The GNU C Preprocessor Internals: Lexer

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## The Lexer

### Overview

The lexer is contained in the file `lex.c`. It is a hand-coded lexer, and not implemented as a state machine. It can understand C, C++ and Objective-C source code, and has been extended to allow reasonably successful preprocessing of assembly language. The lexer does not make an initial pass to strip out trigraphs and escaped newlines, but handles them as they are encountered in a single pass of the input file. It returns preprocessing tokens individually, not a line at a time.

It is mostly transparent to users of the library, since the library’s interface for obtaining the next token, `cpp_get_token`, takes care of lexing new tokens, handling directives, and expanding macros as necessary. However, the lexer does expose some functionality so that clients of the library can easily spell a given token, such as `cpp_spell_token` and `cpp_token_len`. These functions are useful when generating diagnostics, and for emitting the preprocessed output.

### Lexing a token

Lexing of an individual token is handled by `_cpp_lex_direct` and its subroutines. In its current form the code is quite complicated, with read ahead characters and such-like, since it strives to not step back in the character stream in preparation for handling non-ASCII file encodings. The current plan is to convert any such files to UTF-8 before processing them. This complexity is therefore unnecessary and will be removed, so I’ll not discuss it further here.

The job of `_cpp_lex_direct` is simply to lex a token. It is not responsible for issues like directive handling, returning lookahead tokens directly, multiple-include optimization, or conditional block skipping. It necessarily has a minor rôle to play in memory management of lexed lines. I discuss these issues in a separate section (see Lexing a line).

The lexer places the token it lexes into storage pointed to by the variable `cur_token`, and then increments it. This variable is important for correct diagnostic positioning. Unless a specific line and column are passed to the diagnostic routines, they will examine the `line` and `col` values of the token just before the location that `cur_token` points to, and use that location to report the diagnostic.

The lexer does not consider whitespace to be a token in its own right. If whitespace (other than a new line) precedes a token, it sets the `PREV_WHITE` bit in the token’s flags. Each token has its `line` and `col` variables set to the line and column of the first character of the token. This line number is the line number in the translation unit, and can be converted to a source (file, line) pair using the line map code.

The first token on a logical, i.e. unescaped, line has the flag `BOL` set for beginning-of-line. This flag is intended for internal use, both to distinguish a ‘`#`’ that begins a directive from one that doesn’t, and to generate a call-back to clients that want to be notified about the start of every non-directive line with tokens on it. Clients cannot reliably determine this for themselves: the first token might be a macro, and the tokens of a macro expansion do not have the `BOL` flag set. The macro expansion may even be empty, and the next token on the line certainly won’t have the `BOL` flag set.

New lines are treated specially; exactly how the lexer handles them is context-dependent. The C standard mandates that directives are terminated by the first unescaped newline character, even if it appears in the middle of a macro expansion. Therefore, if the state variable `in_directive` is set, the lexer returns a `CPP_EOF` token, which is normally used to indicate end-of-file, to indicate end-of-directive. In a directive a `CPP_EOF` token never means end-of-file. Conveniently, if the caller was `collect_args`, it already handles `CPP_EOF` as if it were end-of-file, and reports an error about an unterminated macro argument list.

The C standard also specifies that a new line in the middle of the arguments to a macro is treated as whitespace. This white space is important in case the macro argument is stringized. The state variable `parsing_args` is nonzero when the preprocessor is collecting the arguments to a macro call. It is set to 1 when looking for the opening parenthesis to a function-like macro, and 2 when collecting the actual arguments up to the closing parenthesis, since these two cases need to be distinguished sometimes. One such time is here: the lexer sets the `PREV_WHITE` flag of a token if it meets a new line when `parsing_args` is set to 2. It doesn’t set it if it meets a new line when `parsing_args` is 1, since then code like

    #define foo() bar
    foo
    baz

would be output with an erroneous space before ‘`baz`’:

    foo
     baz

This is a good example of the subtlety of getting token spacing correct in the preprocessor; there are plenty of tests in the testsuite for corner cases like this.

