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+\input texinfo
+@setfilename cpp.info
+@settitle The C Preprocessor
+@setchapternewpage odd
+@ifinfo
+This file documents the GNU C Preprocessor.
+
+Copyright (C) 1987, 1989 Free Software Foundation, Inc.
+
+Permission is granted to make and distribute verbatim copies of
+this manual provided the copyright notice and this permission notice
+are preserved on all copies.
+
+@ignore
+Permission is granted to process this file through Tex and print the
+results, provided the printed document carries copying permission
+notice identical to this one except for the removal of this paragraph
+(this paragraph not being relevant to the printed manual).
+
+@end ignore
+Permission is granted to copy and distribute modified versions of this
+manual under the conditions for verbatim copying, provided also that
+the entire resulting derived work is distributed under the terms of a
+permission notice identical to this one.
+
+Permission is granted to copy and distribute translations of this manual
+into another language, under the above conditions for modified versions.
+@end ifinfo
+
+@titlepage
+@sp 6
+@center @titlefont{The C Preprocessor}
+@sp 4
+@center Last revised July 1990
+@center for GCC version 1.38
+@sp 5
+@center Richard M. Stallman
+@page
+@vskip 0pt plus 1filll
+Copyright @copyright{} 1987, 1989 Free Software Foundation, Inc.
+
+Permission is granted to make and distribute verbatim copies of
+this manual provided the copyright notice and this permission notice
+are preserved on all copies.
+
+Permission is granted to copy and distribute modified versions of this
+manual under the conditions for verbatim copying, provided also that
+the entire resulting derived work is distributed under the terms of a
+permission notice identical to this one.
+
+Permission is granted to copy and distribute translations of this manual
+into another language, under the above conditions for modified versions.
+@end titlepage
+@page
+
+@node Top, Global Actions,, (DIR)
+@chapter The C Preprocessor
+
+The C preprocessor is a @dfn{macro processor} that is used automatically by
+the C compiler to transform your program before actual compilation.  It is
+called a macro processor because it allows you to define @dfn{macros},
+which are brief abbreviations for longer constructs.
+
+The C preprocessor provides four separate facilities that you can use as
+you see fit:
+
+@itemize @bullet
+@item
+Inclusion of header files.  These are files of declarations that can be
+substituted into your program.
+
+@item
+Macro expansion.  You can define @dfn{macros}, which are abbreviations
+for arbitrary fragments of C code, and then the C preprocessor will
+replace the macros with their definitions throughout the program.
+
+@item
+Conditional compilation.  Using special preprocessor commands, you
+can include or exclude parts of the program according to various
+conditions.
+
+@item
+Line control.  If you use a program to combine or rearrange source files into
+an intermediate file which is then compiled, you can use line control
+to inform the compiler of where each source line originally came from.
+@end itemize
+
+C preprocessors vary in some details.  This manual discusses the GNU C
+preprocessor, the C Compatible Compiler Preprocessor.  The GNU C
+preprocessor provides a superset of the features of ANSI Standard C.
+
+ANSI Standard C requires the rejection of many harmless constructs commonly
+used by today's C programs.  Such incompatibility would be inconvenient for
+users, so the GNU C preprocessor is configured to accept these constructs
+by default.  Strictly speaking, to get ANSI Standard C, you must use the
+options @samp{-trigraphs}, @samp{-undef} and @samp{-pedantic}, but in
+practice the consequences of having strict ANSI Standard C make it
+undesirable to do this.  @xref{Invocation}.
+
+@menu
+* Global Actions::    Actions made uniformly on all input files.
+* Commands::          General syntax of preprocessor commands.
+* Header Files::      How and why to use header files.
+* Macros::            How and why to use macros.
+* Conditionals::      How and why to use conditionals.
+* Combining Sources:: Use of line control when you combine source files.
+* Other Commands::    Miscellaneous preprocessor commands.
+* Output::            Format of output from the C preprocessor.
+* Invocation::        How to invoke the preprocessor; command options.
+* Concept Index::     Index of concepts and terms.
+* Index::             Index of commands, predefined macros and options.
+@end menu
+
+@node Global Actions, Commands, Top, Top
+@section Transformations Made Globally
+
+Most C preprocessor features are inactive unless you give specific commands
+to request their use.  (Preprocessor commands are lines starting with
+@samp{#}; @pxref{Commands}).  But there are three transformations that the
+preprocessor always makes on all the input it receives, even in the absence
+of commands.
+
+@itemize @bullet
+@item
+All C comments are replaced with single spaces.
+
+@item
+Backslash-Newline sequences are deleted, no matter where.  This
+feature allows you to break long lines for cosmetic purposes without
+changing their meaning.
+
+@item
+Predefined macro names are replaced with their expansions
+(@pxref{Predefined}).
+@end itemize
+
+The first two transformations are done @emph{before} nearly all other parsing
+and before preprocessor commands are recognized.  Thus, for example, you
+can split a line cosmetically with Backslash-Newline anywhere (except
+when trigraphs are in use; see below).
+
+@example
+/*
+*/ # /*
+*/ defi\
+ne FO\
+O 10\
+20
+@end example
+
+@noindent
+is equivalent into @samp{#define FOO 1020}.  You can split even an escape
+sequence with Backslash-Newline.  For example, you can split @code{"foo\bar"}
+between the @samp{\} and the @samp{b} to get
+
+@example
+"foo\\
+bar"
+@end example
+
+@noindent
+This behavior is unclean: in all other contexts, a Backslash can be
+inserted in a string constant as an ordinary character by writing a double
+Backslash, and this creates an exception.  But the ANSI C standard requires
+it.  (Strict ANSI C does not allow Newlines in string constants, so they
+do not consider this a problem.)
+
+But there are a few exceptions to all three transformations.
+
+@itemize @bullet
+@item
+C comments and predefined macro names are not recognized inside a
+@samp{#include} command in which the file name is delimited with
+@samp{<} and @samp{>}.
+
+@item
+C comments and predefined macro names are never recognized within a
+character or string constant.  (Strictly speaking, this is the rule,
+not an exception, but it is worth noting here anyway.)
+
+@item
+Backslash-Newline may not safely be used within an ANSI ``trigraph''.
+Trigraphs are converted before Backslash-Newline is deleted.  If you
+write what looks like a trigraph with a Backslash-Newline inside, the
+Backslash-Newline is deleted as usual, but it is then too late to
+recognize the trigraph.
+
+This exception is relevant only if you use the @samp{-trigraphs}
+option to enable trigraph processing.  @xref{Invocation}.
+@end itemize
+
+@node Commands, Header Files, Global Actions, Top
+@section Preprocessor Commands
+
+@cindex preprocessor commands
+@cindex commands
+Most preprocessor features are active only if you use preprocessor commands
+to request their use.
+
+Preprocessor commands are lines in your program that start with @samp{#}.
+The @samp{#} is followed by an identifier that is the @dfn{command name}.
+For example, @samp{#define} is the command that defines a macro.
+Whitespace is also allowed before and after the @samp{#}.
+
+The set of valid command names is fixed.  Programs cannot define new
+preprocessor commands.
+
+Some command names require arguments; these make up the rest of the command
+line and must be separated from the command name by whitespace.  For example,
+@samp{#define} must be followed by a macro name and the intended expansion
+of the macro.
+
+A preprocessor command cannot be more than one line in normal circumstances.
+It may be split cosmetically with Backslash-Newline, but that has no effect
+on its meaning.  Comments containing Newlines can also divide the command into
+multiple lines, but the comments are changed to Spaces before the command
+is interpreted.  The only way a significant Newline can occur in a preprocessor
+command is within a string constant or character constant.  Note that
+most C compilers that might be applied to the output from the preprocessor
+do not accept string or character constants containing Newlines.
+
+The @samp{#} and the command name cannot come from a macro expansion.  For
+example, if @samp{foo} is defined as a macro expanding to @samp{define},
+that does not make @samp{#foo} a valid preprocessor command.
+
+@node Header Files, Macros, Commands, Top
+@section Header Files
+
+@cindex header file
+A header file is a file containing C declarations and macro definitions
+(@pxref{Macros}) to be shared between several source files.  You request
+the use of a header file in your program with the C preprocessor command
+@samp{#include}.
+
+@menu
+* Header Uses::         What header files are used for.
+* Include Syntax::      How to write @samp{#include} commands.
+* Include Operation::   What @samp{#include} does.
+* Once-Only::		Preventing multiple inclusion of one header file.
+@end menu
+
+@node Header Uses, Include Syntax, Header Files, Header Files
+@subsection Uses of Header Files
+
+Header files serve two kinds of purposes.
+
+@itemize @bullet
+@item
+@findex system header files
+System header files declare the interfaces to parts of the operating
+system.  You include them in your program to supply the definitions
+you need to invoke system calls and libraries.
+
+@item
+Your own header files contain declarations for interfaces between the
+source files of your program.  Each time you have a group of related
+declarations and macro definitions all or most of which are needed in
+several different source files, it is a good idea to create a header
+file for them.
+@end itemize
+
+Including a header file produces the same results in C compilation as
+copying the header file into each source file that needs it.  But such
+copying would be time-consuming and error-prone.  With a header file, the
+related declarations appear in only one place.  If they need to be changed,
+they can be changed in one place, and programs that include the header file
+will automatically use the new version when next recompiled.  The header
+file eliminates the labor of finding and changing all the copies as well as
+the risk that a failure to find one copy will result in inconsistencies
+within a program.
+
+The usual convention is to give header files names that end with @file{.h}.
