Old C Changes
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This is the contents of the 'Old C Changes' help file, which is believed to refer to Typesetter C.
C Changes
1. Long integers
The compiler implements 32-bit integers. The associated type
keyword is `long'. The word can act rather like an adjective in
that `long int' means a 32-bit integer and `long float' means the
same as `double.' But plain `long' is a long integer. Essential-
ly all operations on longs are implemented except that
assignment-type operators do not have values, so l1+(l2=+l3)
won't work. Neither will l1 = l2 = 0.
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 ex-
pected.
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 pointer (which in-
cludes its usage as a subscript). The conversion rules for ex-
pressions involving doubles and floats mixed with longs are the
same as those for short integers, _�m_�u_�t_�a_�t_�i_�s _�m_�u_�t_�a_�n_�d_�i_�s.
A point to note is that constant expressions involving longs are
not evaluated at compile time, and may not be used where con-
stants are expected. Thus
long x {5000L*5000L};
is illegal;
long x {5000*5000};
is legal but wrong because the high-order part is lost; but both
long x 25000000L;
and
long x 25.e6;
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 avail-
able. It may be used alone:
unsigned u;
or as an adjective with `int'
unsigned int u;
with the same meaning. There are not yet (or possibly ever) un-
signed longs or chars. The meaning of an unsigned variable is
that of an integer modulo 2^n, where n is 16 on the PDP-11. All
operators whose operands are unsigned produce results consistent
with this interpretation except division and remainder where the
divisor is larger than 32767; then the result is incorrect. The
dividend in an unsigned division may however have any value (i.e.
up to 65535) 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 2^16 and
the result is unsigned. Thus, for example `u = -1' results in
assigning 65535 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 (un-
diagnosed) overflow occurs when the unsigned integer exceeds
2^15-1.
It is intended that unsigned integers be used in contexts where
previously character pointers were used (artificially and non-
portably) 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 lo-
cal 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 ele-
ments 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 ex-
actly 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 ag-
gregate 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 ar-
rays 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 oc-
curring in lexical order so that forward references in initiali-
zations 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 unde-
fined); 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 res-
trictions: 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 vari-
ables.
There are three twoes in the implementation: addition (=+) ap-
plied 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 argu-
ments. 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 de-
finition, with the corresponding _�b_�i actual argument string sub-
stituted for the corresponding _�a_�i 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.
The name `unix' is predefined and replaced by itself to aid writ-
ers 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 re-
placed 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 depen-
dence 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 ordi-
nary declaration. Thus
typedef int *iptr;
makes `iptr' usable in declarations of pointers to integers;
subsequently the declarations
iptr ip;
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 declara-
tions. 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 func-
tions 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 af-
fect 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 pro-
gramming.
The compiler now checks multiple declarations of the same name
more carefully for consistency. It used to be possible to de-
clare 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.
