______________________________________________________________________

  16   Preprocessing directives                          [cpp]

  ______________________________________________________________________

1 A preprocessing directive consists  of  a  sequence  of  preprocessing
  tokens  that  begins  with  a # preprocessing token that is either the
  first character in the source file (optionally after white space  con­
  taining no new-line characters) or that follows white space containing
  at least one new-line character, and is ended  by  the  next  new-line
  character.1)
          preprocessing-file:
                  groupopt
          group:
                  group-part
                  group group-part
          group-part:
                  pp-tokensopt new-line
                  if-section
                  control-line
          if-section:
                  if-group elif-groupsopt else-groupopt endif-line
          if-group:
                  # if     constant-expression new-line groupopt
                  # ifdef  identifier new-line groupopt
                  # ifndef identifier new-line groupopt
          elif-groups:
                  elif-group
                  elif-groups elif-group
          elif-group:
                  # elif   constant-expression new-line groupopt
          else-group:
                  # else   new-line groupopt
          endif-line:
                  # endif  new-line

  _________________________
  1)  Thus,  preprocessing  directives are commonly called lines.  These
  lines have no other syntactic significance,  as  all  white  space  is
  equivalent  except in certain situations during preprocessing (see the
  # character string literal creation operator in  _cpp.stringize_,  for
  example).

          control-line:
                  # include pp-tokens new-line
                  # define  identifier replacement-list new-line
                  # define  identifier lparen identifier-listopt ) replacement-list new-line
                  # undef   identifier new-line
                  # line    pp-tokens new-line
                  # error   pp-tokensopt new-line
                  # pragma  pp-tokensopt new-line
                  #         new-line
          lparen:
                  the left-parenthesis character without preceding white-space
          replacement-list:
                  pp-tokensopt
          pp-tokens:
                  preprocessing-token
                  pp-tokens preprocessing-token
          new-line:
                  the new-line character

2 The  only white-space characters that shall appear between preprocess­
  ing tokens within a  preprocessing  directive  (from  just  after  the
  introducing  # preprocessing token through just before the terminating
  new-line character) are space  and  horizontal-tab  (including  spaces
  that  have  replaced comments or possibly other white-space characters
  in translation phase 3).

3 The implementation can process and skip sections of source files  con­
  ditionally,  include  other  source  files, and replace macros.  These
  capabilities are called preprocessing, because conceptually they occur
  before translation of the resulting translation unit.

4 The preprocessing tokens within a preprocessing directive are not sub­
  ject to macro expansion unless otherwise stated.

  16.1  Conditional inclusion                                 [cpp.cond]

1 The expression that controls conditional inclusion shall be  an  inte­
  gral  constant  expression  except  that: it shall not contain a cast;
  identifiers (including those  lexically  identical  to  keywords)  are
  interpreted  as  described  below;2) and it may contain unary operator
  expressions of the form
          defined identifier
  or
          defined ( identifier )
  which evaluate to 1 if the identifier is currently defined as a  macro
  name  (that is, if it is predefined or if it has been the subject of a
  #define preprocessing directive without an intervening  #undef  direc­
  tive with the same subject identifier), zero if it is not.
  _________________________
  2)  Because  the  controlling  constant expression is evaluated during
  translation phase 4, all identifiers either are or are not macro names
  -- there simply are no keywords, enumeration constants, and so on.

2 Each  preprocessing  token  that  remains after all macro replacements
  have occurred shall be in the lexical form of a token (_lex.token_).

3 Preprocessing directives of the forms
          # if   constant-expression new-line groupopt
          # elif constant-expression new-line groupopt
  check  whether  the  controlling  constant  expression  evaluates   to
  nonzero.

4 Prior  to  evaluation,  macro invocations in the list of preprocessing
  tokens that  will  become  the  controlling  constant  expression  are
  replaced  (except  for those macro names modified by the defined unary
  operator), just as in normal text.  If the token defined is  generated
  as  a  result  of this replacement process or use of the defined unary
  operator does not match one of the two specified forms prior to  macro
  replacement, the behavior is undefined.  After all replacements due to
  macro expansion and the defined unary operator  have  been  performed,
  all  remaining identifiers are replaced with the pp-number 0, and then
  each preprocessing token is converted into  a  token.   The  resulting
  tokens comprise the controlling constant expression which is evaluated
  according to the rules of _expr.const_ using arithmetic  that  has  at
  least  the  ranges  specified in _lib.support.limits_, except that int
  and unsigned int act as if  they  have  the  same  representation  as,
  respectively,  long  and  unsigned  long.   This includes interpreting
  character constants, which may  involve  converting  escape  sequences
  into  execution  character set members.  Whether the numeric value for
  these character constants matches the value obtained when an identical
  character constant occurs in an expression (other than within a #if or
  #elif directive) is implementation-defined.3) Also, whether a  single-
  character  character constant may have a negative value is implementa­
  tion-defined.

