MirOS Manual: 23.Cknr(PSD)

                     C Reference Manual

                     Dennis M. Ritchie

                        May 1, 1977

1.  Introduction

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.

     This manual describes the  current  version  of  the  C
language as it exists on the PDP-11, the Honeywell 6000, the
IBM System/370, and the Interdata  8/32.  Where  differences
exist, it concentrates on the PDP-11, 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

2.  Lexical conventions

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, key-
words, and constants.

     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.


The characters /* introduce a comment, which terminates with
the characters */. Comments do not nest.

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2.2  Identifiers (Names)

An identifier is a sequence of letters and digits; the first
character  must  be alphabetic. The underscore `_' counts as
alphabetic. Upper and lower case letters are considered dif-
ferent.  On the PDP-11, no more than the first eight charac-
ters are significant, and only the first seven for  external

2.3  Keywords

The following identifiers are reserved for use as  keywords,
and may not be used otherwise:

               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

The entry keyword is not currently implemented by  any  com-
piler  but  is reserved for future use. Some implementations
also reserve the word fortran.

2.4  Constants

There are several kinds of constants, as follows:

2.4.1  Integer constants

An integer constant consisting of a sequence  of  digits  is
taken  to be octal if it begins with 0 (digit zero), decimal
otherwise. The digits 8 and 9 have octal  value  10  and  11
respectively.  A  sequence  of  digits  preceded by 0x or 0X
(digit zero) is taken to be a hexadecimal integer. The hexa-
decimal  digits include a or A through f or F with values 10
through 15. A decimal constant whose value exceeds the larg-
est signed machine integer (32767 on the PDP-11) is taken to
be long; an octal or hex constant which exceeds the  largest
unsigned  machine  integer (0177777 or 0xFFFF on the PDP-11)
is likewise taken to be long.

2.4.2  Explicit long constants

A decimal, octal, or hexadecimal  integer  constant  immedi-
ately  followed  by  l (letter ell) or L is a long constant,
which, on the PDP-11, has 32 significant bits. As  discussed

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below, on other machines integer and long values may be con-
sidered identical.

2.4.3  Character constants

A character constant is a sequence of characters enclosed in
single  quotes  `''.  Within  a  character constant a single
quote must be preceded by  a  backslash  `\'.  Certain  non-
graphic characters, and `\' itself, may be escaped according
to the following table:

                       BS        \b
                       NL (LF)   \n
                       CR        \r
                       HT        \t
                       FF        \f
                       ddd       \ddd
                       \         \\

The escape `\ddd' 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  `\0' (not followed by a digit) which indicates the char-
acter NUL. If the character following a backslash is not one
of those specified, the backslash vanishes.

     The value of a single-character constant is the numeri-
cal  value  of  the character in the machine's character set
(ASCII for the PDP-11). On the PDP-11 at most two characters
are permitted in a character constant and the second charac-
ter of a pair is  stored  in  the  high-order  byte  of  the
integer  value. Character constants with more than one char-
acter  are  inherently  machine-dependent  and   should   be

2.4.4  Floating constants

A floating constant consists of an integer part,  a  decimal
point,  a fraction part, an e or E, and an optionally signed
integer exponent. The integer and fraction parts  both  con-
sist of a sequence of digits. Either the integer part or the
fraction part (not both) may be missing; either the  decimal
point  or  the e and the exponent (not both) may be missing.
Every floating constant is taken to be double-precision.

2.5  Strings

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 `\0'
at the end of each string so that programs  which  scan  the

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string can find its end. In a string, the character `"' must
be preceded by a `\';  in  addition,  the  same  escapes  as
described  for  character  constants may be used. Finally, a
`\' and an immediately following new-line are ignored.

     All strings, even when written  identically,  are  dis-

3.  Syntax notation

In the  syntax  notation  used  in  this  manual,  syntactic
categories  are  indicated by italic type, and literal words
and characters in sans-serif type. Alternatives  are  listed
on separate lines. An optional terminal or non-terminal sym-
bol is indicated by the subscript `opt,' so  that          {
expressionopt }

would indicate an optional expression in  braces.  The  com-
plete syntax is given in S16, in the notation of YACC.

4.  What's in a Name?

C bases the interpretation of an identifier upon two  attri-
butes of the identifier: its storage class and its 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.

     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 func-
tions, even separately compiled  functions.  Register  vari-
ables  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.

     C supports several fundamental types of objects:

     Objects declared as characters (char) are large  enough
to  store  any member of the implementation's character set,
and if a genuine character is stored in  a  character  vari-
able,  its  value is equivalent to the integer code for that
character. Other quantities may  be  stored  into  character
variables,  but  the implementation is machine-dependent. On
the PDP-11, characters are stored as signed 8-bit  integers,
and the character set is ASCII.

     Up to three sizes of integer, declared short int,  int,
and  long int are available. Longer integers provide no less

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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 PDP-11, short and
plain integers are both represented in 16-bit 2's complement
notation. Long integers are 32-bit 2's complement.

     Unsigned integers, declared unsigned, obey the laws  of
arithmetic  modulo $2 sup n$ where $n$ is the number of bits
in the representation. (16 on the  PDP-11;  long  and  short
unsigned quantities are not supported.)

     Single precision floating point (float) quantities,  on
the  PDP-11,  have  magnitude in the range approximately $10
sup { ± 38}$ or 0; their precision is 24 bits or about seven
decimal digits.

     Double-precision floating-point (double) quantities  on
the  PDP-11 have the same range as floats and a precision of
56 bits or about 17 decimal digits. Some implementations may
make float and double synonymous.

     Because objects of these types can usefully  be  inter-
preted  as  numbers,  they will be referred to as arithmetic
types. Types char and int of all sizes will collectively  be
called integral types. Float and double will collectively be
called floating types.

     Besides the fundamental arithmetic  types  there  is  a
conceptually  infinite  class  of  derived types constructed
from the fundamental types in the following ways:

     arrays of objects of most types;

     functions which return objects of a given type;

     pointers to objects of a given type;

     structures containing a sequence of objects of  various

     unions capable of containing any one of several objects
     of various types.

In general these methods  of  constructing  objects  can  be
applied recursively.

5.  Objects and lvalues

An object is a manipulatable region of storage; an 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

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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.

6.  Conversions

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.  S6.6  summarizes  the  conversions
demanded by most ordinary operators; it will be supplemented
as required by the discussion of each operator.

6.1  Characters and integers

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 PDP-11, char-
acter 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 `'\214''.

     When a longer integer is converted to a shorter or to a
char, it is truncated on the left.

6.2  Float and double

All floating arithmetic in  C  is  carried  out  in  double-
precision;  whenever  a float appears in an expression it is
lengthened to double by zero-padding its  fraction.  When  a
double must be converted to float, for example by an assign-
ment, the double  is  rounded  before  truncation  to  float

6.3  Floating and integral

Conversions of floating values to integral type tend  to  be
rather  machine-dependent.  On  the  PDP-11,  truncation  is
towards 0. The result is undefined if the value will not fit
in the space provided.

     Conversions of integral values  to  floating  type  are
well  behaved. Some loss of precision occurs if the destina-
tion lacks sufficient bits.

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6.4  Pointers and integers

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.

     Two pointers to objects of the same type  may  be  sub-
tracted;  in this case the result is converted to an integer
as specified in the discussion of the subtraction operator.

6.5  Unsigned

Whenever an unsigned integer and a plain  integer  are  com-
bined,  the  plain  integer is converted to unsigned and the
result is unsigned. The value (on the PDP-11) is  the  least
unsigned  integer congruent to the signed integer (modulo $2
sup 16$). Because  of  the  2's  complement  notation,  this
conversion  is  conceptual  and there is no actual change in
the bit pattern.

     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.

6.6  Arithmetic conversions

A great many operators cause conversions  and  yield  result
types  in  a  similar  way.  This pattern will be called the
`usual arithmetic conversions.'

     First, any operands of type char or short are converted
     to int, and any of type float are converted to double.

     Then, if either operand is double, the  other  is  con-
     verted to double and that is the type of the result.

     Otherwise, if either operand is long, the other is con-
     verted to long and that is the type of the result.