+
+@node Include Syntax, Include Operation, Header Uses, Header Files
+@subsection The @samp{#include} Command
+
+@findex #include
+Both user and system header files are included using the preprocessor
+command @samp{#include}.  It has three variants:
+
+@table @code
+@item #include <@var{file}>
+This variant is used for system header files.  It searches for a file
+named @var{file} in a list of directories specified by you, then in a
+standard list of system directories.  You specify directories to
+search for header files with the command option @samp{-I}
+(@pxref{Invocation}).  The option @samp{-nostdinc} inhibits searching
+the standard system directories; in this case only the directories
+you specify are searched.
+
+The parsing of this form of @samp{#include} is slightly special
+because comments are not recognized within the @samp{<@dots{}>}.
+Thus, in @samp{#include <x/*y>} the @samp{/*} does not start a comment
+and the command specifies inclusion of a system header file named
+@file{x/*y}.  Of course, a header file with such a name is unlikely to
+exist on Unix, where shell wildcard features would make it hard to
+manipulate.@refill
+
+The argument @var{file} may not contain a @samp{>} character.  It may,
+however, contain a @samp{<} character.
+
+@item #include "@var{file}"
+This variant is used for header files of your own program.  It
+searches for a file named @var{file} first in the current directory,
+then in the same directories used for system header files.  The
+current directory is the directory of the current input file.  It is
+tried first because it is presumed to be the location of the files
+that the current input file refers to.  (If the @samp{-I-} option is
+used, the special treatment of the current directory is inhibited.)
+
+The argument @var{file} may not contain @samp{"} characters.  If
+backslashes occur within @var{file}, they are considered ordinary text
+characters, not escape characters.  None of the character escape
+sequences appropriate to string constants in C are processed.  Thus,
+@samp{#include "x\n\\y"} specifies a filename containing three
+backslashes.  It is not clear why this behavior is ever useful, but
+the ANSI standard specifies it.
+
+@item #include @var{anything else}
+This variant is called a @dfn{computed #include}.  Any @samp{#include}
+command whose argument does not fit the above two forms is a computed
+include.  The text @var{anything else} is checked for macro calls,
+which are expanded (@pxref{Macros}).  When this is done, the result
+must fit one of the above two variants.
+
+This feature allows you to define a macro which controls the file name
+to be used at a later point in the program.  One application of this
+is to allow a site-configuration file for your program to specify the
+names of the system include files to be used.  This can help in
+porting the program to various operating systems in which the
+necessary system header files are found in different places.
+@end table
+
+@node Include Operation, Once-Only, Include Syntax, Header Files
+@subsection How @samp{#include} Works
+
+The @samp{#include} command works by directing the C preprocessor to scan
+the specified file as input before continuing with the rest of the current
+file.  The output from the preprocessor contains the output already
+generated, followed by the output resulting from the included file,
+followed by the output that comes from the text after the @samp{#include}
+command.  For example, given two files as follows:
+
+@example
+/* File program.c */
+int x;
+#include "header.h"
+
+main ()
+@{
+  printf (test ());
+@}
+
+
+/* File header.h */
+char *test ();
+@end example
+
+@noindent
+the output generated by the C preprocessor for @file{program.c} as input
+would be
+
+@example
+int x;
+char *test ();
+
+main ()
+@{
+  printf (test ());
+@}
+@end example
+
+Included files are not limited to declarations and macro definitions; they
+are merely the typical use.  Any fragment of a C program can be included
+from another file.  The include file could even contain the beginning of a
+statement that is concluded in the containing file, or the end of a
+statement that was started in the including file.  However, a comment or a
+string or character constant may not start in the included file and finish
+in the including file.  An unterminated comment, string constant or
+character constant in an included file is considered to end (with an error
+message) at the end of the file.
+
+The line following the @samp{#include} command is always treated as a
+separate line by the C preprocessor even if the included file lacks a final
+newline.
+
+@node Once-Only,, Include Operation, Header Files
+@subsection Once-Only Include Files
+@cindex repeated inclusion
+
+Very often, one header file includes another.  It can easily result that a
+certain header file is included more than once.  This may lead to errors,
+if the header file defines structure types or typedefs, and is certainly
+wasteful.  Therefore, we often wish to prevent multiple inclusion of a
+header file.
+
+The standard way to do this is to enclose the entire real contents of the
+file in a conditional, like this:
+
+@example
+#ifndef __FILE_FOO_SEEN__
+#define __FILE_FOO_SEEN__
+
+@var{the entire file}
+
+#endif /* __FILE_FOO_SEEN__ */
+@end example
+
+The macro @code{__FILE_FOO_SEEN__} indicates that the file has been
+included once already; its name should begin with @samp{__}, and should
+contain the name of the file to avoid accidental conflicts.
+
+One drawback of this method is that the preprocessor must scan the input
+file completely in order to determine that all of it is to be ignored.
+This makes compilation slower.  You can avoid the delay by inserting the
+following command near the beginning of file @emph{in addition to the
+conditionals described above}:
+
+@example
+#pragma once
+@end example
+
+This command tells the GNU C preprocessor to ignore any future commands
+to include the same file (whichever file the @samp{#pragma} appears in).
+
+You should not @emph{rely} on @samp{#pragma once} to prevent multiple
+inclusion of the file.  It is just a hint, and a nonstandard one at
+that.  Most C compilers will ignore it entirely.  For this reason, you
+still need the conditionals if you want to make certain that the file's
+contents are not included twice.
+
+Note that @samp{#pragma once} works by file name; if a file has more
+than one name, it can be included once under each name, even in GNU CC,
+despite @samp{#pragma once}.
+
+@node Macros, Conditionals, Header Files, Top
+@section Macros
+
+A macro is a sort of abbreviation which you can define once and then
+use later.  There are many complicated features associated with macros
+in the C preprocessor.
+
+@menu
+* Simple Macros::    Macros that always expand the same way.
+* Argument Macros::  Macros that accept arguments that are substituted
+                       into the macro expansion.
+* Predefined::       Predefined macros that are always available.
+* Stringification::  Macro arguments converted into string constants.
+* Concatenation::    Building tokens from parts taken from macro arguments.
+* Undefining::       Cancelling a macro's definition.
+* Redefining::       Changing a macro's definition.
+* Macro Pitfalls::   Macros can confuse the unwary.  Here we explain
+                       several common problems and strange features.
+@end menu
+
+@node Simple Macros, Argument Macros, Macros, Macros
+@subsection Simple Macros
+
+A @dfn{simple macro} is a kind of abbreviation.  It is a name which stands
+for a fragment of code.
+
+Before you can use a macro, you must @dfn{define} it explicitly with the
+@samp{#define} command.  @samp{#define} is followed by the name of the
+macro and then the code it should be an abbreviation for.  For example,
+
+@example
+#define BUFFER_SIZE 1020
+@end example
+
+@noindent
+defines a macro named @samp{BUFFER_SIZE} as an abbreviation for the text
+@samp{1020}.  Therefore, if somewhere after this @samp{#define} command
+there comes a C statement of the form
+
+@example
+foo = (char *) xmalloc (BUFFER_SIZE);
+@end example
+
+@noindent
+then the C preprocessor will recognize and @dfn{expand} the macro
+@samp{BUFFER_SIZE}, resulting in
+
+@example
+foo = (char *) xmalloc (1020);
+@end example
+
+@noindent
+the definition must be a single line; however, it may not end in the
+middle of a multi-line string constant or character constant.
+
+The use of all upper case for macro names is a standard convention.
+Programs are easier to read when it is possible to tell at a glance which
+names are macros.
+
+Normally, a macro definition must be a single line, like all C preprocessor
+commands.  (You can split a long macro definition cosmetically with
+Backslash-Newline.)  There is one exception: Newlines can be included in
+the macro definition if within a string or character constant.  By the same
+token, it is not possible for a macro definition to contain an unbalanced
+quote character; the definition automatically extends to include the
+matching quote character that ends the string or character constant.
+Comments within a macro definition may contain Newlines, which make no
+difference since the comments are entirely replaced with Spaces regardless
+of their contents.
+
+Aside from the above, there is no restriction on what can go in a macro
+body.  Parentheses need not balance.  The body need not resemble valid C
+code.  (Of course, you might get error messages from the C compiler when
+you use the macro.)
+
+The C preprocessor scans your program sequentially, so macro definitions
+take effect at the place you write them.  Therefore, the following input to
+the C preprocessor
+
+@example
+foo = X;
+#define X 4
+bar = X;
+@end example
+
+@noindent
+produces as output
+
+@example
+foo = X;
+
+bar = 4;
+@end example
+
+After the preprocessor expands a macro name, the macro's definition body is
+appended to the front of the remaining input, and the check for macro calls
+continues.  Therefore, the macro body can contain calls to other macros.
+For example, after
+
+@example
+#define BUFSIZE 1020
+#define TABLESIZE BUFSIZE
+@end example
+
+@noindent
+the name @samp{TABLESIZE} when used in the program would go through two
+stages of expansion, resulting ultimately in @samp{1020}.
+
+This is not at all the same as defining @samp{TABLESIZE} to be @samp{1020}.
+The @samp{#define} for @samp{TABLESIZE} uses exactly the body you
+specify---in this case, @samp{BUFSIZE}---and does not check to see whether
+it too is the name of a macro.  It's only when you @emph{use} @samp{TABLESIZE}
+that the result of its expansion is checked for more macro names.
+@xref{Cascaded Macros}.
+
+@node Argument Macros, Predefined, Simple Macros, Macros
+@subsection Macros with Arguments
+
+A simple macro always stands for exactly the same text, each time it is
+used.  Macros can be more flexible when they accept @dfn{arguments}.
+Arguments are fragments of code that you supply each time the macro is
+used.  These fragments are included in the expansion of the macro according
+to the directions in the macro definition.