5 Preprocessing directives of the forms
          # ifdef  identifier new-line groupopt
          # ifndef identifier new-line groupopt
  check whether the identifier is or is not currently defined as a macro
  name.   Their  conditions are equivalent to #if defined identifier and
  #if !defined identifier respectively.

6 Each directive's condition is checked in order.  If  it  evaluates  to
  false  (zero),  the  group that it controls is skipped: directives are
  processed only through the name that determines the directive in order
  to  keep  track  of  the level of nested conditionals; the rest of the
  directives' preprocessing tokens are ignored, as are the other prepro­
  cessing  tokens in the group.  Only the first group whose control con­
  dition evaluates to true (nonzero)  is  processed.   If  none  of  the
  _________________________
  3) Thus, the constant expression in the following #if directive and if
  statement is not guaranteed to evaluate to the same value in these two
  contexts.
          #if 'z' - 'a' == 25
          if ('z' - 'a' == 25)

  conditions  evaluates  to  true,  and  there is a #else directive, the
  group controlled by the #else is processed; lacking a #else directive,
  all the groups until the #endif are skipped.4)

  16.2  Source file inclusion                              [cpp.include]

1 A #include directive shall identify a header or source file  that  can
  be processed by the implementation.

2 A preprocessing directive of the form
          # include <h-char-sequence> new-line
  searches  a  sequence  of  implementation-defined  places for a header
  identified uniquely by the specified sequence  between  the  <  and  >
  delimiters, and causes the replacement of that directive by the entire
  contents of the header.  How the places are specified  or  the  header
  identified is implementation-defined.

3 A preprocessing directive of the form
          # include "q-char-sequence" new-line
  causes the replacement of that directive by the entire contents of the
  source file identified by the specified sequence between the "  delim­
  iters.   The  named  source file is searched for in an implementation-
  defined manner.  If this search is not supported,  or  if  the  search
  fails, the directive is reprocessed as if it read
          # include <h-char-sequence> new-line
  with the identical contained sequence (including > characters, if any)
  from the original directive.

4 A preprocessing directive of the form
          # include pp-tokens new-line
  (that does not match one of the two previous forms) is permitted.  The
  preprocessing tokens after include in the directive are processed just
  as in normal text.  (Each identifier currently defined as a macro name
  is  replaced  by  its  replacement  list  of preprocessing tokens.)The
  directive resulting after all replacements shall match one of the  two
  previous forms.5) The method by  which  a  sequence  of  preprocessing
  tokens  between  a  <  and a > preprocessing token pair or a pair of "
  characters is combined into a single header name  preprocessing  token
  is implementation-defined.

5 There shall be an implementation-defined mapping between the delimited
  sequence and the external source file name.  The implementation  shall
  provide  unique  mappings  for  sequences  consisting  of  one or more
  _________________________
  4)  As indicated by the syntax, a preprocessing token shall not follow
  a #else or #endif directive before the terminating new-line character.
  However,  comments  may  appear  anywhere  in a source file, including
  within a preprocessing directive.
  5) Note that adjacent string literals are not concatenated into a sin­
  gle string literal (see the translation phases in _lex.phases_); thus,
  an  expansion that results in two string literals is an invalid direc­
  tive.

  nondigits (_lex.name_) followed by a period .)  and a single nondigit.
  The  implementation  may  ignore the distinctions of alphabetical case
  and restrict the mapping to  six  significant  characters  before  the
  period.

  +-------                 BEGIN BOX 1                -------+
  Does this restriction still make sense for C++?
  +-------                  END BOX 1                 -------+

6 A  #include  preprocessing  directive may appear in a source file that
  has been read because of a #include directive in another file,  up  to
  an implementation-defined nesting limit (see <<<<???>>>>).

7 The  most  common  uses of #include preprocessing directives are as in
  the following:
          #include <stdio.h>
          #include "myprog.h"

8 This example illustrates a macro-replaced #include directive:
          #if VERSION == 1
                  #define INCFILE  "vers1.h"
          #elif VERSION == 2
                  #define INCFILE  "vers2.h"   /* and so on */
          #else
                  #define INCFILE  "versN.h"
          #endif
          #include INCFILE

  16.3  Macro replacement                                  [cpp.replace]

1 Two replacement lists are identical if and only if  the  preprocessing
  tokens  in  both  have the same number, ordering, spelling, and white-
  space separation, where all  white-space  separations  are  considered
  identical.