     Otherwise, if either operand is unsigned, the other  is
     converted  to  unsigned  and  that  is  the type of the

     Otherwise, both operands must be int, and that  is  the
     type of the result.

7.  Expressions

The precedence of expression operators is the  same  as  the
order of the major subsections of this section (highest pre-
cedence first). Thus the  expressions  referred  to  as  the
operands  of  +  (S7.4)  are  those  expressions  defined in
SS7.1-7.3. Within each subsection, the  operators  have  the

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same  precedence.  Left- or right-associativity is specified
in each subsection for the operators discussed therein.  The
precedence and associativity of all the expression operators
is summarized in the collected grammar.

     Otherwise the order of  evaluation  of  expressions  is
undefined.  In particular the compiler considers itself free
to compute subexpressions in  the  order  it  believes  most
efficient,  even if the subexpressions involve side effects.
Expressions involving a commutative and associative operator
may  be  rearranged  arbitrarily,  even  in  the presence of
parentheses; to force a particular order  of  evaluation  an
explicit temporary must be used.

7.1  Primary expressions

Primary expressions involving ., ->, subscripting, and func-
tion  calls group left to right.         primary-expression:
                identifier                          constant
                string                    (   expression   )
                primary-expression    [     expression     ]
                primary-expression  (  expression-listopt  )
                primary-lvalue         .          identifier
                primary-expression -> identifier

        expression-list:                          expression
                expression-list , expression

An identifier is a primary expression, provided it has  been
suitably  declared as discussed below. Its type is specified
by its declaration. However, if the type of  the  identifier
is  `array  of  ...',  then  the  value  of  the identifier-
expression is a pointer to the first object  in  the  array,
and  the  type  of the expression is `pointer to ...'. More-
over, an array identifier is not an lvalue expression. Like-
wise,  an  identifier  which is declared `function returning
...', when used except in the function-name  position  of  a
call, is converted to `pointer to function returning ...'.

     A decimal, octal, character, or floating constant is  a
primary  expression.  Its  type  may be int, long, or double
depending on its form.

     A string is a primary expression. Its  type  is  origi-
nally  `array  of  char';  but following the same rule given
above for identifiers, this is modified to `pointer to char'
and  the  result  is a pointer to the first character in the
string. (There is an exception in certain initializers;  see

     A parenthesized  expression  is  a  primary  expression
whose type and value are identical to those of the unadorned
expression. The presence  of  parentheses  does  not  affect
whether the expression is an lvalue.

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     A primary  expression  followed  by  an  expression  in
square brackets is a primary expression. The intuitive mean-
ing is that of a subscript. Usually, the primary  expression
has  type `pointer to ...', the subscript expression is int,
and the type of the result is `...'. The expression `E1[E2]'
is  identical  (by  definition)  to  `*((E1)+(E2))'. All the
clues needed to understand this notation  are  contained  in
this  section  together with the discussions in SS 7.1, 7.2,
and 7.4 on identifiers, *, and + respectively;  S14.3  below
summarizes the implications.

     A function call is a  primary  expression  followed  by
parentheses  containing  a  possibly  empty, comma-separated
list of expressions which constitute the actual arguments to
the  function. The primary expression must be of type `func-
tion returning ...', and the result of the function call  is
of  type  `...'. As indicated below, a hitherto unseen iden-
tifier followed immediately by a left parenthesis is contex-
tually   declared  to  represent  a  function  returning  an
integer; thus in the most common case, integer-valued  func-
tions need not be declared.

     Any actual arguments of type  float  are  converted  to
double  before  the call; any of type char or short are con-
verted to int.

     In preparing for the call to a function, a copy is made
of each actual parameter; thus, all argument-passing in C is
strictly by value. A function may change the values  of  its
formal  parameters,  but  these  changes  cannot  affect the
values of the actual parameters. On the other  hand,  it  is
possible  to  pass  a  pointer on the understanding that the
function may change the value of the  object  to  which  the
pointer  points.  The  order  of  evaluation of arguments is
undefined by the language; take note that the  various  com-
pilers differ.

     Recursive calls to any function are permitted.

     A primary expression followed by a dot followed  by  an
identifier is an expression. The first expression must be an
lvalue naming a structure or union, and the identifier  must
name  a  member  of the structure or union. The result is an
lvalue referring to the named member  of  the  structure  or

     A primary expression followed by an arrow (built from a
`-'  and  a `>') followed by an identifier is an expression.
The first expression must be a pointer to a structure  or  a
union  and  the identifier must name a member of that struc-
ture or union. The result is  an  lvalue  referring  to  the
named  member of the structure or union to which the pointer
expression points.

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     Thus  the  expression  `E1->MOS'   is   the   same   as
`(*E1).MOS'.  Structures  and  unions are discussed in S8.5.
The rules given here for the use of  structures  and  unions
are  not enforced strictly, in order to allow an escape from
the typing mechanism. See S14.1.

7.2  Unary operators

Expressions  with  unary  operators   group   right-to-left.
        unary-expression:                    *    expression
                &   lvalue                   -    expression
                !  expression                  ~  expression
                ++    lvalue                    --    lvalue
                lvalue    ++                    lvalue    --
                (       type-name        )        expression
                sizeof  expression                  sizeof (
type-name )

The unary * operator means indirection: the expression  must
be  a  pointer, and the result is an lvalue referring to the
object to which the expression points. If the  type  of  the
expression  is  `pointer  to ...', the type of the result is

     The result of the unary & operator is a pointer to  the
object  referred to by the lvalue. If the type of the lvalue
is `...', the type of the result is `pointer to ...'.

     The result of the unary - operator is the  negative  of
its operand. The usual arithmetic conversions are performed.
The negative of an unsigned quantity  is  computed  by  sub-
tracting  its  value  from $2 sup n$, where $n$ is 16 on the

     The result of the logical negation operator ! is  1  if
the value of its operand is 0, 0 if the value of its operand
is non-zero. The type of the result is int. It is applicable
to any arithmetic type or to pointers.

     The ~ operator  yields  the  one's  complement  of  its
operand. The usual arithmetic conversions are performed. The
type of the operand must be integral.

     The object referred to by the lvalue operand of  prefix
`++'  is  incremented.  The  value  is  the new value of the
operand, but is not  an  lvalue.  The  expression  `++a'  is
equivalent  to  `(a += 1)'.  See the discussions of addition
(S7.4) and assignment operators (S7.14) for  information  on

     The lvalue operand of prefix `--' is decremented analo-
gously to the ++ operator.

     When postfix `++' is applied to an lvalue the result is

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the value of the object referred to by the lvalue. After the
result is noted, the  object  is  incremented  in  the  same
manner as for the prefix ++ operator. The type of the result
is the same as the type of the lvalue expression.

     When postfix `--' is applied to an lvalue the result is
the value of the object referred to by the lvalue. After the
result is noted, the object is decremented in the manner  as
for  the  prefix  -- operator. The type of the result is the
same as the type of the lvalue expression.

     An expression preceded by the parenthesized name  of  a
data  type  causes conversion of the value of the expression
to the  named  type.  The  construction  of  type  names  is
described in S8.7.

     The sizeof operator yields the size, in bytes,  of  its
operand.  (A  byte  is  undefined  by the language except in
terms of the value of sizeof. However in all existing imple-
mentations  a  byte  is  the space required to hold a char.)
When applied to an array, the result is the total number  of
bytes in the array. The size is determined from the declara-
tions of the objects in the expression. This  expression  is
semantically  an integer constant and may be used anywhere a
constant is required. Its major use is in communication with
routines like storage allocators and I/O systems.

     The  sizeof  operator  may  also  be   applied   to   a
parenthesized type name. In that case it yields the size, in
bytes, of an object of the indicated type.

     The construction `sizeof(type)' is taken to be a  unit,
so   the   expression   `sizeof(type)-2'   is  the  same  as

7.3  Multiplicative operators

The multiplicative operators *,  /,  and  %  group  left-to-
right.  The  usual  arithmetic  conversions  are  performed.
        multiplicative-expression:                   expres-
sion  *  expression                  expression / expression
                expression % expression

The binary * operator indicates multiplication. The * opera-
tor  is associative and expressions with several multiplica-
tions at the same level may be rearranged.