+
+To define a macro that uses arguments, you write a @samp{#define} command
+with a list of @dfn{argument names} in parentheses after the name of the
+macro.  The argument names may be any valid C identifiers, separated by
+commas and optionally whitespace.  The open-parenthesis must follow the
+macro name immediately, with no space in between.
+
+For example, here is a macro that computes the minimum of two numeric
+values, as it is defined in many C programs:
+
+@example
+#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
+@end example
+
+@noindent
+(This is not the best way to define a ``minimum'' macro in GNU C.
+@xref{Side Effects}, for more information.)
+
+To use a macro that expects arguments, you write the name of the macro
+followed by a list of @dfn{actual arguments} in parentheses. separated by
+commas.  The number of actual arguments you give must match the number of
+arguments the macro expects.   Examples of use of the macro @samp{min}
+include @samp{min (1, 2)} and @samp{min (x + 28, *p)}.
+
+The expansion text of the macro depends on the arguments you use.
+Each of the argument names of the macro is replaced, throughout the
+macro definition, with the corresponding actual argument.  Using the
+same macro @samp{min} defined above, @samp{min (1, 2)} expands into
+
+@example
+((1) < (2) ? (1) : (2))
+@end example
+
+@noindent
+where @samp{1} has been substituted for @samp{X} and @samp{2} for @samp{Y}.
+
+Likewise, @samp{min (x + 28, *p)} expands into
+
+@example
+((x + 28) < (*p) ? (x + 28) : (*p))
+@end example
+
+Parentheses in the actual arguments must balance; a comma within
+parentheses does not end an argument.  However, there is no requirement for
+brackets or braces to balance; thus, if you want to supply @samp{array[x =
+y, x + 1]} as an argument, you must write it as @samp{array[(x = y, x +
+1)]}, which is equivalent C code.
+
+After the actual arguments are substituted into the macro body, the entire
+result is appended to the front of the remaining input, and the check for
+macro calls continues.  Therefore, the actual arguments can contain calls
+to other macros, either with or without arguments, or even to the same
+macro.  The macro body can also contain calls to other macros.  For
+example, @samp{min (min (a, b), c)} expands into
+
+@example
+((((a) < (b) ? (a) : (b))) < (c)
+ ? (((a) < (b) ? (a) : (b)))
+ : (c))
+@end example
+
+@noindent
+(Line breaks shown here for clarity would not actually be generated.)
+
+If you use the macro name followed by something other than an
+open-parenthesis (after ignoring any spaces, tabs and comments that
+follow), it is not a call to the macro, and the preprocessor leaves the
+name unaltered.  Therefore, it is possible for the same name to be a
+variable or function in your program as well as a macro, and you can
+choose in each instance whether to refer to the macro (if an actual
+argument list follows) or the variable or function (if an argument list
+does not follow).
+
+Such dual use of one name could be confusing and should be avoided
+except when the two meanings are effectively synonymous: that is, when the
+name is both a macro and a function and the two have similar effects.  You
+can think of the name simply as a function; use of the name for purposes
+other than calling it (such as, to take the address) will refer to the
+function, while calls will expand the macro and generate better but
+equivalent code.  For example, you can use a function named @samp{min} in
+the same source file that defines the macro.  If you write @samp{&min} with
+no argument list, you refer to the function.  If you write @samp{min (x,
+bb)}, with an argument list, the macro is expanded.  If you write
+@samp{(min) (a, bb)}, where the name @samp{min} is not followed by an
+open-parenthesis, the macro is not expanded, so you wind up with a call to
+the function @samp{min}.
+
+It is not allowed to define the same name as both a simple macro and
+a macro with arguments.
+
+In the definition of a macro with arguments, the list of argument names
+must follow the macro name immediately with no space in between.  If there
+is a space after the macro name, the macro is defined as taking no
+arguments, and all the rest of the name is taken to be the expansion.  The
+reason for this is that it is often useful to define a macro that takes no
+arguments and whose definition begins with an identifier in parentheses.
+This rule about spaces makes it possible for you to do either this:
+
+@example
+#define FOO(x) - 1 / (x)
+@end example
+
+@noindent
+(which defines @samp{FOO} to take an argument and expand into minus the
+reciprocal of that argument) or this:
+
+@example
+#define BAR (x) - 1 / (x)
+@end example
+
+@noindent
+(which defines @samp{BAR} to take no argument and always expand into
+@samp{(x) - 1 / (x)}).
+
+Note that the @emph{uses} of a macro with arguments can have spaces before
+the left parenthesis; it's the @emph{definition} where it matters whether
+there is a space.
+
+@node Predefined, Stringification, Argument Macros, Macros
+@subsection Predefined Macros
+
+@cindex predefined macros
+Several simple macros are predefined.  You can use them without giving
+definitions for them.  They fall into two classes: standard macros and
+system-specific macros.
+
+@menu
+* Standard Predefined::     Standard predefined macros.
+* Nonstandard Predefined::  Nonstandard predefined macros.
+@end menu
+
+@node Standard Predefined, Nonstandard Predefined, Predefined, Predefined
+@subsubsection Standard Predefined Macros
+
+The standard predefined macros are available with the same meanings
+regardless of the machine or operating system on which you are using GNU C.
+Their names all start and end with double underscores.  Those preceding
+@code{__GNUC__} in this table are standardized by ANSI C; the rest are
+GNU C extensions.
+
+@table @code
+@item __FILE__
+@findex __FILE__
+This macro expands to the name of the current input file, in the form
+of a C string constant.
+
+@item __BASE_FILE__
+@findex __BASE_FILE__
+This macro expands to the name of the main input file, in the form
+of a C string constant.  This is the source file that was specified
+as an argument when the C compiler was invoked.
+
+@item __LINE__
+@findex __LINE__
+This macro expands to the current input line number, in the form of a
+decimal integer constant.  While we call it a predefined macro, it's
+a pretty strange macro, since its ``definition'' changes with each
+new line of source code.
+
+This and @samp{__FILE__} are useful in generating an error message to
+report an inconsistency detected by the program; the message can state
+the source line at which the inconsistency was detected.  For example,
+
+@example
+fprintf (stderr, "Internal error: negative string length "
+                 "%d at %s, line %d.",
+         length, __FILE__, __LINE__);
+@end example
+
+A @samp{#include} command changes the expansions of @samp{__FILE__}
+and @samp{__LINE__} to correspond to the included file.  At the end of
+that file, when processing resumes on the input file that contained
+the @samp{#include} command, the expansions of @samp{__FILE__} and
+@samp{__LINE__} revert to the values they had before the
+@samp{#include} (but @samp{__LINE__} is then incremented by one as
+processing moves to the line after the @samp{#include}).
+
+The expansions of both @samp{__FILE__} and @samp{__LINE__} are altered
+if a @samp{#line} command is used.  @xref{Combining Sources}.
+
+@item __DATE__
+@findex __DATE__
+This macro expands to a string constant that describes the date on
+which the preprocessor is being run.  The string constant contains
+eleven characters and looks like @samp{"Jan 29 1987"} or @w{@samp{"Apr
+1 1905"}}.
+
+@item __TIME__
+@findex __TIME__
+This macro expands to a string constant that describes the time at
+which the preprocessor is being run.  The string constant contains
+eight characters and looks like @samp{"23:59:01"}.
+
+@item __STDC__
+@findex __STDC__
+This macro expands to the constant 1, to signify that this is ANSI
+Standard C.  (Whether that is actually true depends on what C compiler
+will operate on the output from the preprocessor.)
+
+@item __GNUC__
+This macro is defined if and only if this is GNU C.  This macro is
+defined only when the entire GNU C compiler is in use; if you invoke
+the preprocessor directly, @samp{__GNUC__} is undefined.
+
+@item __STRICT_ANSI__
+This macro is defined if and only if the @samp{-ansi} switch was
+specified when GNU C was invoked.  Its definition is the null string.
+This macro exists primarily to direct certain GNU header files not to
+define certain traditional Unix constructs which are incompatible with
+ANSI C.
+
+@item __VERSION__
+This macro expands to a string which describes the version number of
+GNU C.  The string is normally a sequence of decimal numbers separated
+by periods, such as @samp{"1.18"}.  The only reasonable use of this
+macro is to incorporate it into a string constant.
+
+@item __OPTIMIZE__
+This macro is defined in optimizing compilations.  It causes certain
+GNU header files to define alternative macro definitions for some
+system library functions.  It is unwise to refer to or test the
+definition of this macro unless you make very sure that programs will
+execute with the same effect regardless.
+
+@item __CHAR_UNSIGNED__
+This macro is defined if and only if the data type @code{char} is
+unsigned on the target machine.  It exists to cause the standard
+header file @file{limit.h} to work correctly.  It is bad practice
+to refer to this macro yourself; instead, refer to the standard
+macros defined in @file{limit.h}.
+@end table
+
+@node Nonstandard Predefined,, Standard Predefined, Predefined
+@subsubsection Nonstandard Predefined Macros
+
+The C preprocessor normally has several predefined macros that vary between
+machines because their purpose is to indicate what type of system and
+machine is in use.  This manual, being for all systems and machines, cannot
+tell you exactly what their names are; instead, we offer a list of some
+typical ones.
+
+Some nonstandard predefined macros describe the operating system in use,
+with more or less specificity.  For example,
+
+@table @code
+@item unix
+@findex unix
+@samp{unix} is normally predefined on all Unix systems.
+
+@item BSD
+@findex BSD
+@samp{BSD} is predefined on recent versions of Berkeley Unix
+(perhaps only in version 4.3).
+@end table
+
+Other nonstandard predefined macros describe the kind of CPU, with more or
+less specificity.  For example,
+
+@table @code
+@item vax
+@findex vax
+@samp{vax} is predefined on Vax computers.
+
+@item mc68000
+@findex mc68000
+@samp{mc68000} is predefined on most computers whose CPU is a Motorola
+68000, 68010 or 68020.