The lexer is written to treat each of ‘`\r`’, ‘`\n`’, ‘`\r\n`’ and ‘`\n\r`’ as a single new line indicator. This allows it to transparently preprocess MS-DOS, Macintosh and Unix files without their needing to pass through a special filter beforehand.

We also decided to treat a backslash, either ‘`\`’ or the trigraph ‘`??/`’, separated from one of the above newline indicators by non-comment whitespace only, as intending to escape the newline. It tends to be a typing mistake, and cannot reasonably be mistaken for anything else in any of the C-family grammars. Since handling it this way is not strictly conforming to the ISO standard, the library issues a warning wherever it encounters it.

Handling newlines like this is made simpler by doing it in one place only. The function `handle_newline` takes care of all newline characters, and `skip_escaped_newlines` takes care of arbitrarily long sequences of escaped newlines, deferring to `handle_newline` to handle the newlines themselves.

The most painful aspect of lexing ISO-standard C and C++ is handling trigraphs and backlash-escaped newlines. Trigraphs are processed before any interpretation of the meaning of a character is made, and unfortunately there is a trigraph representation for a backslash, so it is possible for the trigraph ‘`??/`’ to introduce an escaped newline.

Escaped newlines are tedious because theoretically they can occur anywhere—between the ‘`+`’ and ‘`=`’ of the ‘`+=`’ token, within the characters of an identifier, and even between the ‘`*`’ and ‘`/`’ that terminates a comment. Moreover, you cannot be sure there is just one—there might be an arbitrarily long sequence of them.

So, for example, the routine that lexes a number, `parse_number`, cannot assume that it can scan forwards until the first non-number character and be done with it, because this could be the ‘`\`’ introducing an escaped newline, or the ‘`?`’ introducing the trigraph sequence that represents the ‘`\`’ of an escaped newline. If it encounters a ‘`?`’ or ‘`\`’, it calls `skip_escaped_newlines` to skip over any potential escaped newlines before checking whether the number has been finished.

Similarly code in the main body of `_cpp_lex_direct` cannot simply check for a ‘`=`’ after a ‘`+`’ character to determine whether it has a ‘`+=`’ token; it needs to be prepared for an escaped newline of some sort. Such cases use the function `get_effective_char`, which returns the first character after any intervening escaped newlines.

The lexer needs to keep track of the correct column position, including counting tabs as specified by the `-ftabstop=` option. This should be done even within C-style comments; they can appear in the middle of a line, and we want to report diagnostics in the correct position for text appearing after the end of the comment.

Some identifiers, such as `__VA_ARGS__` and poisoned identifiers, may be invalid and require a diagnostic. However, if they appear in a macro expansion we don’t want to complain with each use of the macro. It is therefore best to catch them during the lexing stage, in `parse_identifier`. In both cases, whether a diagnostic is needed or not is dependent upon the lexer’s state. For example, we don’t want to issue a diagnostic for re-poisoning a poisoned identifier, or for using `__VA_ARGS__` in the expansion of a variable-argument macro. Therefore `parse_identifier` makes use of state flags to determine whether a diagnostic is appropriate. Since we change state on a per-token basis, and don’t lex whole lines at a time, this is not a problem.

Another place where state flags are used to change behavior is whilst lexing header names. Normally, a ‘`<`’ would be lexed as a single token. After a `#include` directive, though, it should be lexed as a single token as far as the nearest ‘`>`’ character. Note that we don’t allow the terminators of header names to be escaped; the first ‘`"`’ or ‘`>`’ terminates the header name.

Interpretation of some character sequences depends upon whether we are lexing C, C++ or Objective-C, and on the revision of the standard in force. For example, ‘`::`’ is a single token in C++, but in C it is two separate ‘`:`’ tokens and almost certainly a syntax error. Such cases are handled by `_cpp_lex_direct` based upon command-line flags stored in the `cpp_options` structure.

