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New C Compiler Features

1.  Long and short integers

In the interests of portability, C integers now come in three flavours:
`long', `short', and just plain `int'.
A short integer is 16 bits long on both the Interdata X/32 and X/16, as well
as on the PDP-11;
a long integer is 32 bits long on any machine;
an ordinary int is the "natural" size for the host machine:
16 bits on Interdata X/16 and PDP-11, 32 bits on Interdata X/32.

The new type keywords can act rather like adjectives in that
`short int' means a 16-bit integer and `long float'
means the same as `double'.
Or they may stand alone, implying integer types.

Long constants are written with a terminating `l' or `L'.
E.g. "123L" or "0177777777L" or "0X56789abcdL".
The latter is a hex constant, which could also have been short;
it is marked by starting with "0X".
Every fixed decimal constant larger than 32767 is taken to
be long, and so are octal or hex constants larger than
0177777 (0Xffff, or 0XFFFF if you like).
A warning is given in such a case since this is actually
an incompatibility with the older compiler.
Where the constant is just used as an initializer or
assigned to something it doesn't matter.
If it is passed to a subroutine
then the routine will not get what it expected.

When a short and a long integer are
operands of an arithmetic operator,
the short is converted to long (with sign extension).
This is true also when a short is assigned to a long.
When a long is assigned to a short integer it
is truncated at the high order end with no notice
of possible loss of significant digits.
This is true as well when a long is added to a 16-bit pointer
(which includes its usage as a subscript).
The conversion rules for expressions involving
doubles and floats mixed with longs
are the same as those for short integers,
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mutatis mutandis.

A point to note is that constant expressions involving
longs are not evaluated at compile time,
and may not be used where constants are expected.
Thus
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	long x {5000L*5000L};
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is illegal;
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	long x {5000*5000};
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is legal but wrong because the high-order part is lost;
but both
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	long x 25000000L;
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and
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	long x 25.e6;
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are correct
and have the same meaning
because the double constant is converted to long at compile time.

2.  Unsigned integers

A new fundamental data type with keyword `unsigned,' is
available.  It may be used alone:
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        unsigned u;
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or as an adjective with `int'
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        unsigned int u;
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with the same meaning.  There are not yet (or possibly ever)
unsigned longs, shorts or chars.  The meaning of an unsigned variable is
that of an integer modulo 2^n, where n is wordsize of the machine.  All
operators whose operands are unsigned produce results consistent
with this interpretation except division and remainder where the
divisor is larger than 2^(n-1)-1; then the result is incorrect.  The
dividend in an unsigned division may however have any value (i.e.
up to 2^n-1) with correct results.  Right shifts of unsigned
quantities are guaranteed to be logical shifts.

When an ordinary integer and an unsigned integer are combined
then the ordinary integer is mapped into an integer mod 2^n and
the result is unsigned.  Thus, for example `u = -1' results in
assigning 65535 (on a 16-bit machine) or 4294967295 (on a 32-bit machine) to u.
This is mathematically reasonable, and
also happens to involve no run-time overhead.

When an unsigned integer is assigned to a plain integer, an
(undiagnosed) overflow occurs when the unsigned integer exceeds
2^(n-1)-1.

It is intended that unsigned integers be used in contexts where
previously character pointers were used (artificially and
nonportably) to represent unsigned integers.

3.  Block structure.

A sequence of declarations may now appear at the beginning of any
compound statement in {}.  The variables declared thereby are
local to the compound statement.  Any declarations of the same
name existing before the block was entered are pushed down for
the duration of the block.  Just as in functions, as before, auto
variables disappear and lose their values when the block is left;
static variables retain their values.  Also according to the same
rules as for the declarations previously allowed at the start of
functions, if no storage class is mentioned in a declaration the
default is automatic.

Implementation of inner-block declarations is such that there is
no run-time cost associated with using them.

