Interdata_v6/usr/doc/cman/cman1

.fp 3 G
.TL
C Reference Manual
.AU
Dennis M. Ritchie
.AI
.MH
.sp
May 1, 1977
.PP
.so manmacs
.EQ
delim $$
.EN
.FS
.nh
Revised June, 1978
by R. Miller,
University of Wollongong.
.hy
.FE
.SH
.ti 0
1.  Introduction
.LP
C is a computer language which offers
a rich selection of operators and data types
and the ability to impose useful structure
on both control flow and data.
All the basic operations and data objects
are close to those actually implemented
by most real computers,
so that a very efficient implementation is possible,
but the design is not tied to any particular
machine and with a little care it is possible
to write easily portable programs.
.PP
This manual describes the current version of the C language as it
exists on the \*(pd, the Honeywell 6000, the
\s8IBM\s10 System/370,
.MC
and the \*I 16-bit and 32-bit series.
.mc
Where differences exist, it concentrates on the \*(pd
.MC
and \*I,
.mc
but tries to point out
implementation-dependent
details.
With few exceptions,
these dependencies follow
directly from the underlying properties of the hardware;
the various compilers are generally quite compatible.
.bp
.SH
2.  Lexical conventions
.LP
Blanks, tabs, newlines,
and comments as described below
are ignored except as they serve to separate
tokens.
Some space is required to separate
otherwise adjacent identifiers,
keywords, and constants.
.PP
If the input stream has been parsed into tokens
up to a given character, the next token is taken
to include the longest string of characters
which could possibly constitute a token.
.SH
2.1  Comments
.LP
The characters
.Bd /\**
introduce a comment, which terminates
with the characters
.Bd \**/ "" .
Comments do not nest.
.SH
2.2  Identifiers (Names)
.LP
An identifier is a sequence of letters and digits;
the first character must be alphabetic.
The underscore `\(ru' counts as alphabetic.
Upper and lower case letters
are considered different.
.MC
No more than the first eight characters
are significant
(although more may be used).
External identifiers, which are used by various assemblers
and loaders, are more restricted:

.TS
center;
l l .
DEC \*(pd	7 characters, 2 cases
Honeywell 6000	6 characters, 1 case
IBM 360/370	7 characters, 1 case
\*I \*(I2	8 characters, 2 cases
\*I \*(I1	6 characters, 1 case
.TE
.mc
.SH
2.3  Keywords
.LP
The following identifiers are reserved for use
as keywords, and may not be used otherwise:
.DS L
.TS
center;
LfG LfG LfG .
int	extern	else
char	register	for
float	typedef	do
double	static	while
struct	goto	switch
union	return	case
long	sizeof	default
short	break	entry
unsigned	continue
auto	if
.TE
.DE
The
.Bd entry
keyword is not currently implemented by any compiler but
is reserved for future use.
Some implementations also reserve the words
.Bd fortran
.MC
and
.Bd asm.
.mc
.SH
2.4  Constants
.LP
There are several kinds
of constants,
.MC
as described below.
Hardware differences between implementations
are summarized in \(sc2.6.
.mc
.SH
2.4.1  Integer constants
.LP
An integer constant consisting of a sequence of digits
is taken
to be octal if it begins with \fG0\fR (digit zero), decimal otherwise.
The digits \fG8\fR and \fG9\fR have octal value 10 and 11 respectively.
A sequence of digits preceded by
.Bd 0x
or
.Bd 0X
(digit zero) is taken to be a hexadecimal integer.
The hexadecimal digits include
.Bd a
or
.Bd A
through
.Bd f
or
.Bd F
with values 10 through 15.
A decimal constant whose value exceeds the largest
.MC
signed machine integer is taken to be
.mc
.Bd long "" ;
an octal or hex constant which exceeds the largest unsigned machine integer
.MC
is likewise taken to be
.mc
.Bd long.
.SH
2.4.2  Explicit long constants
.LP
A decimal, octal, or hexadecimal integer constant immediately followed
by
.Bd l
(letter ell)
or
.Bd L
.MC
is a long constant.
As discussed below,
on some machines
.mc
integer and long values may be considered identical.
.SH
2.4.3  Character constants
.LP
A character constant is a sequence of characters enclosed in single quotes
.Bd \|\(aa ` '.
Within a character constant a single quote must be preceded by
a backslash `\e'.
Certain non-graphic characters, and `\e' itself,
may be escaped according to the following table:
.DS L
.TS
center;
L L .
\s8BS\s10	\eb
\s8NL (LF)\s10	\en
\s8CR\s10	\er
\s8HT\s10	\et
\s8FF\s10	\ef
\fIddd\fR	\e\fIddd\fR
\e	\e\e
.TE
.DE
The escape `\e\fIddd\|\fR'
consists of the backslash followed by 1, 2, or 3 octal digits
which are taken to specify the value of the
desired character.