+
+@item m68k
+@findex m68k
+@samp{m68k} is also predefined on most computers whose CPU is a 68000,
+68010 or 68020; however, some makers use @samp{mc68000} and some use
+@samp{m68k}.  Some predefine both names.  What happens in GNU C
+depends on the system you are using it on.
+
+@item M68020
+@findex M68020
+@samp{M68020} has been observed to be predefined on some systems that
+use 68020 CPUs---in addition to @samp{mc68000} and @samp{m68k} that
+are less specific.
+
+@item ns32000
+@findex ns32000
+@samp{ns32000} is predefined on computers which use the National
+Semiconductor 32000 series CPU.
+@end table
+
+Yet other nonstandard predefined macros describe the manufacturer of
+the system.  For example,
+
+@table @code
+@item sun
+@findex sun
+@samp{sun} is predefined on all models of Sun computers.
+
+@item pyr
+@findex pyr
+@samp{pyr} is predefined on all models of Pyramid computers.
+
+@item sequent
+@findex sequent
+@samp{sequent} is predefined on all models of Sequent computers.
+@end table
+
+These predefined symbols are not only nonstandard, they are contrary to the
+ANSI standard because their names do not start with underscores.
+Therefore, the option @samp{-ansi} inhibits the definition of these
+symbols.
+
+This tends to make @samp{-ansi} useless, since many programs depend on the
+customary nonstandard predefined symbols.  Even system header files check
+them and will generate incorrect declarations if they do not find the names
+that are expected.  You might think that the header files supplied for the
+Uglix computer would not need to test what machine they are running on,
+because they can simply assume it is the Uglix; but often they do, and they
+do so using the customary names.  As a result, very few C programs will
+compile with @samp{-ansi}.  We intend to avoid such problems on the GNU
+system.
+
+What, then, should you do in an ANSI C program to test the type of machine
+it is to run on?
+
+GNU C offers a parallel series of symbols for this purpose, whose names
+are made from the customary ones by adding @samp{__} at the beginning
+and end.  Thus, the symbol @code{__vax__} would be available on a vax,
+and so on.
+
+The set of nonstandard predefined names in the GNU C preprocessor is
+controlled by the macro @samp{CPP_PREDEFINES}, which should be a string
+containing @samp{-D} options, separated by spaces.  For example, on the
+Sun 3, we use the following definition:
+
+@example
+#define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k"
+@end example
+
+@node Stringification, Concatenation, Predefined, Macros
+@subsection Stringification
+
+@cindex stringification
+@dfn{Stringification} means turning a code fragment into a string constant
+whose contents are the text for the code fragment.  For example,
+stringifying @samp{foo (z)} results in @samp{"foo (z)"}.
+
+In the C preprocessor, stringification is an option available when macro
+arguments are substituted into the macro definition.  In the body of the
+definition, when an argument name appears, the character @samp{#} before
+the name specifies stringification of the corresponding actual argument
+when it is substituted at that point in the definition.  The same argument
+may be substituted in other places in the definition without
+stringification if the argument name appears in those places with no
+@samp{#}.
+
+Here is an example of a macro definition that uses stringification:
+
+@example
+#define WARN_IF(EXP) \
+do @{ if (EXP) fprintf (stderr, "Warning: " #EXP "\n"); @} while (0)
+@end example
+
+@noindent
+Here the actual argument for @samp{EXP} is substituted once as given,
+into the @samp{if} statement, and once as stringified, into the
+argument to @samp{fprintf}.  The @samp{do} and @samp{while (0)} are
+a kludge to make it possible to write @samp{WARN_IF (@var{arg});},
+which the resemblance of @samp{WARN_IF} to a function would make
+C programmers want to do; @pxref{Swallow Semicolon}).
+
+The stringification feature is limited to transforming one macro argument
+into one string constant: there is no way to combine the argument with
+other text and then stringify it all together.  But the example above shows
+how an equivalent result can be obtained in ANSI Standard C using the
+feature that adjacent string constants are concatenated as one string
+constant.  The preprocessor stringifies @samp{EXP}'s actual argument
+into a separate string constant, resulting in text like
+
+@example
+do @{ if (x == 0) fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0)
+@end example
+
+@noindent
+but the C compiler then sees three consecutive string constants and
+concatenates them into one, producing effectively
+
+@example
+do @{ if (x == 0) fprintf (stderr, "Warning: x == 0\n"); @} while (0)
+@end example
+
+Stringification in C involves more than putting doublequote characters
+around the fragment; it is necessary to put backslashes in front of all
+doublequote characters, and all backslashes in string and character
+constants, in order to get a valid C string constant with the proper
+contents.  Thus, stringifying @samp{p = "foo\n";} results in @samp{"p =
+\"foo\\n\";"}.  However, backslashes that are not inside of string or
+character constants are not duplicated: @samp{\n} by itself stringifies to
+@samp{"\n"}.
+
+Whitespace (including comments) in the text being stringified is handled
+according to precise rules.  All leading and trailing whitespace is ignored.
+Any sequence of whitespace in the middle of the text is converted to
+a single space in the stringified result.
+
+@node Concatenation, Undefining, Stringification, Macros
+@subsection Concatenation
+
+@cindex concatenation
+@dfn{Concatenation} means joining two strings into one.  In the context
+of macro expansion, concatenation refers to joining two lexical units
+into one longer one.  Specifically, an actual argument to the macro can be
+concatenated with another actual argument or with fixed text to produce
+a longer name.  The longer name might be the name of a function,
+variable or type, or a C keyword; it might even be the name of another
+macro, in which case it will be expanded.
+
+When you define a macro, you request concatenation with the special
+operator @samp{##} in the macro body.  When the macro is called,
+after actual arguments are substituted, all @samp{##} operators are
+deleted, and so is any whitespace next to them (including whitespace
+that was part of an actual argument).  The result is to concatenate
+the syntactic tokens on either side of the @samp{##}.
+
+Consider a C program that interprets named commands.  There probably needs
+to be a table of commands, perhaps an array of structures declared as
+follows:
+
+@example
+struct command
+@{
+  char *name;
+  void (*function) ();
+@};
+
+struct command commands[] =
+@{
+  @{ "quit", quit_command@},
+  @{ "help", help_command@},
+  @dots{}
+@};
+@end example
+
+It would be cleaner not to have to give each command name twice, once in
+the string constant and once in the function name.  A macro which takes the
+name of a command as an argument can make this unnecessary.  The string
+constant can be created with stringification, and the function name by
+concatenating the argument with @samp{_command}.  Here is how it is done:
+
+@example
+#define COMMAND(NAME)  @{ #NAME, NAME ## _command @}
+
+struct command commands[] =
+@{
+  COMMAND (quit),
+  COMMAND (help),
+  @dots{}
+@};
+@end example
+
+The usual case of concatenation is concatenating two names (or a name and a
+number) into a longer name.  But this isn't the only valid case.  It is
+also possible to concatenate two numbers (or a number and a name, such as
+@samp{1.5} and @samp{e3}) into a number.  Also, multi-character operators
+such as @samp{+=} can be formed by concatenation.  In some cases it is even
+possible to piece together a string constant.  However, two pieces of text
+that don't together form a valid lexical unit cannot be concatenated.  For
+example, concatenation with @samp{x} on one side and @samp{+} on the other
+is not meaningful because those two characters can't fit together in any
+lexical unit of C.  The ANSI standard says that such attempts at
+concatenation are undefined, but in the GNU C preprocessor it is well
+defined: it puts the @samp{x} and @samp{+} side by side with no particular
+special results.
+
+Keep in mind that the C preprocessor converts comments to whitespace before
+macros are even considered.  Therefore, you cannot create a comment by
+concatenating @samp{/} and @samp{*}: the @samp{/*} sequence that starts a
+comment is not a lexical unit, but rather the beginning of a ``long'' space
+character.  Also, you can freely use comments next to a @samp{##} in a
+macro definition, or in actual arguments that will be concatenated, because
+the comments will be converted to spaces at first sight, and concatenation
+will later discard the spaces.
+
+@node Undefining, Redefining, Concatenation, Macros
+@subsection Undefining Macros
+
+@cindex undefining macros
+To @dfn{undefine} a macro means to cancel its definition.  This is done
+with the @samp{#undef} command.  @samp{#undef} is followed by the macro
+name to be undefined.
+
+Like definition, undefinition occurs at a specific point in the source
+file, and it applies starting from that point.  The name ceases to be a
+macro name, and from that point on it is treated by the preprocessor as if
+it had never been a macro name.
+
+For example,
+
+@example
+#define FOO 4
+x = FOO;
+#undef FOO
+x = FOO;
+@end example
+
+@noindent
+expands into
+
+@example
+x = 4;
+
+x = FOO;
+@end example
+
+@noindent
+In this example, @samp{FOO} had better be a variable or function as well
+as (temporarily) a macro, in order for the result of the expansion to be
+valid C code.
+
+The same form of @samp{#undef} command will cancel definitions with
+arguments or definitions that don't expect arguments.  The @samp{#undef}
+command has no effect when used on a name not currently defined as a macro.
+
+@node Redefining, Macro Pitfalls, Undefining, Macros
+@subsection Redefining Macros
+
+@cindex redefining macros
+@dfn{Redefining} a macro means defining (with @samp{#define}) a name that
+is already defined as a macro.
+
+A redefinition is trivial if the new definition is transparently identical
+to the old one.  You probably wouldn't deliberately write a trivial
+redefinition, but they can happen automatically when a header file is
+included more than once (@pxref{Header Files}), so they are accepted
+silently and without effect.
