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1. Overview

The C preprocessor, often known as cpp, is a macro processor that is used automatically by the C compiler to transform your program before compilation. It is called a macro processor because it allows you to define macros, which are brief abbreviations for longer constructs.

The C preprocessor is intended to be used only with C, C++, and Objective-C source code. In the past, it has been abused as a general text processor. It will choke on input which does not obey C's lexical rules. For example, apostrophes will be interpreted as the beginning of character constants, and cause errors. Also, you cannot rely on it preserving characteristics of the input which are not significant to C-family languages. If a Makefile is preprocessed, all the hard tabs will be removed, and the Makefile will not work.

Having said that, you can often get away with using cpp on things which are not C. Other Algol-ish programming languages are often safe (Pascal, Ada, etc.) So is assembly, with caution. `-traditional-cpp' mode preserves more white space, and is otherwise more permissive. Many of the problems can be avoided by writing C or C++ style comments instead of native language comments, and keeping macros simple.

Wherever possible, you should use a preprocessor geared to the language you are writing in. Modern versions of the GNU assembler have macro facilities. Most high level programming languages have their own conditional compilation and inclusion mechanism. If all else fails, try a true general text processor, such as GNU M4.

C preprocessors vary in some details. This manual discusses the GNU C preprocessor, which provides a small superset of the features of ISO Standard C. In its default mode, the GNU C preprocessor does not do a few things required by the standard. These are features which are rarely, if ever, used, and may cause surprising changes to the meaning of a program which does not expect them. To get strict ISO Standard C, you should use the `-std=c90', `-std=c99' or `-std=c1x' options, depending on which version of the standard you want. To get all the mandatory diagnostics, you must also use `-pedantic'. See section Invocation.

This manual describes the behavior of the ISO preprocessor. To minimize gratuitous differences, where the ISO preprocessor's behavior does not conflict with traditional semantics, the traditional preprocessor should behave the same way. The various differences that do exist are detailed in the section Traditional Mode.

For clarity, unless noted otherwise, references to `CPP' in this manual refer to GNU CPP.


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1.1 Character sets

Source code character set processing in C and related languages is rather complicated. The C standard discusses two character sets, but there are really at least four.

The files input to CPP might be in any character set at all. CPP's very first action, before it even looks for line boundaries, is to convert the file into the character set it uses for internal processing. That set is what the C standard calls the source character set. It must be isomorphic with ISO 10646, also known as Unicode. CPP uses the UTF-8 encoding of Unicode.

The character sets of the input files are specified using the `-finput-charset=' option.

All preprocessing work (the subject of the rest of this manual) is carried out in the source character set. If you request textual output from the preprocessor with the `-E' option, it will be in UTF-8.

After preprocessing is complete, string and character constants are converted again, into the execution character set. This character set is under control of the user; the default is UTF-8, matching the source character set. Wide string and character constants have their own character set, which is not called out specifically in the standard. Again, it is under control of the user. The default is UTF-16 or UTF-32, whichever fits in the target's wchar_t type, in the target machine's byte order.(1) Octal and hexadecimal escape sequences do not undergo conversion; '\x12' has the value 0x12 regardless of the currently selected execution character set. All other escapes are replaced by the character in the source character set that they represent, then converted to the execution character set, just like unescaped characters.

Unless the experimental `-fextended-identifiers' option is used, GCC does not permit the use of characters outside the ASCII range, nor `\u' and `\U' escapes, in identifiers. Even with that option, characters outside the ASCII range can only be specified with the `\u' and `\U' escapes, not used directly in identifiers.


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1.2 Initial processing

The preprocessor performs a series of textual transformations on its input. These happen before all other processing. Conceptually, they happen in a rigid order, and the entire file is run through each transformation before the next one begins. CPP actually does them all at once, for performance reasons. These transformations correspond roughly to the first three "phases of translation" described in the C standard.

  1. The input file is read into memory and broken into lines.

    Different systems use different conventions to indicate the end of a line. GCC accepts the ASCII control sequences LF, CR LF and CR as end-of-line markers. These are the canonical sequences used by Unix, DOS and VMS, and the classic Mac OS (before OSX) respectively. You may therefore safely copy source code written on any of those systems to a different one and use it without conversion. (GCC may lose track of the current line number if a file doesn't consistently use one convention, as sometimes happens when it is edited on computers with different conventions that share a network file system.)

    If the last line of any input file lacks an end-of-line marker, the end of the file is considered to implicitly supply one. The C standard says that this condition provokes undefined behavior, so GCC will emit a warning message.

  2. If trigraphs are enabled, they are replaced by their corresponding single characters. By default GCC ignores trigraphs, but if you request a strictly conforming mode with the `-std' option, or you specify the `-trigraphs' option, then it converts them.