A special case of this construction is `\e0' (not followed
by a digit) which indicates the character
.SM
NUL.
.NL
If the character following a backslash is not one
of those specified, the backslash vanishes.
.PP
The value of a single-character constant is the numerical value of the
.MC
character in the machine's character set (\s8ASCII\s10 for the \*I and \*(pd).
.mc
On the \*(pd at most two characters are permitted
in a character constant and the second character of a pair
is stored in the high-order byte of the integer value.
.MC
On the \*I up to two (\*(I1) or four (\*(I2) characters
are permitted,
and are stored right-justified in a word.
.mc
Character constants with more than one character are
inherently machine-dependent and should
be avoided.
.SH
2.4.4  Floating constants
.LP
A floating constant consists of
an integer part, a decimal point, a fraction part,
an
.Bd e
or
.Bd E,
and an optionally signed integer exponent.
The integer and fraction parts both consist of a sequence
of digits.
Either the integer part or the fraction
part (not both) may be missing;
either the decimal point or
the \fGe\fR and the exponent (not both) may be missing.
Every floating constant is taken to be double-precision.
.SH
2.5  Strings
.LP
A string is a sequence of characters surrounded by
double quotes
`\|"\|'.
A string has type
`array of characters' and storage class `static' (see below)
and is initialized with
the given characters.
The compiler places
a null byte `\|\e0\|'
at the end of each string so that programs
which scan the string can
find its end.
In a string, the character
`\|"\|'
must be preceded by
a
`\e'\|;
in addition, the same escapes as described for character
constants may be used.
Finally, a `\e' and
an immediately following new-line are ignored.
.PP
All strings, even when written identically, are distinct.
.ne 13
.SH
2.6  Hardware Characteristics
.LP
.TS
c c c c c c
c c c c c c
l l l l l l .
	DEC	Honeywell	IBM	\*I	\*I
	\*(pd	6000	370	\*(I1	\*(I2

	ASCII	ASCII	EBCDIC	ASCII	ASCII
char	8 bits	9 bits	8 bits	8 bits	8 bits
int	16	36	32	16	32
short	16	36	16	16	16
long	32	36	32	32	32
float	32	36	32	32	32
double	64	72	64	32	32
range	$\(+-10 sup \(+-38$	$\(+-10 sup \(+-38$	$\(+-10 sup \(+-76$\
	$\(+-10 sup \(+-76$	$\(+-10 sup \(+-76$
.TE
.SH
3.  Syntax notation
.LP
In the syntax notation used in this manual,
syntactic categories are indicated by
.MC
\fIunderlining\fR,
and literal words and characters
in \fGbold\fR type.
.mc
Alternatives are listed on separate lines.
An optional terminal or non-terminal symbol is
indicated by the subscript `opt,' so that
.SY
	{ expression\*(op }
.ES
would indicate an optional expression in braces.
The complete syntax is given in \(sc16, in the notation
of YACC.
.SH
4.  What's in a Name?
.LP
C bases the interpretation of an
identifier upon two attributes of the identifier: its
.It "storage class"
and its
.It type.
The storage class determines the location and lifetime
of the storage associated with an identifier;
the type determines
the meaning of the values
found in the identifier's storage.
.PP
There are four declarable storage classes:
automatic,
static,
external,
and
register.
Automatic variables are local to each invocation of
a block, and are discarded upon exit from the block;
static variables are local to a block, but retain
their values upon reentry to a block even after control
has left the block;
external variables exist and retain their values throughout
the execution of the entire program, and
may be used for communication between
functions, even separately compiled functions.
Register variables are (if possible) stored in the fast registers
of the machine; like automatic
variables they are local to each block and disappear on exit from the block.
.PP
C supports several
fundamental
types of objects:
.PP
Objects declared as characters
.Bd (char)
are large enough to store any member of the implementation's
character set,
and if a genuine character is stored in a character variable,
its value is equivalent to the integer code for that character.
Other quantities may be stored into character variables, but
.MC
the implementation is machine-dependent.
.mc
.PP
Up to three sizes of integer, declared
.Bd "short int,"
.Bd int,
and
.Bd "long int"
are available.
Longer integers provide no less storage than shorter ones,
but the implementation may make either short integers, or long integers,
or both equivalent to plain integers.
`Plain' integers have the natural size suggested
by the host machine architecture;
the other sizes are provided to meet special needs.
On the \*(pd and \*I,
.MC
integers are represented
in 2's complement notation.
.mc
.PP
Unsigned
integers, declared
.Bd unsigned,
obey the laws of arithmetic modulo
$2 sup n$
where $n$ is the number of bits in the representation.
.MC
On the \*I and \*(pd,
long and short unsigned quantities are not supported.