+
+Nontrivial redefinition is considered likely to be an error, so
+it provokes a warning message from the preprocessor.  However, sometimes it
+is useful to change the definition of a macro in mid-compilation.  You can
+inhibit the warning by undefining the macro with @samp{#undef} before the
+second definition.
+
+In order for a redefinition to be trivial, the new definition must
+exactly match the one already in effect, with two possible exceptions:
+
+@itemize @bullet
+@item
+Whitespace may be added or deleted at the beginning or the end.
+
+@item
+Whitespace may be changed in the middle (but not inside strings).
+However, it may not be eliminated entirely, and it may not be added
+where there was no whitespace at all.
+@end itemize
+
+Recall that a comment counts as whitespace.
+
+@node Macro Pitfalls,, Redefining, Macros
+@subsection Pitfalls and Subtleties of Macros
+
+In this section we describe some special rules that apply to macros and
+macro expansion, and point out certain cases in which the rules have
+counterintuitive consequences that you must watch out for.
+
+@menu
+* Misnesting::        Macros can contain unmatched parentheses.
+* Macro Parentheses:: Why apparently superfluous parentheses
+                         may be necessary to avoid incorrect grouping.
+* Swallow Semicolon:: Macros that look like functions
+                         but expand into compound statements.
+* Side Effects::      Unsafe macros that cause trouble when
+                         arguments contain side effects.
+* Self-Reference::    Macros whose definitions use the macros' own names.
+* Argument Prescan::  Actual arguments are checked for macro calls
+                         before they are substituted.
+* Cascaded Macros::   Macros whose definitions use other macros.
+@end menu
+
+@node Misnesting, Macro Parentheses, Macro Pitfalls, Macro Pitfalls
+@subsubsection Improperly Nested Constructs
+
+Recall that when a macro is called with arguments, the arguments are
+substituted into the macro body and the result is checked, together with
+the rest of the input file, for more macro calls.
+
+It is possible to piece together a macro call coming partially from the
+macro body and partially from the actual arguments.  For example,
+
+@example
+#define double(x) (2*(x))
+#define call_with_1(x) x(1)
+@end example
+
+@noindent
+would expand @samp{call_with_1 (double)} into @samp{(2*(1))}.
+
+Macro definitions do not have to have balanced parentheses.  By writing an
+unbalanced open parenthesis in a macro body, it is possible to create a
+macro call that begins inside the macro body but ends outside of it.  For
+example,
+
+@example
+#define strange(file) fprintf (file, "%s %d",
+@dots{}
+strange(stderr) p, 35)
+@end example
+
+@noindent
+This bizarre example expands to @samp{fprintf (stderr, "%s %d", p, 35)}!
+
+@node Macro Parentheses, Swallow Semicolon, Misnesting, Macro Pitfalls
+@subsubsection Unintended Grouping of Arithmetic
+
+You may have noticed that in most of the macro definition examples shown
+above, each occurrence of a macro argument name had parentheses around it.
+In addition, another pair of parentheses usually surround the entire macro
+definition.  Here is why it is best to write macros that way.
+
+Suppose you define a macro as follows,
+
+@example
+#define ceil_div(x, y) (x + y - 1) / y
+@end example
+
+@noindent
+whose purpose is to divide, rounding up.  (One use for this
+operation is to compute how many @samp{int}'s are needed to hold
+a certain number of @samp{char}'s.)  Then suppose it is used as follows:
+
+@example
+a = ceil_div (b & c, sizeof (int));
+@end example
+
+@noindent
+This expands into
+
+@example
+a = (b & c + sizeof (int) - 1) / sizeof (int);
+@end example
+
+@noindent
+which does not do what is intended.  The operator-precedence rules of
+C make it equivalent to this:
+
+@example
+a = (b & (c + sizeof (int) - 1)) / sizeof (int);
+@end example
+
+@noindent
+But what we want is this:
+
+@example
+a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
+@end example
+
+@noindent
+Defining the macro as
+
+@example
+#define ceil_div(x, y) ((x) + (y) - 1) / (y)
+@end example
+
+@noindent
+provides the desired result.
+
+However, unintended grouping can result in another way.  Consider
+@samp{sizeof ceil_div(1, 2)}.  That has the appearance of a C expression
+that would compute the size of the type of @samp{ceil_div (1, 2)}, but in
+fact it means something very different.  Here is what it expands to:
+
+@example
+sizeof ((1) + (2) - 1) / (2)
+@end example
+
+@noindent
+This would take the size of an integer and divide it by two.  The precedence
+rules have put the division outside the @samp{sizeof} when it was intended
+to be inside.
+
+Parentheses around the entire macro definition can prevent such problems.
+Here, then, is the recommended way to define @samp{ceil_div}:
+
+@example
+#define ceil_div(x, y) (((x) + (y) - 1) / (y))
+@end example
+
+@node Swallow Semicolon, Side Effects, Macro Parentheses, Macro Pitfalls
+@subsubsection Swallowing the Semicolon
+
+@cindex semicolons (after macro calls)
+Often it is desirable to define a macro that expands into a compound
+statement.  Consider, for example, the following macro, that advances a
+pointer (the argument @samp{p} says where to find it) across whitespace
+characters:
+
+@example
+#define SKIP_SPACES (p, limit)  \
+@{ register char *lim = (limit); \
+  while (p != lim) @{            \
+    if (*p++ != ' ') @{          \
+      p--; break; @}@}@}
+@end example
+
+@noindent
+Here Backslash-Newline is used to split the macro definition, which must
+be a single line, so that it resembles the way such C code would be
+laid out if not part of a macro definition.
+
+A call to this macro might be @samp{SKIP_SPACES (p, lim)}.  Strictly
+speaking, the call expands to a compound statement, which is a complete
+statement with no need for a semicolon to end it.  But it looks like a
+function call.  So it minimizes confusion if you can use it like a function
+call, writing a semicolon afterward, as in @samp{SKIP_SPACES (p, lim);}
+
+But this can cause trouble before @samp{else} statements, because the
+semicolon is actually a null statement.  Suppose you write
+
+@example
+if (*p != 0)
+  SKIP_SPACES (p, lim);
+else @dots{}
+@end example
+
+@noindent
+The presence of two statements---the compound statement and a null
+statement---in between the @samp{if} condition and the @samp{else}
+makes invalid C code.
+
+The definition of the macro @samp{SKIP_SPACES} can be altered to solve
+this problem, using a @samp{do @dots{} while} statement.  Here is how:
+
+@example
+#define SKIP_SPACES (p, limit)     \
+do @{ register char *lim = (limit); \
+     while (p != lim) @{            \
+       if (*p++ != ' ') @{          \
+         p--; break; @}@}@}           \
+while (0)
+@end example
+
+Now @samp{SKIP_SPACES (p, lim);} expands into
+
+@example
+do @{@dots{}@} while (0);
+@end example
+
+@noindent
+which is one statement.
+
+@node Side Effects, Self-Reference, Swallow Semicolon, Macro Pitfalls
+@subsubsection Duplication of Side Effects
+
+@cindex side effects (in macro arguments)
+@cindex unsafe macros
+Many C programs define a macro @samp{min}, for ``minimum'', like this:
+
+@example
+#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
+@end example
+
+When you use this macro with an argument containing a side effect,
+as shown here,
+
+@example
+next = min (x + y, foo (z));
+@end example
+
+@noindent
+it expands as follows:
+
+@example
+next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
+@end example
+
+@noindent
+where @samp{x + y} has been substituted for @samp{X} and @samp{foo (z)}
+for @samp{Y}.
+
+The function @samp{foo} is used only once in the statement as it appears
+in the program, but the expression @samp{foo (z)} has been substituted
+twice into the macro expansion.  As a result, @samp{foo} might be called
+two times when the statement is executed.  If it has side effects or
+if it takes a long time to compute, the results might not be what you
+intended.  We say that @samp{min} is an @dfn{unsafe} macro.
+
+The best solution to this problem is to define @samp{min} in a way that
+computes the value of @samp{foo (z)} only once.  The C language offers no
+standard way to do this, but it can be done with GNU C extensions as
+follows:
+
+@example
+#define min(X, Y)                     \
+(@{ typeof (X) __x = (X), __y = (Y);   \
+   (__x < __y) ? __x : __y; @})
+@end example
+
+If you do not wish to use GNU C extensions, the only solution is to be
+careful when @emph{using} the macro @samp{min}.  For example, you can
+calculate the value of @samp{foo (z)}, save it in a variable, and use that
+variable in @samp{min}:
+
+@example
+#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
+@dots{}
+@{
+  int tem = foo (z);
+  next = min (x + y, tem);
+@}
+@end example
+
+@noindent
+(where I assume that @samp{foo} returns type @samp{int}).
+
+@node Self-Reference, Argument Prescan, Side Effects, Macro Pitfalls
+@subsubsection Self-Referential Macros
+
+@cindex self-reference
+A @dfn{self-referential} macro is one whose name appears in its definition.
+A special feature of ANSI Standard C is that the self-reference is not
+considered a macro call.  It is passed into the preprocessor output
+unchanged.
+
+Let's consider an example:
+
+@example
+#define foo (4 + foo)
+@end example
+
+@noindent
+where @samp{foo} is also a variable in your program.
+
+Following the ordinary rules, each reference to @samp{foo} will expand into
+@samp{(4 + foo)}; then this will be rescanned and will expand into @samp{(4
++ (4 + foo))}; and so on until it causes a fatal error (memory full) in the
+preprocessor.
+
+However, the special rule about self-reference cuts this process short
+after one step, at @samp{(4 + foo)}.  Therefore, this macro definition
+has the possibly useful effect of causing the program to add 4 to
+the value of @samp{foo} wherever @samp{foo} is referred to.