4.  Initialization (part 1)

This compiler properly handles initialization of structures
so the construction

 	struct { char name[8]; char type; float val; } x
 		{ "abc", 'a', 123.4 };

compiles correctly.
In particular it is recognized that the string is supposed
to fill an 8-character array, the `a' goes into a character,
and that the 123.4 must be rounded and placed in a single-precision
cell.
Structures of arrays, arrays of structures, and the like all work;
a more formal description of what is done follows.

<initializer> ::= <element>

<element> ::= <expression> | <element> , <element> |
 		{ <element> } | { <element> , }

An element is an expression or a comma-separated sequence of
elements possibly enclosed in braces.
In a brace-enclosed
sequence, a comma is optional after the last element.
This very
ambiguous definition is parsed as described below.
"Expression"
must of course be a constant expression within the previous
meaning of the Act.

An initializer for a non-structured scalar is an element with
exactly one expression in it.

An "aggregate" is a structure or an array.
If the initializer
for an aggregate begins with a left brace, then the succeeding
comma-separated sequence of elements initialize the members of
the aggregate.
It is erroneous for the number of members in the
sequence to exceed the number of elements in the aggregate.
If
the sequence has too few members the aggregate is padded.

If the initializer for an aggregate does not begin with a left
brace, then the members of the aggregate are initialized with
successive elements from the succeeding comma-separated sequence.
If the sequence terminates before the aggregate is filled the
aggregate is padded.

The "top level" initializer is the object which initializes an
external object itself, as opposed to one of its members.
The
top level initializer for an aggregate must begin with a left
brace.

If the top-level object being initialized is an array and if its
size is omitted in the declaration, e.g. "int a[]", then the size
is calculated from the number of elements which initialized it.

Short of complete assimilation of this description, there are two
simple approaches to the initialization of complicated objects.
First, observe that it is always legal to initialize any object
with a comma-separated sequence of expressions.
The members of
every structure and array are stored in a specified order, so the
expressions which initialize these members may if desired be laid
out in a row to successively, and recursively, initialize the
members.

Alternatively, the sequences of expressions which initialize
arrays or structures may uniformly be enclosed in braces.

5.  Initialization (part 2)

Declarations, whether external, at the head of functions, or
in inner blocks may have initializations whose syntax is the same
as previous external declarations with initializations.  The only
restrictions are that automatic structures and arrays may not be
initialized (they can't be assigned either); nor, for the moment
at least, may external variables when declared inside a function.

The declarations and initializations should be thought of as
occurring in lexical order so that forward references in
initializations are unlikely to work.  E.g.,

        { int a a;
          int b c;
          int c 5;
          ...
        }

Here a is initialized by itself (and its value is thus
undefined); b is initialized with the old value of c (which is
either undefined or any c declared in an outer block).

6.  Bit fields

A declarator inside a structure may have the form

	<declarator> : <constant>

which specifies that the object declared is stored in a field
the number of bits in which is specified by the constant.
If several such things are stacked up next to each other
then the compiler allocates the fields from right to left,
going to the next word
when the new field will not fit.
The declarator may also have the form

	: <constant>

which allocates an unnamed field to simplify accurate
modelling of things like hardware formats where there are unused
fields.
Finally,

	: 0

means to force the next field to start on a word boundary.

The types of bit fields can be only "int" or "char".
The only difference between the two
is in the alignment and length restrictions:
no int field can be longer than 16 bits, nor any char longer
than 8 bits.
If a char field will not fit into the current character,
then it is moved up to the next character boundary.

Both int and char fields
are taken to be unsigned (non-negative)
integers.

Bit-field variables are not quite full-class citizens.
Although most operators can be applied to them,
including assignment operators,
they do not have addresses (i.e. there are no bit pointers)
so the unary & operator cannot be applied to them.
For essentially this reason there are no arrays of bit field
variables.

There are three twoes in the implementation:
addition (=+) applied to fields
can result in an overflow into the next field;
it is not possible to initialize bit fields.