2 An  identifier  currently defined as a macro without use of lparen (an
  object-like macro) may be redefined by another  #define  preprocessing
  directive  provided that the second definition is an object-like macro
  definition and the two replacement lists are identical.

3 An identifier currently defined as a macro using lparen  (a  function-
  like  macro)  may be redefined by another #define preprocessing direc­
  tive provided that the second definition is a function-like macro def­
  inition  that  has the same number and spelling of parameters, and the
  two replacement lists are identical.

4 The number of arguments in an  invocation  of  a  function-like  macro
  shall agree with the number of parameters in the macro definition, and
  there shall exist a ) preprocessing token that terminates the  invoca­
  tion.

5 A  parameter  identifier  in  a  function-like macro shall be uniquely
  declared within its scope.

6 The identifier immediately following the define is  called  the  macro
  name.  There is one name space for macro names.  Any white-space char­
  acters preceding or following the replacement  list  of  preprocessing
  tokens are not considered part of the replacement list for either form
  of macro.

7 If a # preprocessing token, followed by an  identifier,  occurs  lexi­
  cally at the point at which a preprocessing directive could begin, the
  identifier is not subject to macro replacement.

8 A preprocessing directive of the form
          # define identifier replacement-list new-line
  defines an object-like macro that causes each subsequent  instance  of
  the macro name6) to be replaced by the replacement list of preprocess­
  ing  tokens  that  constitute  the  remainder  of  the directive.  The
  replacement list is then rescanned for more macro names  as  specified
  below.

9 A preprocessing directive of the form
          # define identifier lparen identifier-listopt ) replacement-list new-line
  defines  a  function-like macro with parameters, similar syntactically
  to a function call.  The parameters are specified by the optional list
  of  identifiers,  whose  scope  extends  from their declaration in the
  identifier list until  the  new-line  character  that  terminates  the
  #define  preprocessing  directive.   Each  subsequent  instance of the
  function-like macro name followed by a (  as  the  next  preprocessing
  token introduces the sequence of preprocessing tokens that is replaced
  by the replacement list  in  the  definition  (an  invocation  of  the
  macro).   The  replaced sequence of preprocessing tokens is terminated
  by the matching ) preprocessing token,  skipping  intervening  matched
  pairs  of left and right parenthesis preprocessing tokens.  Within the
  sequence of preprocessing tokens making up an invocation  of  a  func­
  tion-like  macro,  new-line is considered a normal white-space charac­
  ter.

10The sequence of  preprocessing  tokens  bounded  by  the  outside-most
  matching parentheses forms the list of arguments for the function-like
  macro.  The individual arguments within  the  list  are  separated  by
  comma  preprocessing  tokens,  but  comma preprocessing tokens between
  matching inner parentheses do  not  separate  arguments.   If  (before
  argument  substitution)  any  argument  consists  of  no preprocessing
  tokens, the behavior is undefined.  If there are sequences of  prepro­
  cessing  tokens  within the list of arguments that would otherwise act
  as preprocessing directives, the behavior is undefined.
  _________________________
  6)  Since,  by  macro-replacement  time,  all  character constants and
  string literals are preprocessing tokens, not sequences possibly  con­
  taining  identifier-like  subsequences (see 2.1.1.2, translation phas­
  es), they are never scanned for macro names or parameters.

  16.3.1  Argument substitution                              [cpp.subst]

1 After the arguments for the invocation of a function-like  macro  have
  been  identified,  argument  substitution takes place.  A parameter in
  the replacement list, unless preceded by a # or ## preprocessing token
  or  followed  by  a ## preprocessing token (see below), is replaced by
  the corresponding argument after all  macros  contained  therein  have
  been  expanded.  Before being substituted, each argument's preprocess­
  ing tokens are completely macro replaced as if they formed the rest of
  the translation unit; no other preprocessing tokens are available.

  16.3.2  The # operator                                 [cpp.stringize]

1 Each # preprocessing token in the replacement list for a function-like
  macro shall be followed by a parameter as the next preprocessing token
  in the replacement list.