+
+In most cases, it is a bad idea to take advantage of this feature.  A
+person reading the program who sees that @samp{foo} is a variable will
+not expect that it is a macro as well.  The reader will come across the
+identifier @samp{foo} in the program and think its value should be that
+of the variable @samp{foo}, whereas in fact the value is four greater.
+
+The special rule for self-reference applies also to @dfn{indirect}
+self-reference.  This is the case where a macro @var{x} expands to use a
+macro @samp{y}, and @samp{y}'s expansion refers to the macro @samp{x}.  The
+resulting reference to @samp{x} comes indirectly from the expansion of
+@samp{x}, so it is a self-reference and is not further expanded.  Thus,
+after
+
+@example
+#define x (4 + y)
+#define y (2 * x)
+@end example
+
+@noindent
+@samp{x} would expand into @samp{(4 + (2 * x))}.  Clear?
+
+But suppose @samp{y} is used elsewhere, not from the definition of @samp{x}.
+Then the use of @samp{x} in the expansion of @samp{y} is not a self-reference
+because @samp{x} is not ``in progress''.  So it does expand.  However,
+the expansion of @samp{x} contains a reference to @samp{y}, and that
+is an indirect self-reference now because @samp{y} is ``in progress''.
+The result is that @samp{y} expands to @samp{(2 * (4 + y))}.
+
+It is not clear that this behavior would ever be useful, but it is specified
+by the ANSI C standard, so you need to understand it.
+
+@node Argument Prescan, Cascaded Macros, Self-Reference, Macro Pitfalls
+@subsubsection Separate Expansion of Macro Arguments
+
+We have explained that the expansion of a macro, including the substituted
+actual arguments, is scanned over again for macro calls to be expanded.
+
+What really happens is more subtle: first each actual argument text is scanned
+separately for macro calls.  Then the results of this are substituted into
+the macro body to produce the macro expansion, and the macro expansion
+is scanned again for macros to expand.
+
+The result is that the actual arguments are scanned @emph{twice} to expand
+macro calls in them.
+
+Most of the time, this has no effect.  If the actual argument contained
+any macro calls, they are expanded during the first scan.  The result
+therefore contains no macro calls, so the second scan does not change it.
+If the actual argument were substituted as given, with no prescan,
+the single remaining scan would find the same macro calls and produce
+the same results.
+
+You might expect the double scan to change the results when a
+self-referential macro is used in an actual argument of another macro
+(@pxref{Self-Reference}): the self-referential macro would be expanded once
+in the first scan, and a second time in the second scan.  But this is not
+what happens.  The self-references that do not expand in the first scan are
+marked so that they will not expand in the second scan either.
+
+The prescan is not done when an argument is stringified or concatenated.
+Thus,
+
+@example
+#define str(s) #s
+#define foo 4
+str (foo)
+@end example
+
+@noindent
+expands to @samp{"foo"}.  Once more, prescan has been prevented from
+having any noticeable effect.
+
+More precisely, stringification and concatenation use the argument as
+written, in un-prescanned form.  The same actual argument would be used in
+prescanned form if it is substituted elsewhere without stringification or
+concatenation.
+
+@example
+#define str(s) #s lose(s)
+#define foo 4
+str (foo)
+@end example
+
+expands to @samp{"foo" lose(4)}.
+
+You might now ask, ``Why mention the prescan, if it makes no difference?
+And why not skip it and make the preprocessor faster?''  The answer is
+that the prescan does make a difference in three special cases:
+
+@itemize @bullet
+@item
+Nested calls to a macro.
+
+@item
+Macros that call other macros that stringify or concatenate.
+
+@item
+Macros whose expansions contain unshielded commas.
+@end itemize
+
+We say that @dfn{nested} calls to a macro occur when a macro's actual
+argument contains a call to that very macro.  For example, if @samp{f}
+is a macro that expects one argument, @samp{f (f (1))} is a nested
+pair of calls to @samp{f}.  The desired expansion is made by
+expanding @samp{f (1)} and substituting that into the definition of
+@samp{f}.  The prescan causes the expected result to happen.
+Without the prescan, @samp{f (1)} itself would be substituted as
+an actual argument, and the inner use of @samp{f} would appear
+during the main scan as an indirect self-reference and would not
+be expanded.  Here, the prescan cancels an undesirable side effect
+(in the medical, not computational, sense of the term) of the special
+rule for self-referential macros.
+
+But prescan causes trouble in certain other cases of nested macro calls.
+Here is an example:
+
+@example
+#define foo  a,b
+#define bar(x) lose(x)
+#define lose(x) (1 + (x))
+
+bar(foo)
+@end example
+
+@noindent
+We would like @samp{bar(foo)} to turn into @samp{(1 + (foo))}, which
+would then turn into @samp{(1 + (a,b))}.  But instead, @samp{bar(foo)}
+expands into @samp{lose(a,b)}, and you get an error because @code{lose}
+requires a single argument.  In this case, the problem is easily solved
+by the same parentheses that ought to be used to prevent misnesting of
+arithmetic operations:
+
+@example
+#define foo (a,b)
+#define bar(x) lose((x))
+@end example
+
+The problem is more serious when the operands of the macro are not
+expressions; for example, when they are statements.  Then parentheses
+are unacceptable because they would make for invalid C code:
+
+@example
+#define foo @{ int a, b; @dots{} @}
+@end example
+
+@noindent
+In GNU C you can shield the commas using the @samp{(@{@dots{}@})}
+construct which turns a compound statement into an expression:
+
+@example
+#define foo (@{ int a, b; @dots{} @})
+@end example
+
+Or you can rewrite the macro definition to avoid such commas:
+
+@example
+#define foo @{ int a; int b; @dots{} @}
+@end example
+
+There is also one case where prescan is useful.  It is possible
+to use prescan to expand an argument and then stringify it---if you use
+two levels of macros.  Let's add a new macro @samp{xstr} to the
+example shown above:
+
+@example
+#define xstr(s) str(s)
+#define str(s) #s
+#define foo 4
+xstr (foo)
+@end example
+
+This expands into @samp{"4"}, not @samp{"foo"}.  The reason for the
+difference is that the argument of @samp{xstr} is expanded at prescan
+(because @samp{xstr} does not specify stringification or concatenation of
+the argument).  The result of prescan then forms the actual argument for
+@samp{str}.  @samp{str} uses its argument without prescan because it
+performs stringification; but it cannot prevent or undo the prescanning
+already done by @samp{xstr}.
+
+@node Cascaded Macros,, Argument Prescan, Macro Pitfalls
+@subsubsection Cascaded Use of Macros
+
+@cindex cascaded macros
+@cindex macro body uses macro
+A @dfn{cascade} of macros is when one macro's body contains a reference
+to another macro.  This is very common practice.  For example,
+
+@example
+#define BUFSIZE 1020
+#define TABLESIZE BUFSIZE
+@end example
+
+This is not at all the same as defining @samp{TABLESIZE} to be @samp{1020}.
+The @samp{#define} for @samp{TABLESIZE} uses exactly the body you
+specify---in this case, @samp{BUFSIZE}---and does not check to see whether
+it too is the name of a macro.
+
+It's only when you @emph{use} @samp{TABLESIZE} that the result of its expansion
+is checked for more macro names.
+
+This makes a difference if you change the definition of @samp{BUFSIZE}
+at some point in the source file.  @samp{TABLESIZE}, defined as shown,
+will always expand using the definition of @samp{BUFSIZE} that is
+currently in effect:
+
+@example
+#define BUFSIZE 1020
+#define TABLESIZE BUFSIZE
+#undef BUFSIZE
+#define BUFSIZE 37
+@end example
+
+@noindent
+Now @samp{TABLESIZE} expands (in two stages) to @samp{37}.
+
+@node Conditionals, Combining Sources, Macros, Top
+@section Conditionals
+
+@cindex conditionals
+In a macro processor, a @dfn{conditional} is a command that allows a part
+of the program to be ignored during compilation, on some conditions.
+In the C preprocessor, a conditional can test either an arithmetic expression
+or whether a name is defined as a macro.
+
+A conditional in the C preprocessor resembles in some ways an @samp{if}
+statement in C, but it is important to understand the difference between
+them.  The condition in an @samp{if} statement is tested during the execution
+of your program.  Its purpose is to allow your program to behave differently
+from run to run, depending on the data it is operating on.  The condition
+in a preprocessor conditional command is tested when your program is compiled.
+Its purpose is to allow different code to be included in the program depending
+on the situation at the time of compilation.
+
+@menu
+* Uses: Conditional Uses.       What conditionals are for.
+* Syntax: Conditional Syntax.   How conditionals are written.
+* Deletion: Deleted Code.       Making code into a comment.
+* Macros: Conditionals-Macros.  Why conditionals are used with macros.
+* Errors: #error Command.       Detecting inconsistent compilation parameters.
+@end menu
+
+@node Conditional Uses, Conditional Syntax, Conditionals, Conditionals
+@subsection Why Conditionals are Used
+
+Generally there are three kinds of reason to use a conditional.
+
+@itemize @bullet
+@item
+A program may need to use different code depending on the machine or
+operating system it is to run on.  In some cases the code for one
+operating system may be erroneous on another operating system; for
+example, it might refer to library routines that do not exist on the
+other system.  When this happens, it is not enough to avoid executing
+the invalid code: merely having it in the program makes it impossible
+to link the program and run it.  With a preprocessor conditional, the
+offending code can be effectively excised from the program when it is
+not valid.
+
+@item
+You may want to be able to compile the same source file into two
+different programs.  Sometimes the difference between the programs is
+that one makes frequent time-consuming consistency checks on its
+intermediate data while the other does not.
+
+@item
+A conditional whose condition is always false is a good way to exclude
+code from the program but keep it as a sort of comment for future
+reference.