     The binary / operator indicates division. When positive
integers are divided truncation is toward 0, but the form of
truncation is machine-dependent if either operand  is  nega-
tive.  In  all cases it is true that $(a/b) * b~+~ a%b ~ = ~
a$. On the PDP-11, the remainder has the same  sign  as  the

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     The binary % operator yields  the  remainder  from  the
division  of  the  first expression by the second. The usual
arithmetic conversions are performed.  On  the  PDP-11,  the
remainder  has  the  same sign as the dividend. The operands
must not be floating.

7.4  Additive operators

The additive operators +  and  -  group  left-to-right.  The
usual  arithmetic  conversions are performed. There are some
additional   type   possibilities   for    each    operator.
        additive-expression:                   expression  +
expression                 expression - expression

The result of the `+' operator is the sum of the operands. A
pointer to an object in an array and a value of any integral
type may be added. The latter is in all cases  converted  to
an  address  offset  by  multiplying it by the length of the
object to which the pointer points. The result is a  pointer
of  the  same type as the original pointer, and which points
to another object in the same  array,  appropriately  offset
from  the  original  object.  Thus  if  P is a pointer to an
object in an array, the expression `P+1' is a pointer to the
next object in the array.

     No further type combinations are allowed.

     The + operator  is  associative  and  expressions  with
several additions at the same level may be rearranged.

     The result of the `-' operator is the difference of the
operands.  The  usual  arithmetic conversions are performed.
Additionally, a value of any integral type may be subtracted
from  a  pointer, and then the same conversions as for addi-
tion apply.

     If two pointers to objects of the same  type  are  sub-
tracted,  the result is converted (by division by the length
of the object) to an int representing the number of  objects
separating  the  pointed-to objects. This conversion will in
general give unexpected results unless the pointers point to
objects  in  the same array, since pointers, even to objects
of the same type, do not necessarily differ by a multiple of
the object-length.

7.5  Shift operators

The shift operators << and >> group left-to-right. Both per-
form  the  usual  arithmetic  conversions on their operands,
each of which must be integral. Then the  right  operand  is
converted to int; the type of the result is that of the left
operand. The result is undefined if  the  right  operand  is
negative  or  larger  than the number of bits in the object.
        shift-expression:                   expression    <<

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expression                 expression >> expression

The value of `E1<<E2' is E1 (interpreted as a  bit  pattern)
left-shifted  E2  bits; vacated bits are 0-filled. The value
of `E1>>E2' is E1 right-shifted E2 bit positions. The  shift
is guaranteed to be logical (0-fill) if E1 is unsigned; oth-
erwise it may be (and is, on the PDP-11) arithmetic (fill by
a copy of the sign bit).

7.6  Relational operators

The relational operators group left-to-right, but this  fact
is  not very useful; `a<b<c' does not mean what it seems to.
        relational-expression:                 expression  <
expression                     expression    >    expression
                expression           <=           expression
                expression >= expression

The operators < (less than), > (greater than), <= (less than
or  equal  to) and >= (greater than or equal to) all yield 0
if the specified relation is false and 1 if it is true.  The
type  of the result is int. The usual arithmetic conversions
are performed. Two pointers may be compared, and the  result
depends  on  the  relative locations in the address space of
the pointed-to objects. Pointer comparison is portable  only
when the pointers point to objects in the same array.

7.7  Equality operators

        equality-expression:                  expression  ==
expression                 expression != expression

The == (equal to) and the != (not equal  to)  operators  are
exactly  analogous  to  the  relational operators except for
their lower precedence. (Thus `a<b == c<d' is 1 whenever a<b
and c<d have the same truth-value).

     A pointer may be compared to an integer, but the result
is machine dependent unless the integer is the constant 0. A
pointer to which 0 has been assigned is  guaranteed  not  to
point  to  any  object, and will appear to be equal to 0; in
conventional usage, such a pointer is considered to be null.

7.8  Bitwise and operator

        and-expression:                 expression & expres-

The & operator is associative and  expressions  involving  &
may be rearranged. The usual arithmetic conversions are per-
formed; the result is the bit-wise  `and'  function  of  the
operands. The operator applies only to integral operands.

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7.9  Bitwise exclusive or operator

        exclusive-or-expression:                  expression
^ expression

The ^ operator is associative and  expressions  involving  ^
may be rearranged. The usual arithmetic conversions are per-
formed; the result is is the bit-wise `exclusive  or'  func-
tion  of the operands. The operator applies only to integral

7.10  Bitwise inclusive or operator

        inclusive-or-expression:                  expression
| expression

The | operator is associative and expressions with | may  be
rearranged.  The usual arithmetic conversions are performed;
the result is the bit-wise `inclusive or'  function  of  its
operands. The operator applies only to integral operands.

7.11  Logical and operator

        logical-and-expression:                   expression
&& expression

The && operator groups left-to-right. It returns 1  if  both
its operands are non-zero, 0 otherwise. Unlike &, && guaran-
tees left-to-right evaluation; moreover the  second  operand
is not evaluated if the first operand is 0.

     The operands need not have the same type, but each must
have  one  of  the  fundamental  types  or be a pointer. The
result is always int.

7.12  Logical or operator

        logical-or-expression:                 expression ||

The || operator groups left-to-right. It returns 1 if either
of  its  operands is non-zero, and 0 otherwise. Unlike |, ||
guarantees left-to-right evaluation;  moreover,  the  second
operand  is  not evaluated if the value of the first operand
is non-zero.

     The operands need not have the same type, but each must
have  one  of  the  fundamental  types  or be a pointer. The
result is always int.

7.13  Conditional operator

        conditional-expression:                 expression ?

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expression : expression

Conditional  expressions  group  right-to-left.  The   first
expression is evaluated and if it is non-zero, the result is
the value of the second expression, otherwise that of  third
expression.  If  possible,  the usual arithmetic conversions
are performed to bring the second and third expressions to a
common  type;  otherwise,  if  both are pointers of the same
type, the result has the common type; otherwise, one must be
a  pointer  and the other the constant 0, and the result has
the type of the pointer. Only one of the  second  and  third
expressions is evaluated.

7.14  Assignment operators

There are a number of assignment  operators,  all  of  which
group  right-to-left.  All  require  an lvalue as their left
operand, and the type of an assignment expression is that of
its  left operand. The value is the value stored in the left
operand after the assignment has taken place. The two  parts
of  a  compound  assignment  operator  are  separate tokens.
        assignment-expression:                   lvalue    =
expression                    lvalue    +    =    expression
                lvalue - = expression                 lvalue
*   =   expression                  lvalue  /  =  expression
                lvalue % = expression                 lvalue
>>  =  expression                  lvalue  <<  =  expression
                lvalue & = expression                 lvalue
^ = expression                 lvalue | = expression

Notice that the representation of  the  compound  assignment
operators  has  changed; formerly the `=' came first and the
other operator came second (without any space). The compiler
continues to accept the previous notation.

     In the simple assignment with `=',  the  value  of  the
expression  replaces  that  of the object referred to by the
lvalue. If both operands have  arithmetic  type,  the  right
operand  is converted to the type of the left preparatory to
the assignment.

     The behavior of an expression of the form `E1 op  = E2'
may   be   inferred   by   taking   it   as   equivalent  to
`E1 = E1 op E2'; however, E1 is evaluated only once.  In  +=
and -=, the left operand may be a pointer, in which case the
(integral) right operand is converted as explained in  S7.4;
all  right  operands  and all non-pointer left operands must
have arithmetic type.

     The compiler currently allows a pointer to be  assigned
to  an  integer, an integer to a pointer, and a pointer to a
pointer of another type.  The  assignment  is  a  pure  copy
operation,  with  no  conversion. This usage is nonportable,
and may produce pointers which cause  addressing  exceptions

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                           - 16 -

when  used. However, it is guaranteed that assignment of the
constant 0 to a pointer will produce a null pointer  distin-
guishable from a pointer to any object.