2 If,  in the replacement list, a parameter is immediately preceded by a
  # preprocessing token, both are replaced by a single character  string
  literal  preprocessing token that contains the spelling of the prepro­
  cessing token sequence for the corresponding  argument.   Each  occur­
  rence  of  white  space  between  the  argument's preprocessing tokens
  becomes a single space character  in  the  character  string  literal.
  White  space  before  the first preprocessing token and after the last
  preprocessing token comprising the argument  is  deleted.   Otherwise,
  the  original  spelling of each preprocessing token in the argument is
  retained in the character string literal, except for special  handling
  for producing the spelling of string literals and character constants:
  a \ character is inserted before each " and \ character of a character
  constant  or  string  literal (including the delimiting " characters).
  If the replacement that results is not a valid character  string  lit­
  eral,  the behavior is undefined.  The order of evaluation of # and ##
  operators is unspecified.

  16.3.3  The ## operator                                   [cpp.concat]

1 A ## preprocessing token shall not occur at the beginning  or  at  the
  end of a replacement list for either form of macro definition.

2 If,  in  the  replacement list, a parameter is immediately preceded or
  followed by a ## preprocessing token, the parameter is replaced by the
  corresponding argument's preprocessing token sequence.

3 For  both  object-like and function-like macro invocations, before the
  replacement list is reexamined for more macro names to  replace,  each
  instance of a ## preprocessing token in the replacement list (not from
  an argument) is deleted and the preceding preprocessing token is  con­
  catenated  with  the  following preprocessing token.  If the result is
  not a valid preprocessing  token,  the  behavior  is  undefined.   The
  resulting token is available for further macro replacement.  The order
  of evaluation of ## operators is unspecified.

  16.3.4  Rescanning and further replacement                [cpp.rescan]

1 After all parameters in the replacement list  have  been  substituted,
  the  resulting preprocessing token sequence is rescanned with all sub­
  sequent preprocessing tokens of the source file for more  macro  names
  to replace.

2 If  the  name of the macro being replaced is found during this scan of
  the replacement list (not including the rest of the source file's pre­
  processing  tokens),  it  is  not  replaced.   Further,  if any nested
  replacements encounter the name of the macro being replaced, it is not
  replaced.   These  nonreplaced  macro name preprocessing tokens are no
  longer available for  further  replacement  even  if  they  are  later
  (re)examined  in contexts in which that macro name preprocessing token
  would otherwise have been replaced.

3 The resulting completely macro-replaced preprocessing  token  sequence
  is  not  processed  as  a preprocessing directive even if it resembles
  one.

  16.3.5  Scope of macro definitions                         [cpp.scope]

1 A macro definition lasts (independent of block structure) until a cor­
  responding #undef directive is encountered or (if none is encountered)
  until the end of the translation unit.

2 A preprocessing directive of the form
          # undef identifier new-line
  causes the specified identifier no longer to be  defined  as  a  macro
  name.   It  is  ignored  if  the specified identifier is not currently
  defined as a macro name.

3 The simplest use of this facility is to define a manifest constant, as
  in
          #define TABSIZE 100
          int table[TABSIZE];

4 The following defines a function-like macro whose value is the maximum
  of its arguments.  It has the advantages of working for any compatible
  types  of  the  arguments  and  of generating in-line code without the
  overhead of function calling.  It has the disadvantages of  evaluating
  one  or  the  other  of  its  arguments  a second time (including side
  effects) and generating more code than a function if  invoked  several
  times.  It also cannot have its address taken, as it has none.
          #define max(a, b) ((a) > (b) ? (a) : (b))
  The parentheses ensure that the arguments and the resulting expression
  are bound properly.

5 To illustrate  the  rules  for  redefinition  and  reexamination,  the
  sequence

          #define x    3
          #define f(a) f(x * (a))
          #undef  x
          #define x    2
          #define g    f
          #define z    z[0]
          #define h    g(~
          #define m(a) a(w)
          #define w    0,1
          #define t(a) a
          f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
          g(x+(3,4)-w) | h 5) & m
                  (f)^m(m);
  results in
          f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
          f(2 * (2+(3,4)-0,1)) | f(2 * (~5)) & f(2 * (0,1))^m(0,1);

6 To  illustrate  the  rules  for creating character string literals and
  concatenating tokens, the sequence
          #define str(s)      # s
          #define xstr(s)     str(s)
          #define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \
                                          x ## s, x ## t)
          #define INCFILE(n)  vers ## n  /* from previous #include example */
          #define glue(a, b)  a ## b
          #define xglue(a, b) glue(a, b)
          #define HIGHLOW     "hello"
          #define LOW         LOW ", world"
          debug(1, 2);
          fputs(str(strncmp("abc\0d", "abc", '\4')  /* this goes away */
                  == 0) str(: @\n), s);
          #include xstr(INCFILE(2).h)
          glue(HIGH, LOW);
          xglue(HIGH, LOW)
  results in
          printf("x" "1" "= %d, x" "2" "= %s", x1, x2);
          fputs("strncmp(\"abc\\0d\", \"abc\", '\\4') == 0" ": @\n", s);
          #include "vers2.h"    (after macro replacement, before file access)
          "hello";
          "hello" ", world"
  or, after concatenation of the character string literals,
          printf("x1= %d, x2= %s", x1, x2);
          fputs("strncmp(\"abc\\0d\", \"abc\", '\\4') == 0: @\n", s);
          #include "vers2.h"    (after macro replacement, before file access)
          "hello";
          "hello, world"
  Space around the # and ## tokens in the macro definition is  optional.