    These are nine three-character sequences, all starting with `??', that are defined by ISO C to stand for single characters. They permit obsolete systems that lack some of C's punctuation to use C. For example, `??/' stands for `\', so '??/n' is a character constant for a newline.

    Trigraphs are not popular and many compilers implement them incorrectly. Portable code should not rely on trigraphs being either converted or ignored. With `-Wtrigraphs' GCC will warn you when a trigraph may change the meaning of your program if it were converted. See Wtrigraphs.

    In a string constant, you can prevent a sequence of question marks from being confused with a trigraph by inserting a backslash between the question marks, or by separating the string literal at the trigraph and making use of string literal concatenation. "(??\?)" is the string `(???)', not `(?]'. Traditional C compilers do not recognize these idioms.

    The nine trigraphs and their replacements are

     
    Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
    Replacement:      [    ]    {    }    #    \    ^    |    ~
    
  3. Continued lines are merged into one long line.

    A continued line is a line which ends with a backslash, `\'. The backslash is removed and the following line is joined with the current one. No space is inserted, so you may split a line anywhere, even in the middle of a word. (It is generally more readable to split lines only at white space.)

    The trailing backslash on a continued line is commonly referred to as a backslash-newline.

    If there is white space between a backslash and the end of a line, that is still a continued line. However, as this is usually the result of an editing mistake, and many compilers will not accept it as a continued line, GCC will warn you about it.

  4. All comments are replaced with single spaces.

    There are two kinds of comments. Block comments begin with `/*' and continue until the next `*/'. Block comments do not nest:

     
    /* this is /* one comment */ text outside comment
    

    Line comments begin with `//' and continue to the end of the current line. Line comments do not nest either, but it does not matter, because they would end in the same place anyway.

     
    // this is // one comment
    text outside comment
    

It is safe to put line comments inside block comments, or vice versa.

 
/* block comment
   // contains line comment
   yet more comment
 */ outside comment

// line comment /* contains block comment */

But beware of commenting out one end of a block comment with a line comment.

 
 // l.c.  /* block comment begins
    oops! this isn't a comment anymore */

Comments are not recognized within string literals. "/* blah */" is the string constant `/* blah */', not an empty string.

Line comments are not in the 1989 edition of the C standard, but they are recognized by GCC as an extension. In C++ and in the 1999 edition of the C standard, they are an official part of the language.

Since these transformations happen before all other processing, you can split a line mechanically with backslash-newline anywhere. You can comment out the end of a line. You can continue a line comment onto the next line with backslash-newline. You can even split `/*', `*/', and `//' onto multiple lines with backslash-newline. For example:

 
/\
*
*/ # /*
*/ defi\
ne FO\
O 10\
20

is equivalent to #define FOO 1020. All these tricks are extremely confusing and should not be used in code intended to be readable.

There is no way to prevent a backslash at the end of a line from being interpreted as a backslash-newline. This cannot affect any correct program, however.


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1.3 Tokenization

After the textual transformations are finished, the input file is converted into a sequence of preprocessing tokens. These mostly correspond to the syntactic tokens used by the C compiler, but there are a few differences. White space separates tokens; it is not itself a token of any kind. Tokens do not have to be separated by white space, but it is often necessary to avoid ambiguities.

When faced with a sequence of characters that has more than one possible tokenization, the preprocessor is greedy. It always makes each token, starting from the left, as big as possible before moving on to the next token. For instance, a+++++b is interpreted as a ++ ++ + b, not as a ++ + ++ b, even though the latter tokenization could be part of a valid C program and the former could not.

Once the input file is broken into tokens, the token boundaries never change, except when the `##' preprocessing operator is used to paste tokens together. See section Concatenation. For example,

 
#define foo() bar
foo()baz
     → bar baz
not
     → barbaz

The compiler does not re-tokenize the preprocessor's output. Each preprocessing token becomes one compiler token.

Preprocessing tokens fall into five broad classes: identifiers, preprocessing numbers, string literals, punctuators, and other. An identifier is the same as an identifier in C: any sequence of letters, digits, or underscores, which begins with a letter or underscore. Keywords of C have no significance to the preprocessor; they are ordinary identifiers. You can define a macro whose name is a keyword, for instance. The only identifier which can be considered a preprocessing keyword is defined. See section Defined.

This is mostly true of other languages which use the C preprocessor. However, a few of the keywords of C++ are significant even in the preprocessor. See section C++ Named Operators.

In the 1999 C standard, identifiers may contain letters which are not part of the "basic source character set", at the implementation's discretion (such as accented Latin letters, Greek letters, or Chinese ideograms). This may be done with an extended character set, or the `\u' and `\U' escape sequences. The implementation of this feature in GCC is experimental; such characters are only accepted in the `\u' and `\U' forms and only if `-fextended-identifiers' is used.