Once a token has been lexed, it leads an independent existence. The spelling of numbers, identifiers and strings is copied to permanent storage from the original input buffer, so a token remains valid and correct even if its source buffer is freed with `_cpp_pop_buffer`. The storage holding the spellings of such tokens remains until the client program calls cpp_destroy, probably at the end of the translation unit.

### Lexing a line

When the preprocessor was changed to return pointers to tokens, one feature I wanted was some sort of guarantee regarding how long a returned pointer remains valid. This is important to the stand-alone preprocessor, the future direction of the C family front ends, and even to cpplib itself internally.

Occasionally the preprocessor wants to be able to peek ahead in the token stream. For example, after the name of a function-like macro, it wants to check the next token to see if it is an opening parenthesis. Another example is that, after reading the first few tokens of a `#pragma` directive and not recognizing it as a registered pragma, it wants to backtrack and allow the user-defined handler for unknown pragmas to access the full `#pragma` token stream. The stand-alone preprocessor wants to be able to test the current token with the previous one to see if a space needs to be inserted to preserve their separate tokenization upon re-lexing (paste avoidance), so it needs to be sure the pointer to the previous token is still valid. The recursive-descent C++ parser wants to be able to perform tentative parsing arbitrarily far ahead in the token stream, and then to be able to jump back to a prior position in that stream if necessary.

The rule I chose, which is fairly natural, is to arrange that the preprocessor lex all tokens on a line consecutively into a token buffer, which I call a token run, and when meeting an unescaped new line (newlines within comments do not count either), to start lexing back at the beginning of the run. Note that we do not lex a line of tokens at once; if we did that `parse_identifier` would not have state flags available to warn about invalid identifiers (see Invalid identifiers).

In other words, accessing tokens that appeared earlier in the current line is valid, but since each logical line overwrites the tokens of the previous line, tokens from prior lines are unavailable. In particular, since a directive only occupies a single logical line, this means that the directive handlers like the `#pragma` handler can jump around in the directive’s tokens if necessary.

Two issues remain: what about tokens that arise from macro expansions, and what happens when we have a long line that overflows the token run?

Since we promise clients that we preserve the validity of pointers that we have already returned for tokens that appeared earlier in the line, we cannot reallocate the run. Instead, on overflow it is expanded by chaining a new token run on to the end of the existing one.

The tokens forming a macro’s replacement list are collected by the `#define` handler, and placed in storage that is only freed by `cpp_destroy`. So if a macro is expanded in the line of tokens, the pointers to the tokens of its expansion that are returned will always remain valid. However, macros are a little trickier than that, since they give rise to three sources of fresh tokens. They are the built-in macros like `__LINE__`, and the ‘`#`’ and ‘`##`’ operators for stringizing and token pasting. I handled this by allocating space for these tokens from the lexer’s token run chain. This means they automatically receive the same lifetime guarantees as lexed tokens, and we don’t need to concern ourselves with freeing them.

Lexing into a line of tokens solves some of the token memory management issues, but not all. The opening parenthesis after a function-like macro name might lie on a different line, and the front ends definitely want the ability to look ahead past the end of the current line. So cpplib only moves back to the start of the token run at the end of a line if the variable `keep_tokens` is zero. Line-buffering is quite natural for the preprocessor, and as a result the only time cpplib needs to increment this variable is whilst looking for the opening parenthesis to, and reading the arguments of, a function-like macro. In the near future cpplib will export an interface to increment and decrement this variable, so that clients can share full control over the lifetime of token pointers too.

The routine `_cpp_lex_token` handles moving to new token runs, calling `_cpp_lex_direct` to lex new tokens, or returning previously-lexed tokens if we stepped back in the token stream. It also checks each token for the `BOL` flag, which might indicate a directive that needs to be handled, or require a start-of-line call-back to be made. `_cpp_lex_token` also handles skipping over tokens in failed conditional blocks, and invalidates the control macro of the multiple-include optimization if a token was successfully lexed outside a directive. In other words, its callers do not need to concern themselves with such issues.

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