+@end itemize
+
+Most simple programs that are intended to run on only one machine will
+not need to use preprocessor conditionals.
+
+@node Conditional Syntax, Deleted Code, Conditional Uses, Conditionals
+@subsection Syntax of Conditionals
+
+@findex #if
+A conditional in the C preprocessor begins with a @dfn{conditional
+command}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}.
+@xref{Conditionals-Macros}, for info on @samp{#ifdef} and
+@samp{#ifndef}; only @samp{#if} is explained here.
+
+@menu
+* If: #if Command.     Basic conditionals using @samp{#if} and @samp{#endif}.
+* Else: #else Command. Including some text if the condition fails.
+* Elif: #elif Command. Testing several alternative possibilities.
+@end menu
+
+@node #if Command, #else Command, Conditional Syntax, Conditional Syntax
+@subsubsection The @samp{#if} Command
+
+The @samp{#if} command in its simplest form consists of
+
+@example
+#if @var{expression}
+@var{controlled text}
+#endif /* @var{expression} */
+@end example
+
+The comment following the @samp{#endif} is not required, but it is a good
+practice because it helps people match the @samp{#endif} to the
+corresponding @samp{#if}.  Such comments should always be used, except in
+short conditionals that are not nested.  In fact, you can put anything at
+all after the @samp{#endif} and it will be ignored by the GNU C preprocessor,
+but only comments are acceptable in ANSI Standard C.
+
+@var{expression} is a C expression of integer type, subject to stringent
+restrictions.  It may contain
+
+@itemize @bullet
+@item
+Integer constants, which are all regarded as @code{long} or
+@code{unsigned long}.
+
+@item
+Character constants, which are interpreted according to the character
+set and conventions of the machine and operating system on which the
+preprocessor is running.  The GNU C preprocessor uses the C data type
+@samp{char} for these character constants; therefore, whether some
+character codes are negative is determined by the C compiler used to
+compile the preprocessor.  If it treats @samp{char} as signed, then
+character codes large enough to set the sign bit will be considered
+negative; otherwise, no character code is considered negative.
+
+@item
+Arithmetic operators for addition, subtraction, multiplication,
+division, bitwise operations, shifts, comparisons, and @samp{&&} and
+@samp{||}.
+
+@item
+Identifiers that are not macros, which are all treated as zero(!).
+
+@item
+Macro calls.  All macro calls in the expression are expanded before
+actual computation of the expression's value begins.
+@end itemize
+
+Note that @samp{sizeof} operators and @code{enum}-type values are not allowed.
+@code{enum}-type values, like all other identifiers that are not taken
+as macro calls and expanded, are treated as zero.
+
+The text inside of a conditional can include preprocessor commands.  Then
+the commands inside the conditional are obeyed only if that branch of the
+conditional succeeds.  The text can also contain other conditional groups.
+However, the @samp{#if}'s and @samp{#endif}'s must balance.
+
+@node #else Command, #elif Command, #if Command, Conditional Syntax
+@subsubsection The @samp{#else} Command
+
+@findex #else
+The @samp{#else} command can be added to a conditional to provide alternative
+text to be used if the condition is false.  This looks like
+
+@example
+#if @var{expression}
+@var{text-if-true}
+#else /* Not @var{expression} */
+@var{text-if-false}
+#endif /* Not @var{expression} */
+@end example
+
+If @var{expression} is nonzero, and the @var{text-if-true} is considered
+included, then @samp{#else} acts like a failing conditional and the
+@var{text-if-false} is ignored.  Contrariwise, if the @samp{#if}
+conditional fails, the @var{text-if-false} is considered included.
+
+@node #elif Command,, #else Command, Conditional Syntax
+@subsubsection The @samp{#elif} Command
+
+@findex #elif
+One common case of nested conditionals is used to check for more than two
+possible alternatives.  For example, you might have
+
+@example
+#if X == 1
+@dots{}
+#else /* X != 1 */
+#if X == 2
+@dots{}
+#else /* X != 2 */
+@dots{}
+#endif /* X != 2 */
+#endif /* X != 1 */
+@end example
+
+Another conditional command, @samp{#elif}, allows this to be abbreviated
+as follows:
+
+@example
+#if X == 1
+@dots{}
+#elif X == 2
+@dots{}
+#else /* X != 2 and X != 1*/
+@dots{}
+#endif /* X != 2 and X != 1*/
+@end example
+
+@samp{#elif} stands for ``else if''.  Like @samp{#else}, it goes in the
+middle of a @samp{#if}-@samp{#endif} pair and subdivides it; it does not
+require a matching @samp{#endif} of its own.  Like @samp{#if}, the
+@samp{#elif} command includes an expression to be tested.
+
+The text following the @samp{#elif} is processed only if the original
+@samp{#if}-condition failed and the @samp{#elif} condition succeeeds.  More
+than one @samp{#elif} can go in the same @samp{#if}-@samp{#endif} group.
+Then the text after each @samp{#elif} is processed only if the @samp{#elif}
+condition succeeds after the original @samp{#if} and any previous
+@samp{#elif}'s within it have failed.  @samp{#else} is equivalent to
+@samp{#elif 1}, and @samp{#else} is allowed after any number of
+@samp{#elif}'s, but @samp{#elif} may not follow a @samp{#else}.
+
+@node Deleted Code, Conditionals-Macros, Conditional Syntax, Conditionals
+@subsection Keeping Deleted Code for Future Reference
+
+If you replace or delete a part of the program but want to keep the old
+code around as a comment for future reference, the easy way to do this is
+to put @samp{#if 0} before it and @samp{#endif} after it.
+
+This works even if the code being turned off contains conditionals, but
+they must be entire conditionals (balanced @samp{#if} and @samp{#endif}).
+
+@node Conditionals-Macros, #error Command, Deleted Code, Conditionals
+@subsection Conditionals and Macros
+
+Conditionals are rarely useful except in connection with macros.  A
+@samp{#if} command whose expression uses no macros is equivalent to
+@samp{#if 1} or @samp{#if 0}; you might as well determine which one, by
+computing the value of the expression yourself, and then simplify the
+program.  But when the expression uses macros, its value can vary from
+compilation to compilation.
+
+For example, here is a conditional that tests the expression
+@samp{BUFSIZE == 1020}, where @samp{BUFSIZE} must be a macro.
+
+@example
+#if BUFSIZE == 1020
+  printf ("Large buffers!\n");
+#endif /* BUFSIZE is large */
+@end example
+
+@findex defined
+The special operator @samp{defined} may be used in @samp{#if} expressions
+to test whether a certain name is defined as a macro.  Either @samp{defined
+@var{name}} or @samp{defined (@var{name})} is an expression whose value is
+1 if @var{name} is defined as macro at the current point in the program,
+and 0 otherwise.  For the @samp{defined} operator it makes no difference
+what the definition of the macro is; all that matters is whether there is a
+definition.  Thus, for example,@refill
+
+@example
+#if defined (vax) || defined (ns16000)
+@end example
+
+@noindent
+would include the following code if either of the names @samp{vax} and
+@samp{ns16000} is defined as a macro.
+
+If a macro is defined and later undefined with @samp{#undef},
+subsequent use of the @samp{defined} operator will return 0, because
+the name is no longer defined.  If the macro is defined again with
+another @samp{#define}, @samp{defined} will recommence returning 1.
+
+@findex #ifdef
+@findex #ifndef
+Conditionals that test just the definedness of one name are very common, so
+there are two special short conditional commands for this case.  They are
+
+@table @code
+@item #ifdef @var{name}
+is equivalent to @samp{#if defined (@var{name})}.
+
+@item #ifndef @var{name}
+is equivalent to @samp{#if ! defined (@var{name})}.
+@end table
+
+Macro definitions can vary between compilations for several reasons.
+
+@itemize @bullet
+@item
+Some macros are predefined on each kind of machine.  For example, on a
+Vax, the name @samp{vax} is a predefined macro.  On other machines, it
+would not be defined.
+
+@item
+Many more macros are defined by system header files.  Different
+systems and machines define different macros, or give them different
+values.  It is useful to test these macros with conditionals to avoid
+using a system feature on a machine where it is not implemented.
+
+@item
+Macros are a common way of allowing users to customize a program for
+different machines or applications.  For example, the macro
+@samp{BUFSIZE} might be defined in a configuration file for your
+program that is included as a header file in each source file.  You
+would use @samp{BUFSIZE} in a preprocessor conditional in order to
+generate different code depending on the chosen configuration.
+
+@item
+Macros can be defined or undefined with @samp{-D} and @samp{-U}
+command options when you compile the program.  You can arrange to
+compile the same source file into two different programs by choosing
+a macro name to specify which program you want, writing conditionals
+to test whether or how this macro is defined, and then controlling
+the state of the macro with compiler command options.
+@xref{Invocation}.
+@end itemize
+
+@node #error Command,, Conditionals-Macros, Conditionals
+@subsection The @samp{#error} Command
+
+@findex #error
+The command @samp{#error} causes the preprocessor to report a fatal
+error.  The rest of the line that follows @samp{#error} is used as the
+error message.
+
+You would use @samp{#error} inside of a conditional that detects a
+combination of parameters which you know the program does not properly
+support.  For example, if you know that the program will not run
+properly on a Vax, you might write
+
+@example
+#ifdef vax
+#error Won't work on Vaxen.  See comments at get_last_object.
+#endif
+@end example
+
+@noindent
+@xref{Nonstandard Predefined}, for why this works.
+
+If you have several configuration parameters that must be set up by
+the installation in a consistent way, you can use conditionals to detect
+an inconsistency and report it with @samp{#error}.  For example,
+
+@example
+#if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \
+    || HASH_TABLE_SIZE % 5 == 0
+#error HASH_TABLE_SIZE should not be divisible by a small prime
+#endif
+@end example
+
+@node Combining Sources, Other Commands, Conditionals, Top
+@section Combining Source Files
+
+@cindex line control
+@findex #line
+One of the jobs of the C preprocessor is to inform the C compiler of where
+each line of C code came from: which source file and which line number.