7.15  Comma operator

        comma-expression:                    expression    ,

A pair of expressions separated  by  a  comma  is  evaluated
left-to-right  and  the value of the left expression is dis-
carded. The type and value of the result are  the  type  and
value  of  the  right operand. This operator groups left-to-
right. In contexts where comma is given a  special  meaning,
for  example  in  a  list  of  actual arguments to functions
(S7.1) and lists of initializers (S8.6), the comma  operator
as described in this section can only appear in parentheses;
for example, `f(a, (t = 3, t+2), c)'  has  three  arguments,
the second of which has the value 5.

8.  Declarations

Declarations are used within function definitions to specify
the interpretation which C gives to each identifier; they do
not necessarily reserve storage associated with the identif-
ier.   Declarations   have   the  form          declaration:
                decl-specifiers declarator-listopt  ;

The declarators in the declarator-list contain the  identif-
iers  being  declared.  The  decl-specifiers  consist  of  a
sequence   of   type   and   storage    class    specifiers.
        decl-specifiers:                      type-specifier
decl-specifiersopt                    sc-specifier     decl-
The list must be self-consistent in a way described below.

8.1  Storage class specifiers

The      sc-specifiers      are:               sc-specifier:
                auto                                  static
                extern                              register

The typedef specifier does not reserve storage and is called
a  `storage class specifier' only for syntactic convenience;
it is discussed in S8.8.

     The meanings of the various storage classes  were  dis-
cussed in S4.

     The auto, static, and register declarations also  serve
as  definitions  in that they cause an appropriate amount of
storage to be reserved. In the extern case there must be  an

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                           - 17 -

external  definition  (S10)  for the given identifiers some-
where outside the function in which they are declared.

     A register declaration is best thought of  as  an  auto
declaration,  together  with a hint to the compiler that the
variables declared will be heavily used. Only the first  few
(three,  for  the  PDP-11)  such declarations are effective.
Moreover, only variables of certain types will be stored  in
registers;  on  the  PDP-11, they are int, char, or pointer.
One restriction applies to register variables: the  address-
of  operator  &  cannot  be applied to them. Smaller, faster
programs can be expected if register declarations  are  used
appropriately,  but  future  developments  may  render  them

     At most one sc-specifier may be given in a declaration.
If  the  sc-specifier  is  missing from a declaration, it is
taken to be auto inside a function, extern  outside.  Excep-
tion: functions are always extern.

8.2  Type specifiers

The     type-specifiers     are              type-specifier:
                char                                   short
                int                                     long
                unsigned                               float
                double                      struct-or-union-
specifier                 typedef-name

The words long, short, and unsigned may  be  thought  of  as
adjectives;  the  following  combinations are acceptable (in
any order).                 short  int                  long
int                 unsigned int                 long float

The meaning of the last is the same as double. Otherwise, at
most  one  type-specifier  may be given in a declaration. If
the type-specifier is missing  from  a  declaration,  it  is
taken to be int.

     Specifiers for structures and unions are  discussed  in
S8.5; declarations with typedef names are discussed in S8.8.

8.3  Declarators

The declarator-list appearing in a declaration is  a  comma-
separated sequence of declarators, each of which may have an
initializer.         declarator-list:                  init-
declarator                 init-declarator , declarator-list

        init-declarator:                          declarator
Initializers are discussed in S8.6. The  specifiers  in  the
declaration  indicate  the  type  and  storage  class of the

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                           - 18 -

objects to which the declarators refer. Declarators have the
syntax:            declarator:                    identifier
                ( declarator )                 *  declarator
                declarator  ( )                 declarator [
constant-expressionopt ]

The grouping is the same as in expressions.

8.4  Meaning of declarators

Each declarator is taken to be an assertion that when a con-
struction  of  the same form as the declarator appears in an
expression, it yields an object of the  indicated  type  and
storage class. Each declarator contains exactly one identif-
ier; it is this identifier that is declared.

     If an unadorned identifier  appears  as  a  declarator,
then  it has the type indicated by the specifier heading the

     A declarator in parentheses is identical  to  the  una-
dorned  declarator,  but  the binding of complex declarators
may be altered by parentheses. See the examples below.

     If a declarator has the form

                * D

for D a declarator, then the contained  identifier  has  the
type  `pointer  to  ...',  where `...' is the type which the
identifier would have had if the declarator had been  simply

     If a declarator has the form


then the contained identifier has the type `function return-
ing ...', where `...' is the type which the identifier would
have had if the declarator had been simply D.

     A declarator may have the form




Such declarators make the  contained  identifier  have  type
`array.'  If the unadorned declarator D would specify a non-
array of type `...', then the declarator `D[i]' yields a  1-
dimensional  array  with rank i of objects of type `...'. If
the unadorned declarator D would  specify  an  n-dimensional

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                           - 19 -

array with rank $i sub 1 times i sub 2 times ... times i sub
n$, then the declarator $roman D [ i sub { n+1 }  ]$  yields
an  $(n+1)$-dimensional array with rank $i sub 1 times i sub
2 times ... times i sub n times i sub { n+1}$.

     In the first case the constant expression is an expres-
sion  whose value is determinable at compile time, and whose
type is int. (Constant expressions are defined precisely  in
S15.)  The constant expression of an array declarator may be
missing only for the first dimension. This notation is  use-
ful  when  the array is external and the actual declaration,
which allocates storage, is given elsewhere.  The  constant-
expression  may  also be omitted when the declarator is fol-
lowed by initialization. In this case the size is calculated
from the number of initial elements supplied.

     An array may be  constructed  from  one  of  the  basic
types,  from  a  pointer, from a structure or union, or from
another array (to generate a multi-dimensional array).

     Not all the possibilities allowed by the  syntax  above
are  actually  permitted.  The  restrictions are as follows:
functions may not return arrays,  structures  or  functions,
although  they may return pointers to such things; there are
no arrays of functions, although  there  may  be  arrays  of
pointers  to functions. Likewise a structure may not contain
a function, but it may contain a pointer to a function.

     As an example, the declaration

                int i, *ip, f(), *fip(), (*pfi)();

declares an integer i, a pointer ip to an integer,  a  func-
tion  f  returning  an  integer,  a function fip returning a
pointer to an integer, and a pointer pfi to a function which
returns  an  integer. It is especially useful to compare the
last two. The binding of `*fip()' is `*(fip())', so that the
declaration  suggests,  and  the  same  construction  in  an
expression requires, the calling of a function fip, and then
using  indirection  through the (pointer) result to yield an
integer. In the declarator `(*pfi)()', the extra parentheses
are  necessary,  as they are also in an expression, to indi-
cate that indirection through a pointer to a function yields
a function, which is then called.

     As another example,

                float fa[17], *afp[17];

declares an array of float numbers and an array of  pointers
to float numbers. Finally,

                static int x3d[3][5][7];

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                           - 20 -

declares a static three-dimensional array of integers,  with
rank  3x5x7.  In  complete  detail, x3d is an array of three
items: each item is an array of five  arrays;  each  of  the
latter  arrays  is  an  array  of seven integers. Any of the
expressions `x3d', `x3d[i]', `x3d[i][j]', `x3d[i][j][k]' may
reasonably  appear  in  an  expression. The first three have
type `array', the last has type int.

8.5  Structure and union declarations

A structure is an object consisting of a sequence  of  named
members. Each member may have any type. A union is an object
which may, at a given  time,  contain  any  one  of  several
members.  Structure and union specifiers have the same form.
                struct-or-union    {    struct-decl-list   }
                struct-or-union  identifier  {  struct-decl-
list }                 struct-or-union identifier

        struct-or-union:                              struct

The struct-decl-list is a sequence of declarations  for  the
members of the structure or union:         struct-decl-list:
                struct-declaration                   struct-
declaration struct-decl-list

        struct-declaration:                   type-specifier
struct-declarator-list  ;

        struct-declarator-list:                      struct-
declarator                   struct-declarator   ,   struct-

In the usual case, a struct-declarator is just a  declarator
for a member of a structure or union. A structure member may
also consist of a specified number of bits. Such a member is
also  called  a  field; its length is set off from the field
name     by     a     colon.              struct-declarator:
                declarator                   declarator    :
constant-expression                 :  constant-expression

Within a structure,  the  objects  declared  have  addresses
which increase as their declarations are read left-to-right.
Each non-field member of a structure begins on an addressing
boundary  appropriate  to  its  type. On the PDP-11 the only
requirement is that non-characters begin on a word boundary;
therefore,  there  may  be 1-byte, unnamed holes in a struc-
ture. Field members are packed into machine  integers;  they
do  not  straddle words. A field which does not fit into the
space remaining in a word is put  into  the  next  word.  No
field  may  be  wider than a word. On the PDP-11, fields are
assigned right-to-left.

