(muk.
.ft R
If the \fG\**\fR operator, either explicitly
or implicitly as a result of subscripting,
is applied to this pointer,
the result is the pointed-to (\fIn\fR\(mi1)-dimensional array,
which itself is immediately converted into a pointer.
.pg
For example, consider
.sp .7
.bG
int x[3][5];
.sp .7
.eG
Here \fIx\fR is a 3\(mu5 array of integers.
When \fIx\fR appears in an expression, it is converted
to a pointer to (the first of three) 5-membered arrays of integers.
In the expression ``x[^i^]'',
which is equivalent to ``\**(x+i)'',
\fIx\fR is first converted to a pointer as described;
then \fIi\fR is converted to the type of \fIx\fR,
which involves multiplying \fIi\fR by the
length the object to which the pointer points,
namely 5 integer objects.
The results are added and indirection applied to
yield an array (of 5 integers) which in turn is converted to
a pointer to the first of the integers.
If there is another subscript the same argument applies
again; this time the result is an integer.
.pg
It follows from all this that arrays in C are stored
row-wise (last subscript varies fastest)
and that the first subscript in the declaration helps determine
the amount of storage consumed by an array
but plays no other part in subscript calculations.
.ms
14.4 Labels
.et
Labels do not
have a type of their own; they are
treated
as having type ``array of \fGint\fR''.
Label variables should be declared ``pointer
to \fGint\fR'';
before execution of a \fGgoto\fR referring to the
variable,
a label (or an expression
deriving from a label) should be assigned to the
variable.
.pg
Label variables are a bad idea in general;
the \fGswitch\fR statement makes them
almost always unnecessary.
.ul
15. Constant expressions
.et
In several places C requires expressions which evaluate to
a constant:
after
.ft G
case,
.ft R
as array bounds, and in initializers.
In the first two cases, the expression can
involve only integer constants, character constants,
and
.ft G
sizeof
.ft R
expressions, possibly
connected by the binary operators
.tr ^^
.dp
+ \(mi \** / % & \(or ^ << >>
.ed
or by the unary operators
.dp
\(mi \*~
.ed
.tr ^\|
Parentheses can be used for grouping,
but not for function calls.
.pg
A bit more latitude is permitted for initializers;
besides constant expressions as discussed above,
one can also apply the unary \fG&\fR
operator to external scalars,
and to external arrays subscripted
with a constant expression.
The unary \fG&\fR can also
be applied implicitly
by appearance of unsubscripted external arrays.
The rule here is that initializers must
evaluate either to a constant or to the address
of an external identifier plus or minus a constant.