+
+C code can come from multiple source files if you use @samp{#include};
+both @samp{#include} and the use of conditionals and macros can cause
+the line number of a line in the preprocessor output to be different
+from the line's number in the original source file.  You will appreciate
+the value of making both the C compiler (in error messages) and symbolic
+debuggers such as GDB use the line numbers in your source file.
+
+The C preprocessor builds on this feature by offering a command by which
+you can control the feature explicitly.  This is useful when a file for
+input to the C preprocessor is the output from another program such as the
+@code{bison} parser generator, which operates on another file that is the
+true source file.  Parts of the output from @code{bison} are generated from
+scratch, other parts come from a standard parser file.  The rest are copied
+nearly verbatim from the source file, but their line numbers in the
+@code{bison} output are not the same as their original line numbers.
+Naturally you would like compiler error messages and symbolic debuggers to
+know the original source file and line number of each line in the
+@code{bison} output.
+
+@code{bison} arranges this by writing @samp{#line} commands into the output
+file.  @samp{#line} is a command that specifies the original line number
+and source file name for subsequent input in the current preprocessor input
+file.  @samp{#line} has three variants:
+
+@table @code
+@item #line @var{linenum}
+Here @var{linenum} is a decimal integer constant.  This specifies that
+the line number of the following line of input, in its original source file,
+was @var{linenum}.
+
+@item #line @var{linenum} @var{filename}
+Here @var{linenum} is a decimal integer constant and @var{filename}
+is a string constant.  This specifies that the following line of input
+came originally from source file @var{filename} and its line number there
+was @var{linenum}.  Keep in mind that @var{filename} is not just a
+file name; it is surrounded by doublequote characters so that it looks
+like a string constant.
+
+@item #line @var{anything else}
+@var{anything else} is checked for macro calls, which are expanded.
+The result should be a decimal integer constant followed optionally
+by a string constant, as described above.
+@end table
+
+@samp{#line} commands alter the results of the @samp{__FILE__} and
+@samp{__LINE__} predefined macros from that point on.  @xref{Standard
+Predefined}.
+
+@node Other Commands, Output, Combining Sources, Top
+@section Miscellaneous Preprocessor Commands
+
+@findex #pragma
+@findex #ident
+@cindex null command
+This section describes three additional preprocessor commands.  They are
+not very useful, but are mentioned for completeness.
+
+The @dfn{null command} consists of a @samp{#} followed by a Newline, with
+only whitespace (including comments) in between.  A null command is
+understood as a preprocessor command but has no effect on the preprocessor
+output.  The primary significance of the existence of the null command is
+that an input line consisting of just a @samp{#} will produce no output,
+rather than a line of output containing just a @samp{#}.  Supposedly
+some old C programs contain such lines.
+
+The @samp{#pragma} command is specified in the ANSI standard to have an
+arbitrary implementation-defined effect.  In the GNU C preprocessor,
+@samp{#pragma} commands are ignored, except for @samp{#pragma once}
+(@pxref{Once-Only}).
+
+The @samp{#ident} command is supported for compatibility with certain
+other systems.  It is followed by a line of text.  On certain systems,
+the text is copied into a special place in the object file; on most
+systems, the text is ignored and this directive has no effect.
+
+@node Output, Invocation, Other Commands, Top
+@section C Preprocessor Output
+
+@cindex output format
+The output from the C preprocessor looks much like the input, except
+that all preprocessor command lines have been replaced with blank lines
+and all comments with spaces.  Whitespace within a line is not altered;
+however, a space is inserted after the expansions of most macro calls.
+
+Source file name and line number information is conveyed by lines of
+the form
+
+@example
+# @var{linenum} @var{filename} @var{flag}
+@end example
+
+@noindent
+which are inserted as needed into the middle of the input (but never
+within a string or character constant).  Such a line means that the
+following line originated in file @var{filename} at line @var{linenum}.
+
+The third field, @var{flag}, may be a number, or may be absent.  It is
+@samp{1} for the beginning of a new source file, and @samp{2} for return
+to an old source file at the end of an included file.  It is absent
+otherwise.
+
+@node Invocation, Concept Index, Output, Top
+@section Invoking the C Preprocessor
+
+Most often when you use the C preprocessor you will not have to invoke it
+explicitly: the C compiler will do so automatically.  However, the
+preprocessor is sometimes useful individually.
+
+The C preprocessor expects two file names as arguments, @var{infile} and
+@var{outfile}.  The preprocessor reads @var{infile} together with any other
+files it specifies with @samp{#include}.  All the output generated by the
+combined input files is written in @var{outfile}.
+
+Either @var{infile} or @var{outfile} may be @samp{-}, which as @var{infile}
+means to read from standard input and as @var{outfile} means to write to
+standard output.  Also, if @var{outfile} or both file names are omitted,
+the standard output and standard input are used for the omitted file names.
+
+@cindex options
+Here is a table of command options accepted by the C preprocessor.  Most
+of them can also be given when compiling a C program; they are passed along
+automatically to the preprocessor when it is invoked by the compiler.
+
+@table @samp
+@item -P
+@findex -P
+Inhibit generation of @samp{#}-lines with line-number information in
+the output from the preprocessor (@pxref{Output}).  This might be
+useful when running the preprocessor on something that is not C code
+and will be sent to a program which might be confused by the
+@samp{#}-lines
+
+@item -C
+@findex -C
+Do not discard comments: pass them through to the output file.
+Comments appearing in arguments of a macro call will be copied to the
+output before the expansion of the macro call.
+
+@item -trigraphs
+@findex -trigraphs
+Process ANSI standard trigraph sequences.  These are three-character
+sequences, all starting with @samp{??}, that are defined by ANSI C to
+stand for single characters.  For example, @samp{??/} stands for
+@samp{\}, so @samp{'??/n'} is a character constant for a newline.
+Strictly speaking, the GNU C preprocessor does not support all
+programs in ANSI Standard C unless @samp{-trigraphs} is used, but if
+you ever notice the difference it will be with relief.
+
+You don't want to know any more about trigraphs.
+
+@item -pedantic
+@findex -pedantic
+Issue warnings required by the ANSI C standard in certain cases such
+as when text other than a comment follows @samp{#else} or @samp{#endif}.
+
+@item -I @var{directory}
+@findex -I
+Add the directory @var{directory} to the end of the list of
+directories to be searched for header files (@pxref{Include Syntax}).
+This can be used to override a system header file, substituting your
+own version, since these directories are searched before the system
+header file directories.  If you use more than one @samp{-I} option,
+the directories are scanned in left-to-right order; the standard
+system directories come after.
+
+@item -I-
+Any directories specified with @samp{-I} options before the @samp{-I-}
+option are searched only for the case of @samp{#include "@var{file}"};
+they are not searched for @samp{#include <@var{file}>}.
+
+If additional directories are specified with @samp{-I} options after
+the @samp{-I-}, these directories are searched for all @samp{#include}
+directives.
+
+In addition, the @samp{-I-} option inhibits the use of the current
+directory as the first search directory for @samp{#include "@var{file}"}.
+Therefore, the current directory is searched only if it is requested
+explicitly with @samp{-I.}.  Specifying both @samp{-I-} and @samp{-I.}
+allows you to control precisely which directories are searched before
+the current one and which are searched after.
+
+@item -nostdinc
+Do not search the standard system directories for header files.
+Only the directories you have specified with @samp{-I} options
+(and the current directory, if appropriate) are searched.
+
+@item -D @var{name}
+@findex -D
+Predefine @var{name} as a macro, with definition @samp{1}.
+
+@item -D @var{name}=@var{definition}
+Predefine @var{name} as a macro, with definition @var{definition}.
+There are no restrictions on the contents of @var{definition}, but if
+you are invoking the preprocessor from a shell or shell-like program
+you may need to use the shell's quoting syntax to protect characters
+such as spaces that have a meaning in the shell syntax.
+
+@item -U @var{name}
+@findex -U
+Do not predefine @var{name}.  If both @samp{-U} and @samp{-D} are
+specified for one name, the @samp{-U} beats the @samp{-D} and the name
+is not predefined.
+
+@item -undef
+@findex -undef
+Do not predefine any nonstandard macros.
+
+@item -d
+@findex -d
+Instead of outputting the result of preprocessing, output a list of
+@samp{#define} commands for all the macros defined during the
+execution of the preprocessor.
+
+@item -M
+@findex -M
+Instead of outputting the result of preprocessing, output a rule
+suitable for @code{make} describing the dependencies of the main
+source file.  The preprocessor outputs one @code{make} rule containing
+the object file name for that source file, a colon, and the names of
+all the included files.  If there are many included files then the
+rule is split into several lines using @samp{\}-newline.
+
+This feature is used in automatic updating of makefiles.
+
+@item -MM
+@findex -MM
+Like @samp{-M} but mention only the files included with @samp{#include
+"@var{file}"}.  System header files included with @samp{#include
+<@var{file}>} are omitted.
+
+@item -i @var{file}
+@findex -i
+Process @var{file} as input, discarding the resulting output, before
+processing the regular input file.  Because the output generated from
+@var{file} is discarded, the only effect of @samp{-i @var{file}} is to
+make the macros defined in @var{file} available for use in the main
+input.
+@end table
+
+@node Concept Index, Index, Invocation, Top
+@unnumbered Concept Index
+@printindex cp
+
+@node Index,, Concept Index, Top
+@unnumbered Index of Commands, Macros and Options
+@printindex fn
+
+@contents
+@bye