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                           - 21 -

     A struct-declarator with no declarator,  only  a  colon
and  a  width, indicates an unnamed field useful for padding
to conform to externally-imposed layouts. As a special case,
an  unnamed  field  with a width of 0 specifies alignment of
the next field at a word boundary. The `next field'  presum-
ably  is  a field, not an ordinary structure member, because
in the latter case the alignment would have been automatic.

     The language does not restrict the types of things that
are declared as fields, but implementations are not required
to support any but integer fields. Moreover, even int fields
may  be considered to be unsigned. On the PDP-11, fields are
not signed and have only integer values.

     A union may be thought of as a structure all  of  whose
members  begin  at  offset 0 and whose size is sufficient to
contain any of its members. At most one of the  members  can
be stored in a union at any time.

     A structure or union specifier of the second form, that
is,  one of                 struct identifier { struct-decl-
list }                 union identifier { struct-decl-list }

declares the identifier to be the structure  tag  (or  union
tag)  of  the  structure specified by the list. A subsequent
declaration may then use the third form of specifier, one of
                struct    identifier                   union

Structure tags allow definition of  self-referential  struc-
tures;  they also permit the long part of the declaration to
be given once and used several times. It is  however  absurd
to  declare  a structure or union which contains an instance
of itself, as distinct from a  pointer  to  an  instance  of

     The names of members and tags may be the same as  ordi-
nary  variables.  However, names of tags and members must be
mutually distinct.

     Two structures may share a common initial  sequence  of
members;  that  is,  the  same member may appear in two dif-
ferent structures if it has the same type in both and if all
previous  members  are the same in both. (Actually, the com-
piler checks only that a name in  two  different  structures
has  the  same  type  and  offset  in both, but if preceding
members differ the construction is nonportable.)

     A simple example of a structure declaration is

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                           - 22 -

                struct tnode {
                        char tword[20];
                        int count;
                        struct tnode *left;
                        struct tnode *right;

which contains an array of 20 characters,  an  integer,  and
two  pointers  to  similar structures. Once this declaration
has been given, the following declaration makes sense:

                struct tnode s, *sp;

which declares s to be a structure of the given sort and  sp
to be a pointer to a structure of the given sort. With these
declarations, the expression


refers to the count field  of  the  structure  to  which  sp


refers to the left  subtree  pointer  of  the  structure  s.


refers to the first character of the  tword  member  of  the
right subtree of s.

8.6  Initialization

A declarator may specify an initial value for the identifier
being declared. The initializer is preceded by `=', and con-
sists of an expression or a list of values nested in braces.
        initializer:                       =      expression
                = { initializer-list }                  =  {
initializer-list , }

        initializer-list:                         expression
                initializer-list      ,     initializer-list
                { initializer-list }

The `=' is a new addition to the syntax, intended to allevi-
ate potential ambiguities. The current compiler allows it to
be omitted when the rest of the initializer is a very simple
expression  (just  a  name, string, or constant) or when the
rest of the initializer is enclosed in braces.

     All the expressions in an initializer for a  static  or
external  variable  must  be constant expressions, which are

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                           - 23 -

described in S15, or expressions which reduce to the address
of a previously declared variable, possibly offset by a con-
stant expression. Automatic or  register  variables  may  be
initialized  by  arbitrary  expressions involving previously
declared variables.

     When an initializer applies to a scalar (a  pointer  or
an  object  of  arithmetic  type),  it  consists of a single
expression, perhaps in braces.  The  initial  value  of  the
object is taken from the expression; the same conversions as
for assignment are performed.

     When the declared variable is an aggregate (a structure
or array) then the initializer consists of a brace-enclosed,
comma-separated list of initializers for the members of  the
aggregate,  written in increasing subscript or member order.
If the aggregate contains subaggregates, this  rule  applies
recursively  to  the  members of the aggregate. If there are
fewer initializers in the list than there are members of the
aggregate,  then the aggregate is padded with 0's. It is not
permitted to  initialize  unions  or  automatic  aggregates.
Currently,  the  PDP-11  compiler  also forbids initializing
fields in structures.

     Braces may be elided as  follows.  If  the  initializer
begins with a left brace, then the succeding comma-separated
list of initializers initialize the members  of  the  aggre-
gate; it is erroneous for there to be more initializers than
members. If, however, the initializer does not begin with  a
left  brace,  then  only  enough  elements from the list are
taken to account for  the  members  of  the  aggregate;  any
remaining  members are left to initialize the next member of
the aggregate of which the current aggregate is a part.

     A final abbreviation allows a char array to be initial-
ized  by  a  string.  In this case successive members of the
string initialize the members of the array.

     For example,

        int x[ ] = { 1, 3, 5 };

declares and initializes x as a  1-dimensional  array  which
has three members, since no size was specified and there are
three initializers.

        float   y[4][3] = {
                { 1, 3, 5 },
                { 2, 4, 6 },
                { 3, 5, 7 },

is a completely-bracketed initialization: 1, 3, and  5  ini-
tialize  the first row of the array $y [ 0 ]$, namely $y [ 0

                       April 3, 2017

                           - 24 -

] [ 0 ]$, $y [ 0 ] [ 1 ]$, and $y [ 0 ] [ 2 ]$. Likewise the
next  two lines initialize $y [ 1 ]$ and $y [ 2 ]$. The ini-
tializer ends early and therefore $y [ 3 ]$  is  initialized
with  0.  Precisely the same effect could have been achieved

        float   y[4][3] = {
                1, 3, 5, 2, 4, 6, 3, 5, 7,

The initializer for $y$ begins with a left brace,  but  that
for  $y  [ 0 ]$ does not, therefore 3 elements from the list
are used. Likewise the next three are taken successively for
$y [ 1 ]$ and $y [ 2 ]$. Also,

        float   y[4][3] = {
                { 1 }, { 2 }, { 3 }, { 4 }

initializes the first  column  of  y  (regarded  as  a  two-
dimensional array) and leaves the rest 0.


        char    msg[] = "Syntax error on line %s\n";

shows a character array whose members are initialized with a

8.7  Type names

In two contexts (to specify type conversions explicitly, and
as  an  argument of sizeof) it is desired to supply the name
of a data type. This is accomplished using  a  `type  name,'
which in essence is a declaration for an object of that type
which omits  the  name  of  the  object.          type-name:
                type-specifier abstract-declarator

        abstract-declarator:                           empty
                (  abstract-declarator  )                  *
abstract-declarator                 abstract-declarator (  )
                abstract-declarator [ constant-expressionopt

To avoid ambiguity, in  the  construction                  (
abstract-declarator )

the abstract-declarator is required to  be  nonempty.  Under
this  restriction,  it  is possible to identify uniquely the
location in the  abstract-declarator  where  the  identifier
would  appear  if  the  construction  were a declarator in a
declaration. The named type is then the same as the type  of
the hypothetical identifier. For example,

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                           - 25 -

                int *
                int *[3]
                int (*)[3]

name respectively the types `integer,' `pointer to integer,'
`array  of 3 pointers to integers,' and `pointer to an array
of 3 integers.' As another example,

                int i;
                . . .
                sin( (double) i);

calls the sin routine (which accepts a double argument) with
an argument appropriately converted.

8.8  Typedef

Declarations whose `storage class' is typedef do not  define
storage,  but  instead  define identifiers which can be used
later as if they were type keywords  naming  fundamental  or
derived  types.  Within the scope of a declaration involving
typedef, each of the identifiers appearing as  part  of  any
declarators  therein become syntactically equivalent to type
keywords naming the type associated with the identifiers  in
the    way    described   in   S8.4.           typedef-name:

For example, after

                typedef int MILES, *KLICKSP;
                typedef struct { double re, im;} complex;

the constructions

                MILES distance;
                extern KLICKSP metricp;
                complex z, *zp;

are all legal declarations; the type of distance  is  `int',
that  of  metricp  is `pointer to int,' and that of z is the
specified structure. Zp is a pointer to such a structure.