.mc
.PP
Single precision floating point (\fGfloat\fR)
.MC
and double-precision floating-point (\fGdouble\fR) quantities
are available.
The \*I implementations currently make
.mc
.Bd float
and
.Bd double
synonymous.
.PP
Because objects of these types can usefully be interpreted
as numbers, they will be referred to as
.It arithmetic
types.
Types
.Bd char
and
.Bd int
of all sizes will collectively be called
.It integral
types.
.Bd Float
and
.Bd double
will collectively be called
.It floating
types.
.PP
Besides the fundamental arithmetic types there is a
conceptually infinite class of derived types constructed
from the fundamental types in the following ways:
.IP
.It arrays
of objects of most types;
.IP
.It functions
which return objects of a given type;
.IP
.It pointers
to objects of a given type;
.IP
.It structures
containing a sequence of objects of various types;
.IP
.It unions
capable of containing any one of several objects of various types.
.LP
In general these methods
of constructing objects can
be applied recursively.
.SH
5.  Objects and lvalues
.LP
An
.It object
is a manipulatable region of storage;
an
.It lvalue
is an expression referring to an object.
An obvious example of an lvalue
expression is an identifier.
There are operators which yield lvalues:
for example,
if E is an expression of pointer type, then \**E is an lvalue
expression referring to the object to which E points.
The name `lvalue' comes from the assignment expression
`$E1@=@E2$' in which the left operand E1 must be
an lvalue expression.
The discussion of each operator
below indicates whether it expects lvalue operands and whether it
yields an lvalue.
.SH
6.  Conversions
.LP
A number of operators may, depending on their operands,
cause conversion of the value of an operand from one type to another.
This section explains the result to be expected from such
conversions.
\(sc6.6 summarizes the conversions demanded by
most ordinary operators; it will be supplemented
as required by the discussion
of each operator.
.SH
6.1  Characters and integers
.LP
A character or a short integer may be used wherever an
integer may be used.
In all cases
the value is converted to an integer.
Conversion of a short integer always involves
sign extension; short integers are signed quantities.
Whether or not sign-extension occurs for characters is machine
dependent, but it is guaranteed that a member of the
standard character set is non-negative.
On the \*(pd,
character variables range in value from \-128 to 127;
a character constant specified using an octal escape
also suffers sign extension and may appear negative, for example
.MC
\|\(aa\|\e377\(aa\| when converted to an integer
becomes -1.
.mc
.PP
When a longer integer is converted to a shorter
or to a
.Bd char,
it is truncated on the left.
.SH
6.2  Float and double
.LP
All floating arithmetic in C is carried out in double-precision;
whenever a \fGfloat\fR
appears in an expression it is lengthened to \fGdouble\fR
by zero-padding its fraction.
When a \fGdouble\fR must be
converted to \fGfloat\fR, for example by an assignment,
the \fGdouble\fR is rounded before
truncation to \fGfloat\fR length.
.SH
6.3  Floating and integral
.LP
Conversions of floating values to integral type
.MC
tend to be rather machine-dependent;
in particular, the direction of truncation of negative numbers
varies from machine to machine.
.mc
The result is undefined if the
value will not fit in the space provided.
.PP
Conversions of integral values to floating type
are well behaved.
Some loss of precision occurs
if the destination lacks sufficient bits.
.SH
6.4  Pointers and integers
.LP
An integer or long integer may be added to or subtracted from
a pointer; in such a case
the first is converted as
specified in the discussion of the addition operator.
.PP
Two pointers to objects of the same type may be subtracted;
in this case the result is converted to an integer
as specified in the discussion of the subtraction
operator.
.SH
6.5  Unsigned
.LP
Whenever an unsigned integer and a plain integer
are combined, the plain integer is converted to unsigned
and the result is unsigned.
.MC
The value
is the least unsigned integer congruent to the signed
integer (modulo $2 sup wordsize$).
In a 2's complement representation
.mc
this conversion is conceptual and there is no actual change in the
bit pattern.
.PP
When an unsigned integer is converted to long,
the value of the result is the same numerically as that of the
unsigned integer.
Thus the conversion amounts to padding with zeros on the left.
.SH
6.6  Arithmetic conversions
.LP
A great many operators cause conversions
and yield result types in a similar way.
This pattern will be called the `usual arithmetic conversions.'
.IP
First, any operands of type
.Bd char
or
.Bd short
are converted to
.Bd int,
and any of type
.Bd float
are converted to
.Bd double.
.IP
Then, if either operand is
.Bd double,
the other is converted to
.Bd double
and that is the type of the result.
.IP
Otherwise, if either operand is
.Bd long,
the other is converted to
.Bd long
and that is the type of the result.
.IP
Otherwise, if either operand is
.Bd unsigned,
the other is converted to
.Bd unsigned
and that is the type of the result.
.IP
Otherwise, both operands must be
.Bd int,
and that is the type of the result.