C编译器如何解析以下C语句？

Question

Consider the following lines:

int i;
printf("%d",i);

Will the lexical analyzer go into the string to parse % and d as separate tokens, or will it parse "%d" as one token?

Answer 1

There are two parsers at work here: first, the C compiler, that will parse the C file and basically ignore the content of the string (though modern C compilers will parse the string as well to help catch bad format strings — mismatches between the % conversion specifier and the corresponding argument passed to printf() to be converted).

The next parser is the string format parser built into the C runtime library. This will be called at runtime to parse the format string when you call printf . This parser is of course very simple in comparison.

I have not checked, but I would guess that the C compilers that help checking for bad format strings will implement a printf -like parser as a post-processing step (ie using its own lexer).

Answer 2

A string literal is a single token. The above code will be tokenized like this:

int     keyword "int"
i       identifier
;       semicolon
printf  identifier
(       open paren
"%d"    string literal
,       comma
i       identifier
)       closing paren
;       semicolon

Answer 3

"%d" is a string literal and it will be seen as one token by both the C preprocessor and also by the compiler, we can see this by going to draft C99 standard section 6.4 Lexical elements which defines the following tokens:

token:
  keyword
  identifier
  constant
  string-literal
  punctuator

and the following proprocessing tokens:

preprocessing-token:
  header-name
  identifier
  pp-number
  character-constant
  string-literal
  punctuator
  each non-white-space character that cannot be one of the above

and says:

A token is the minimal lexical element of the language in translation phases 7 and 8. The categories of tokens are: keywords, identifiers, constants, string literals , and punctuators. A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6. The categories of preprocessing tokens are : header names, identifiers, preprocessing numbers, character constants, string literals , punctuators, and single non-white-space characters that do not lexically match the other preprocessing token categories.58) [...]

The different phases of translation are covered in section 5.1.1.2 Translation phases and I will highlight some of the relevant ones here:

[...]

3 The source file is decomposed into preprocessing tokens 6) and sequences of white-space characters (including comments).

[...]

6 Adjacent string literal tokens are concatenated.

7 White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token . The resulting tokens are syntactically and semantically analyzed and translated as a translation unit.

[...]

The distinction between pre-processor tokens and tokens may seem irrelevant but we can see that in at least one case such as in adjacent string literals for example "%d" "\\n" you would have two pre-processor tokens while after phase 6 there would be only one token .