     Typedef  does  not  introduce  brand  new  types,  only
synonyms  for types which could be specified in another way.
Thus in the example above distance  is  considered  to  have
exactly the same type as any other int variable.

9.  Statements

Except as indicated, statements are executed in sequence.

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                           - 26 -

9.1  Expression statement

Most statements are expression statements,  which  have  the
form         expression ;

Usually expression statements are  assignments  or  function

9.2  Compound statement, or block

So  that  several  statements  can  be  used  where  one  is
expected,  the  compound  statement (also, and equivalently,
called  `block')  is  provided:          compound-statement:
                { declaration-listopt statement-listopt }

        declaration-list:                        declaration
                declaration declaration-list

        statement-list:                            statement
                statement statement-list

If any of the identifiers in the declaration-list were  pre-
viously  declared,  the outer declaration is pushed down for
the duration of the block, at  which  time  it  resumes  its

     Any initializations of auto or register  variables  are
performed  each  time the block is entered at the top. It is
currently possible (but a bad practice) to transfer  into  a
block;  in  that case the initializations are not performed.
Initializations of static variables are performed only  once
when  the program begins execution. Inside a block, external
declarations do not reserve storage so initialization is not

9.3  Conditional statement

The two forms of the conditional statement are         if  (
expression  )  statement         if ( expression ) statement
else statement

In both cases the expression is evaluated and if it is  non-
zero, the first substatement is executed. In the second case
the second substatement is executed if the expression is  0.
As  usual  the `else' ambiguity is resolved by connecting an
else with the last encountered elseless if.

9.4  While statement

The while statement has the form         while (  expression
) statement

The substatement is executed repeatedly so long as the value
of  the  expression  remains  non-zero. The test takes place

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                           - 27 -

before each execution of the statement.

9.5  Do statement

The do statement has the form         do statement  while  (
expression ) ;

The substatement is executed repeatedly until the  value  of
the expression becomes zero. The test takes place after each
execution of the statement.

9.6  For statement

The for statement has the form         for ( expression-1opt
; expression-2opt  ; expression-3opt ) statement

This  statement  is  equivalent   to           expression-1;
        while   (expression-2)  {                  statement
                expression-3;         }

Thus the first expression specifies initialization  for  the
loop;  the  second specifies a test, made before each itera-
tion, such that the  loop  is  exited  when  the  expression
becomes  0;  the  third  expression  typically  specifies an
incrementation which is performed after each iteration.

     Any or all of the expressions may be dropped. A missing
expression-2  makes  the  implied while clause equivalent to
`while(1)'; other missing  expressions  are  simply  dropped
from the expansion above.

9.7  Switch statement

The switch statement causes control to be transferred to one
of  several  statements depending on the value of an expres-
sion. It has the form         switch ( expression  )  state-

The usual arithmetic conversion is performed on the  expres-
sion, but the result must be int. The statement is typically
compound. Any statement within the statement may be labelled
with  one  or  more  case  prefixes as follows:         case
constant-expression :

where the constant expression must be int.  No  two  of  the
case  constants  in the same switch may have the same value.
Constant expressions are precisely defined in S15.

     There may also be at most one statement prefix  of  the
form         default :

When the switch statement is  executed,  its  expression  is
evaluated and compared with each case constant If one of the
case constants is equal to  the  value  of  the  expression,

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                           - 28 -

control  is  passed  to  the statement following the matched
case prefix. If no case constant matches the expression, and
if there is a default prefix, control passes to the prefixed
statement. If no case matches and if  there  is  no  default
then none of the statements in the switch is executed.

     Case and Bd default prefixes in themselves do not alter
the  flow  of control, which continues unimpeded across such
prefixes. To exit from a switch, see break, S9.8.

     Usually the statement that is the subject of  a  switch
is  compound.  Declarations  may  appear at the head of this
statement, initializations of automatic  or  register  vari-
ables are ineffective.

9.8  Break statement

The statement         break ;

causes termination of the smallest enclosing while, do, for,
or switch statement; control passes to the statement follow-
ing the terminated statement.

9.9  Continue statement

The statement         continue ;

causes control to pass to the loop-continuation  portion  of
the  smallest enclosing while, do, or for statement; that is
to the end of the loop.  More  precisely,  in  each  of  the
statements       while (...) {       do {                for
(...) {        ...                  ...                  ...
     contin:;            contin:;            contin:;
     }                   } while (...);      }

a continue is equivalent to `goto  contin'.  (Following  the
`contin:' is a null statement, S9.13.)

9.10  Return statement

A function returns to its caller  by  means  of  the  return
statement,  which  has  one  of  the  forms         return ;
        return expression ;

In the first case the returned value is  undefined.  In  the
second  case, the value of the expression is returned to the
caller of the function. If required, the expression is  con-
verted,  as if by assignment, to the type of the function in
which it appears. Flowing off  the  end  of  a  function  is
equivalent to a return with no returned value.

9.11  Goto statement

Control may be transferred unconditionally by means  of  the

                       April 3, 2017

                           - 29 -

statement         goto identifier ;

The identifier must  be  a  label  (S9.12)  located  in  the
current function. Previous versions of C had an incompletely
implemented notion of label variable, which has  been  with-

9.12  Labelled statement

Any statement may be preceded by label prefixes of the  form
        identifier :

which serve to declare the identifier as a label.  The  only
use  of  a  label  is  as a target of a goto. The scope of a
label is the current function, excluding any  sub-blocks  in
which the same identifier has been redeclared. See S11.

9.13  Null statement

The null statement has the form         ;

A null statement is useful to carry a label just before  the
`}'  of  a  compound statement or to supply a null body to a
looping statement such as while.

10. External definitions

A C program consists of a sequence of external  definitions.
An  external  definition  declares  an  identifier  to  have
storage class extern (by default) or perhaps static,  and  a
specified type. The type-specifier (S8.2) may also be empty,
in which case the type is taken to  be  int.  The  scope  of
external  definitions  persists  to  the  end of the file in
which they are declared just as the effect  of  declarations
persists  to  the  end  of  a  block. The syntax of external
definitions is the same as that of all declarations,  except
that only at this level may the code for functions be given.

10.1  External function definitions

Function  definitions  have   the   form           function-
definition:                   decl-specifiersopt   function-
declarator function-body

The only sc-specifiers allowed among the decl-specifiers are
extern  or  static;  See  S11.2  for the distinction between
them. A function declarator is similar to a declarator for a
`function  returning  ...'  except  that it lists the formal
parameters of the function being defined.          function-
declarator:                 declarator ( parameter-listopt )

        parameter-list:                           identifier
                identifier , parameter-list

                       April 3, 2017

                           - 30 -

The  function-body  has  the   form           function-body:
                declaration-list compound-statement

The identifiers in the parameter list, and only those  iden-
tifiers,  may be declared in the declaration list. Any iden-
tifiers whose type is not given are taken  to  be  int.  The
only storage class which may be specified is register; if it
is specified, the corresponding  actual  parameter  will  be
copied,  if  possible,  into a register at the outset of the

     A simple example of a complete function definition is

                int max(a, b, c)
                int a, b, c;
                        int m;
                        m = (a>b)? a:b;
                        return(m>c? m:c);

Here `int' is  the  type-specifier;  `max(a, b, c)'  is  the
function-declarator;  `int a, b, c;' is the declaration-list
for the formal parameters; `{ ... }' is the block giving the
code  for  the  statement. The parentheses in the return are
not required.

     C converts all float actual parameters  to  double,  so
formal  parameters  declared  float  have  their declaration
adjusted to read double. 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 nei-
ther  structures  nor functions can be passed to a function,
it is useless to declare a formal parameter to be  a  struc-
ture or function (pointers to structures or functions are of
course permitted).

     A free return 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.

10.2  External data definitions

An external  data  definition  has  the  form          data-
definition:                 declaration

The storage class of such data may be extern (which  is  the
default) or static, but not auto or register.

