Initial processing - The C Preprocessor

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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.

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.
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:      [    ]    {    }    #    \    ^    |    ~
```
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.
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.