7.  Macro preprocessor

The proprocessor handles `define' statements with formal arguments.
The line

	#define macro(a1,...,an) ...a1...an...

is recognized by the presence of a left parenthesis
following the defined name.
When the form

	macro(b1,...,bn)

is recognized in normal C program text,
it is replaced by the definition, with the corresponding
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bi
actual argument string substituted for the corresponding
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formal arguments.
Both actual and formal arguments are separated by
commas not included in parentheses; the formal arguments
have the syntax of names.

Macro expansions are no longer surrounded by spaces.
Lines in which a replacement has taken place are rescanned until
no macros remain.

The preprocessor has a rudimentary conditional facility.
A line of the form

	#ifdef name

is ignored if
`name' is defined to the preprocessor
(i.e. was the subject of a `define' line).
If name is not defined then all lines through
a line of the form

	#endif

are ignored.
A corresponding
form is

	#ifndef name
 	...
 	#endif

which ignores the intervening lines unless `name' is defined.
Under unix the name `unix' is predefined and replaced by itself
to aid writers of C programs which are expected to be transported
to other machines with C compilers.

In connection with this, there is a new option to the cc command:

	cc -Dname

which causes `name' to be defined to the preprocessor (and replaced by
itself).
This can be used together with conditional preprocessor
statements to select variant versions of a program at compile time.

The previous two facilities (macros with arguments, conditional
compilation)
were actually available in the 6th Edition system, but
undocumented.
New in this release of the cc command is the ability to
nest `include' files.
Preprocessor include lines may have the new form

	#include <file>

where the angle brackets replace double quotes.
In this case, the file name is prepended with a standard prefix,
namely `/usr/include'.
In is intended that commonly-used include files be placed
in this directory;
the convention reduces the dependence on system-specific
naming conventions.
The standard prefix can be replaced by
the cc command option `-I':

	cc -Iotherdirectory

8.  Registers

A formal argument may be given the storage class `register.'
When this occurs the save sequence copies it
from the place
the caller left it into a fast register;
all usual restrictions on its use are the same
as for ordinary register variables.

Now any variable inside a function may be declared `register;'
if the type is unsuitable, or if
there are more than three register declarations,
then the compiler makes it `auto' instead.
The restriction that the & operator may not be applied
to a register remains.

9.  Mode declarations

A declaration of the form

	typedef_______ type-specifier declarator ;_

makes the name given in the declarator into the equivalent
of a keyword specifying the type which the name would have
in an ordinary declaration.
Thus

	typedef int *iptr;

makes `iptr' usable in declarations of pointers to integers;
subsequently the declarations

	iptr ip;
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	int *ip;

would mean the same thing.
Type names introduced in this way
obey the same scope rules as ordinary variables.
The facility is new, experimental, and probably buggy.

10.  Top-level static

The storage class `static' can be specified in top-level
declarations.
Names declared thereby are global to the rest of the file in which
they appear (except as modified by block structure, see 3 above)
but are unconnected with names in programs in other files.
This may be useful to systems of library
routines which want to keep their internal interfaces hidden.

11. Restrictions

The compiler is somewhat stickier about
some constructions that used to be accepted.

One difference is that external declarations made inside
functions are remembered to the end of the file,
that is even past the end of the function.
The most frequent problem that this causes is that
implicit declaration of a function as an integer in one
routine,
and subsequent explicit declaration
of it as another type,
is not allowed.
This turned out to affect
several source programs
distributed with the system.

It is now required that all forward references to labels
inside a function be the subject of a `goto.'
This has turned out to affect mainly people who
pass a label to the routine `setexit.'
In fact a routine is supposed to be passed here,
and why a label worked I do not know.

In general this compiler makes it more difficult
to use label variables.
Think of this as a contribution to structured programming.

The compiler now checks multiple declarations of the same name
more carefully for consistency.
It used to be possible to declare the same name to
be a pointer to different structures;
this is caught.
So too are declarations of the same array as having different
sizes.
The exception is that array declarations with empty brackets
may be used in conjunction with a declaration with a specified size.
Thus

	int a[];
	int a[50];

is acceptable (in either order).

An external array all of whose definitions
involve empty brackets is diagnosed as `undefined'
by the loader;
it used to be taken as having 1 element.