7 And  finally,  to  demonstrate  the  redefinition rules, the following
  sequence is valid.

          #define OBJ_LIKE      (1-1)
          #define OBJ_LIKE      /* white space */ (1-1) /* other */
          #define FTN_LIKE(a)   ( a )
          #define FTN_LIKE( a )(          /* note the white space */ \
                                          a /* other stuff on this line
                                            */ )
  But the following redefinitions are invalid:
          #define OBJ_LIKE    (0)     /* different token sequence */
          #define OBJ_LIKE    (1 - 1) /* different white space */
          #define FTN_LIKE(b) ( a )   /* different parameter usage */
          #define FTN_LIKE(b) ( b )   /* different parameter spelling */

  16.4  Line control                                          [cpp.line]

1 The string literal of a #line directive, if present, shall be a  char­
  acter string literal.

2 The  line  number  of  the current source line is one greater than the
  number of new-line characters read or introduced in translation  phase
  1  (_lex.phases_)  while  processing  the  source  file to the current
  token.

3 A preprocessing directive of the form
          # line digit-sequence new-line
  causes the implementation to behave as if the  following  sequence  of
  source lines begins with a source line that has a line number as spec­
  ified by the digit sequence (interpreted as a decimal  integer).   The
  digit  sequence  shall  not  specify  zero,  nor a number greater than
  32767.

4 A preprocessing directive of the form
          # line digit-sequence "s-char-sequenceopt" new-line
  sets the line number similarly and changes the presumed  name  of  the
  source file to be the contents of the character string literal.

5 A preprocessing directive of the form
          # line pp-tokens new-line
  (that does not match one of the two previous forms) is permitted.  The
  preprocessing tokens after line on the directive are processed just as
  in  normal  text (each identifier currently defined as a macro name is
  replaced by its replacement list of preprocessing tokens).  The direc­
  tive  resulting after all replacements shall match one of the two pre­
  vious forms and is then processed as appropriate.

  16.5  Error directive                                      [cpp.error]

1 A preprocessing directive of the form
          # error pp-tokensopt new-line
  causes  the  implementation  to  produce  a  diagnostic  message  that
  includes the specified sequence of preprocessing tokens.

  16.6  Pragma directive                                    [cpp.pragma]

1 A preprocessing directive of the form
          # pragma pp-tokensopt new-line
  causes  the implementation to behave in an implementation-defined man­
  ner.  Any pragma that is  not  recognized  by  the  implementation  is
  ignored.

  16.7  Null directive                                        [cpp.null]

1 A preprocessing directive of the form
          # new-line
  has no effect.

  16.8  Predefined macro names                          [cpp.predefined]

1 The following macro names shall be defined by the implementation:

  __LINE__
    The line number of the current source line (a decimal constant).

  __FILE__
    The presumed name of the source file (a character string literal).

  __DATE__
    The  date of translation of the source file (a character string lit­
    eral of the form "Mmm dd yyyy", where the names of  the  months  are
    the  same  as those generated by the asctime function, and the first
    character of dd is a space character if the value is less than  10).
    If  the  date  of  translation  is not available, an implementation-
    defined valid date shall be supplied.

  __TIME__
    The time of translation of the source file (a character string  lit­
    eral  of the form "hh:mm:ss" as in the time generated by the asctime
    function).  If the time of translation is not available,  an  imple­
    mentation-defined valid time shall be supplied.

  __STDC__
    Whether __STDC__ is defined and if so, what its value is, are imple­
    mentation dependent.

  __cplusplus
    The name __cplusplus is defined (to an unspecified value) when  com­
    piling a C++ translation unit.

2 The values of the predefined macros (except for __LINE__ and __FILE__)
  remain constant throughout the translation unit.

3 None of these macro names, nor the identifier defined,  shall  be  the
  subject  of a #define or a #undef preprocessing directive.  All prede­
  fined macro names shall begin with a leading underscore followed by an

  uppercase letter or a second underscore.