11.  Scope rules

A C program need not all be compiled at the same  time:  the

                       April 3, 2017

                           - 31 -

source text of the program may be kept in several files, and
precompiled routines may be loaded from libraries.  Communi-
cation  among  the functions of a program may be carried out
both through explicit  calls  and  through  manipulation  of
external data.

     Therefore, there are two kinds of  scope  to  consider:
first,  what  may be called the lexical scope of an identif-
ier, 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 refer-
ences to the same external identifier are references to  the
same object.

11.1  Lexical scope

The lexical scope of identifiers declared in external defin-
itions  persists  from the definition through the end of the
file in which they appear. The lexical scope of  identifiers
which  are  formal  parameters persists through the function
with which they are associated. The lexical scope  of  iden-
tifiers  declared  at  the head of blocks persists until the
end of the block. The lexical scope of labels is  the  whole
of the function in which they appear.

     Because all references to the same external  identifier
refer to the same object (see S11.2) the compiler checks all
declarations of the same external identifier for compatibil-
ity; in effect their scope is increased to the whole file in
which they appear.

     In all cases, however, if an identifier  is  explicitly
declared at the head of a block, including the block consti-
tuting a function, any declaration of that  identifier  out-
side the block is suspended until the end of the block.

     Remember also (S8.5) that identifiers  associated  with
ordinary variables on the one hand and those associated with
structure and union members and tags on the other  form  two
disjoint classes which do not conflict. Typedef names are in
the same class as ordinary identifiers. They  may  be  rede-
clared  in  inner blocks, but an explicit type must be given
in the inner declaration:

        typedef float distance;
        . . .
        {       auto int distance;
                . . .

The int must be present in the  second  declaration,  or  it
would  be  taken to be a declaration with no declarators and
type distance.*

                       April 3, 2017

                           - 32 -

11.2  Scope of externals

If a function declares an  identifier  to  be  extern,  then
somewhere among the files or libraries constituting the com-
plete 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 func-
tion which references the data.

     In PDP-11 C, compatible  external  definitions  of  the
same   identifier   may   be   present  in  several  of  the
separately-compiled pieces of a complete  program,  or  even
twice within the same program file, with the limitation that
the identifier may be initialized in  at  most  one  of  the
definitions.  In  other operating systems, however, the com-
piler must know in just which file the storage for the iden-
tifier  is  allocated,  and  in which file the identifier is
merely being referred to. The appearance of the extern  key-
word  in  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 defini-
tion without the extern 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 extern in the
definition.  The  identifier  can be initialized only in the
declaration where storage is allocated.

     Identifiers declared static at the top level in  exter-
nal definitions are not visible in other files.

12.  Compiler control lines

The C compiler contains a preprocessor capable of macro sub-
stitution,  conditional  compilation, and inclusion of named
files.  Lines  beginning  with  `#'  communicate  with  this
preprocessor.  These  lines  have  syntax independent of the
rest of the language; they  may  appear  anywhere  and  have
effect  which  lasts (independent of scope) until the end of
the source program file.

12.1  Token replacement

A compiler-control line of the form         #  define  iden-
tifier token-string

(note: no trailing semicolon)  causes  the  preprocessor  to
replace  subsequent  instances  of  the  identifier with the
given string of tokens. A line of the form         #  define
identifier( identifier , ... , identifier ) token-string
*It is agreed that the ice is thin here.

                       April 3, 2017

                           - 33 -

where there is no space between the first identifier and the
`(',  is  a  macro  definition  with  arguments.  Subsequent
instances of the first  identifier  followed  by  a  `(',  a
sequence  of  tokens  delimited  by  commas,  and  a `)' are
replaced  by  the  token  string  in  the  definition.  Each
occurrence  of an identifier mentioned in the formal parame-
ter list of the definition is replaced by the  corresponding
token string from the call. The actual arguments in the call
are token strings separated by  commas;  however  commas  in
quoted  strings  or protected by parentheses do not separate
arguments. The number of formal and actual  parameters  must
be the same. Text inside a string or a character constant is
not subject to replacement.

     In both forms the replacement string is  rescanned  for
more  defined  identifiers.  In both forms a long definition
may be continued on another line by writing `\' at  the  end
of the line to be continued.

     This facility is most valuable for definition of `mani-
fest constants', as in

                # define TABSIZE 100
                int table[TABSIZE];

A control line of the form         # undef identifier

causes the identifier's preprocessor definition to  be  for-

12.2  File inclusion

A compiler  control  line  of  the  form          #  include

causes the replacement of that line by the  entire  contents
of the file filename.

     The named file is searched for first in  the  directory
of the original source file, and then in a sequence of stan-
dard places. Alternatively,  a  control  line  of  the  form
        # include <filename>

searches only the standard places, and not the directory  of
the source file.

     Includes may be nested.

12.3  Conditional compilation

A compiler control line of the form         #  if  constant-

                       April 3, 2017

                           - 34 -

checks whether the constant expression (see  S15)  evaluates
to  non-zero.  A  control  line  of the form         # ifdef

checks whether the identifier is currently  defined  in  the
preprocessor;  that is, whether it has been the subject of a
#define control line. A control line of the  form          #
ifndef identifier

checks whether the identifier is currently undefined in  the

     All three forms are followed by an arbitrary number  of
lines, possibly containing a control line         # else

and then by a control line         # endif

If the checked condition is  true  then  any  lines  between
#else  and  #endif  are ignored. If the checked condition is
false then any lines between the test and an #else or, lack-
ing an #else, the #endif, are ignored.

     These constructions may be nested.

12.4  Line control

For the benefit of other preprocessors which generate C pro-
grams, a line of the form         # line constant identifier

causes the compiler to believe, for purposes of error  diag-
nostics,  that the next line number is given by the constant
and the current input file is named by  the  identifier.  If
the  identifier  is absent the remembered file name does not

13.  Implicit declarations

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 defin-
itions,  and in declarations of formal parameters and struc-
ture members. In a  declaration  inside  a  function,  if  a
storage  class  but  no  type  is  given,  the identifier is
assumed to be int; if a type but no storage class  is  indi-
cated, the identifier is assumed to be auto. An exception to
the latter rule is made for functions, since auto  functions
are  meaningless  (C  being incapable of compiling code into
the stack). If  the  type  of  an  identifier  is  `function
returning ...', it is implicitly declared to be extern.

     In an expression, an identifier followed by (  and  not
currently  declared is contextually declared to be `function
returning int'.

                       April 3, 2017

                           - 35 -

14.  Types revisited

This section summarizes the operations  which  can  be  per-
formed on objects of certain types.

14.1  Structures and unions

There are only two things that can be done with a  structure
or  union: name one of its members (by means of the . opera-
tor); or take its address (by unary  &).  Other  operations,
such  as  assigning from or to it or passing it as a parame-
ter, draw an error message. In the future,  it  is  expected
that  these  operations, but not necessarily others, will be

     S7.1 says that in a direct or indirect structure refer-
ence  (with  . or ->) the name on the right must be a member
of the structure named or pointed to by  the  expression  on
the  left.  To  allow  an escape from the typing rules, this
restriction is not firmly enforced by the compiler. In fact,
any  lvalue  is  allowed before `.', and that lvalue is then
assumed to have the form of the structure of which the  name
on the right is a member. Also, the expression before a `->'
is required only to  be  a  pointer  or  an  integer.  If  a
pointer,  it is assumed to point to a structure of which the
name on the right is a member. If an integer, it is taken to
be  the  absolute  address, in machine storage units, of the
appropriate structure.

     Such constructions are non-portable.

14.2  Functions

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

                int f();

Then the definition of g might read

                int (*funcp)();

Notice that f was declared explicitly in the calling routine

                       April 3, 2017

                           - 36 -

since its first appearance was not followed by (.