As an extension, GCC treats `$' as a letter. This is for compatibility with some systems, such as VMS, where `$' is commonly used in system-defined function and object names. `$' is not a letter in strictly conforming mode, or if you specify the `-$' option. See section Invocation.

A preprocessing number has a rather bizarre definition. The category includes all the normal integer and floating point constants one expects of C, but also a number of other things one might not initially recognize as a number. Formally, preprocessing numbers begin with an optional period, a required decimal digit, and then continue with any sequence of letters, digits, underscores, periods, and exponents. Exponents are the two-character sequences `e+', `e-', `E+', `E-', `p+', `p-', `P+', and `P-'. (The exponents that begin with `p' or `P' are new to C99. They are used for hexadecimal floating-point constants.)

The purpose of this unusual definition is to isolate the preprocessor from the full complexity of numeric constants. It does not have to distinguish between lexically valid and invalid floating-point numbers, which is complicated. The definition also permits you to split an identifier at any position and get exactly two tokens, which can then be pasted back together with the `##' operator.

It's possible for preprocessing numbers to cause programs to be misinterpreted. For example, 0xE+12 is a preprocessing number which does not translate to any valid numeric constant, therefore a syntax error. It does not mean 0xE + 12, which is what you might have intended.

String literals are string constants, character constants, and header file names (the argument of `#include').(2) String constants and character constants are straightforward: "…" or '…'. In either case embedded quotes should be escaped with a backslash: '\'' is the character constant for `''. There is no limit on the length of a character constant, but the value of a character constant that contains more than one character is implementation-defined. See section Implementation Details.

Header file names either look like string constants, "…", or are written with angle brackets instead, <…>. In either case, backslash is an ordinary character. There is no way to escape the closing quote or angle bracket. The preprocessor looks for the header file in different places depending on which form you use. See section Include Operation.

No string literal may extend past the end of a line. Older versions of GCC accepted multi-line string constants. You may use continued lines instead, or string constant concatenation. See section Differences from previous versions.

Punctuators are all the usual bits of punctuation which are meaningful to C and C++. All but three of the punctuation characters in ASCII are C punctuators. The exceptions are `@', `$', and ``'. In addition, all the two- and three-character operators are punctuators. There are also six digraphs, which the C++ standard calls alternative tokens, which are merely alternate ways to spell other punctuators. This is a second attempt to work around missing punctuation in obsolete systems. It has no negative side effects, unlike trigraphs, but does not cover as much ground. The digraphs and their corresponding normal punctuators are:

 
Digraph:        <%  %>  <:  :>  %:  %:%:
Punctuator:      {   }   [   ]   #    ##

Any other single character is considered "other". It is passed on to the preprocessor's output unmolested. The C compiler will almost certainly reject source code containing "other" tokens. In ASCII, the only other characters are `@', `$', ``', and control characters other than NUL (all bits zero). (Note that `$' is normally considered a letter.) All characters with the high bit set (numeric range 0x7F-0xFF) are also "other" in the present implementation. This will change when proper support for international character sets is added to GCC.

NUL is a special case because of the high probability that its appearance is accidental, and because it may be invisible to the user (many terminals do not display NUL at all). Within comments, NULs are silently ignored, just as any other character would be. In running text, NUL is considered white space. For example, these two directives have the same meaning.

 
#define X^@1
#define X 1

(where `^@' is ASCII NUL). Within string or character constants, NULs are preserved. In the latter two cases the preprocessor emits a warning message.


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1.4 The preprocessing language

After tokenization, the stream of tokens may simply be passed straight to the compiler's parser. However, if it contains any operations in the preprocessing language, it will be transformed first. This stage corresponds roughly to the standard's "translation phase 4" and is what most people think of as the preprocessor's job.

The preprocessing language consists of directives to be executed and macros to be expanded. Its primary capabilities are:

There are a few more, less useful, features.

Except for expansion of predefined macros, all these operations are triggered with preprocessing directives. Preprocessing directives are lines in your program that start with `#'. Whitespace is allowed before and after the `#'. The `#' is followed by an identifier, the directive name. It specifies the operation to perform. Directives are commonly referred to as `#name' where name is the directive name. For example, `#define' is the directive that defines a macro.

The `#' which begins a directive cannot come from a macro expansion. Also, the directive name is not macro expanded. Thus, if foo is defined as a macro expanding to define, that does not make `#foo' a valid preprocessing directive.

The set of valid directive names is fixed. Programs cannot define new preprocessing directives.

Some directives require arguments; these make up the rest of the directive line and must be separated from the directive name by whitespace. For example, `#define' must be followed by a macro name and the intended expansion of the macro.

A preprocessing directive cannot cover more than one line. The line may, however, be continued with backslash-newline, or by a block comment which extends past the end of the line. In either case, when the directive is processed, the continuations have already been merged with the first line to make one long line.


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