14.3  Arrays, pointers, and subscripting

Every time an identifier of array type appears in an expres-
sion,  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 [] is inter-
preted  in  such  a  way  that  `E1[E2]'  is  identical   to
`*((E1)+(E2))'.  Because of the conversion rules which apply
to +, 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  opera-

     A consistent rule is followed in  the  case  of  multi-
dimensional  arrays.  If E is an n-dimensional array of rank
$i times j times ...  times  k$,  then  E  appearing  in  an
expression is converted to a pointer to an (n-1)-dimensional
array with rank $j times ... times k$. If  the  *  operator,
either explicitly or implicitly as a result of subscripting,
is applied to this pointer, the  result  is  the  pointed-to
(n-1)-dimensional  array,  which  itself is immediately con-
verted into a pointer.

     For example, consider

                int x[3][5];

Here x is a 3x5 array of integers.  When  x  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)', x is first con-
verted to a pointer as described; then i is converted to the
type  of  x,  which involves multiplying i 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.

     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 sub-
script calculations.

15.  Constant expressions

In several places C requires expressions which evaluate to a
constant:  after case, as array bounds, and in initializers.
In the first two cases,  the  expression  can  involve  only
integer   constants,   character   constants,   and   sizeof

                       April 3, 2017

                           - 37 -

expressions, possibly  connected  by  the  binary  operators
        +  -  *  /  %  &  |  ^  <<  >>  ==  !=  <  >  <=  >=

or by the unary operators         -   ~

or by the ternary operator         ? :

Parentheses can be used for grouping, but not  for  function

     A bit more  latitude  is  permitted  for  initializers;
besides  constant  expressions  as  discussed above, one can
also apply the  unary  &  operator  to  external  or  static
objects, and to external or static arrays subscripted with a
constant expression. The unary & can also be applied  impli-
citly  by  appearance of unsubscripted arrays and functions.
The basic rule is that initializers must evaluate either  to
a constant or to the address of a previously declared exter-
nal or static object plus or minus a constant.

                       April 3, 2017

                           - 38 -

16.  Grammar revisited.

     This section repeats the grammar of C in notation some-
what  different  than given before. The description below is
adapted directly  from  a  YACC  grammar  actually  used  by
several  compilers; thus it may (aside from possible editing
errors) be regarded as authentic. The notation is pure  YACC
with  the exception that the `|' separating alternatives for
a production is omitted, since alternatives  are  always  on
separate  lines;  the  `;' separating productions is omitted
since a blank line is left between productions.

     The lines with `%term' name the terminal symbols, which
are  either  commented  upon  or should be self-evident. The
lines with `%left,' `%right,' and `%binary' indicate whether
the    listed   terminals   are   left-associative,   right-
associative, or non-associative, and describe  a  precedence
structure.  The  precedence  (binding strength) increases as
one reads down the page. When  the  construction  `%prec  x'
appears  the  precedence of the rule is that of the terminal
x; otherwise the precedence of the rule is that of its left-
most terminal.

%term NAME
%term STRING
%term ICON
%term FCON
%term PLUS
%term MINUS
%term MUL
%term AND
%term QUEST
%term COLON
%term ANDAND
%term OROR
%term ASOP    /* old-style =+ etc. */
%term RELOP   /* <= >= < > */
%term EQUOP   /* == != */
%term DIVOP   /* / % */
%term OR      /* | */
%term EXOR    /* ^ */
%term SHIFTOP /* << >> */
%term INCOP   /* ++ -- */
%term UNOP    /* ! ~ */
%term STROP   /* . -> */

%term TYPE     /* int, char, long, float, double, unsigned, short */
%term CLASS    /* extern, register, auto, static, typedef */
%term STRUCT   /* struct or union */
%term RETURN
%term GOTO
%term IF

                       April 3, 2017

                           - 39 -

%term ELSE
%term SWITCH
%term BREAK
%term WHILE
%term DO
%term FOR
%term CASE
%term SIZEOF

%term LP       /* ( */
%term RP       /* ) */
%term LC       /* { */
%term RC       /* } */
%term LB       /* [ */
%term RB       /* ] */
%term CM       /* , */
%term SM       /* ; */
%term ASSIGN   /* = */

%left   CM
%right  ASOP    ASSIGN
%right  QUEST   COLON
%left   OROR
%left   ANDAND
%left   OROP
%left   AND
%binary EQUOP
%binary RELOP
%left   SHIFTOP
%left   PLUS    MINUS
%left   MUL     DIVOP
%right  UNOP
%right  INCOP   SIZEOF
%left   LB      LP     STROP

program:          ext_def_list

ext_def_list:     ext_def_list external_def
                  /* empty */

external_def:     optattrib SM
                  optattrib init_dcl_list SM
                  optattrib fdeclarator function_body

function_body:    dcl_list compoundstmt

dcl_list:         dcl_list declaration
                  /* empty */

                       April 3, 2017

                           - 40 -

declaration:      specifiers declarator_list SM
                  specifiers SM

optattrib:        specifiers
                  /* empty */

specifiers:       CLASS type
                  type CLASS

type:             TYPE
                  TYPE TYPE

struct_dcl:       STRUCT NAME LC type_dcl_list RC
                  STRUCT LC type_dcl_list RC
                  STRUCT NAME

type_dcl_list:    type_declaration
                  type_dcl_list type_declaration

type_declaration: type declarator_list SM
                  struct_dcl SM
                  type SM

declarator_list:  declarator
                  declarator_list CM declarator

declarator:       fdeclarator
                  nfdeclarator COLON con_e %prec CM
                  COLON con_e %prec CM

nfdeclarator:     MUL nfdeclarator
                  nfdeclarator LP RP
                  nfdeclarator LB RB
                  nfdeclarator LB con_e RB
                  LP nfdeclarator RP

fdeclarator:      MUL fdeclarator
                  fdeclarator LP RP
                  fdeclarator LB RB
                  fdeclarator LB con_e RB
                  LP fdeclarator RP
                  NAME LP name_list RP
                  NAME LP RP

name_list:        NAME
                  name_list CM NAME

init_dcl_list:    init_declarator %prec CM

                       April 3, 2017

                           - 41 -

                  init_dcl_list CM init_declarator

init_declarator:  nfdeclarator
                  nfdeclarator ASSIGN initializer
                  nfdeclarator initializer

init_list:        initializer %prec CM
                  init_list CM initializer

initializer:      e %prec CM
                  LC init_list RC
                  LC init_list CM RC

compoundstmt:     LC dcl_list stmt_list RC

stmt_list:        stmt_list statement
                  /* empty */

statement:        e SM
                  IF LP e RP statement
                  IF LP e RP statement ELSE statement
                  WHILE LP e RP statement
                  DO statement WHILE LP e RP SM
                  FOR LP opt_e SM opt_e SM opt_e RP statement
                  SWITCH LP e RP statement
                  BREAK SM
                  CONTINUE SM
                  RETURN SM
                  RETURN e SM
                  GOTO NAME SM
                  label statement

label:            NAME COLON
                  CASE con_e COLON
                  DEFAULT COLON

con_e:            e %prec CM

opt_e:            e
                  /* empty */

elist:            e %prec CM
                  elist CM e

e:                e MUL e
                  e CM e
                  e DIVOP e
                  e PLUS e
                  e MINUS e
                  e SHIFTOP e
                  e RELOP e

                       April 3, 2017

                           - 42 -

                  e EQUOP e
                  e AND e
                  e OROP e
                  e ANDAND e
                  e OROR e
                  e MUL ASSIGN e
                  e DIVOP ASSIGN e
                  e PLUS ASSIGN e
                  e MINUS ASSIGN e
                  e SHIFTOP ASSIGN e
                  e AND ASSIGN e
                  e OROP ASSIGN e
                  e QUEST e COLON e
                  e ASOP e
                  e ASSIGN e

term:             term INCOP
                  MUL term
                  AND term
                  MINUS term
                  UNOP term
                  INCOP term
                  SIZEOF term
                  LP type_name RP term %prec STROP
                  SIZEOF LP type_name RP %prec SIZEOF
                  term LB e RB
                  term LP RP
                  term LP elist RP
                  term STROP NAME
                  LP e RP

type_name:        type abst_decl

abst_decl:        /* empty */
                  LP RP
                  LP abst_decl RP LP RP
                  MUL abst_decl
                  abst_decl LB RB
                  abst_decl LB con_e RB
                  LP abst_decl RP

                       April 3, 2017

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