| Stringification

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Sometimes you may want to convert a macro argument into a string constant. Parameters are not replaced inside string constants, but you can use the `#` preprocessing operator instead. When a macro parameter is used with a leading `#`, the preprocessor replaces it with the literal text of the actual argument, converted to a string constant. Unlike normal parameter replacement, the argument is not macro-expanded first. This is called stringification.
There is no way to combine an argument with surrounding text and stringify it all together. Instead, you can write a series of adjacent string constants and stringified arguments. The preprocessor will replace the stringified arguments with string constants. The C compiler will then combine all the adjacent string constants into one long string.
Here is an example of a macro definition that uses stringification:

    #define WARN_IF(EXP) \
    do { if (EXP) \
            fprintf (stderr, "Warning: " #EXP "\n"); } \
    while (0)
    WARN_IF (x == 0);
         expands to do { if (x == 0)
               fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0);

The argument for `EXP` is substituted once, as-is, into the `if` statement, and once, stringified, into the argument to `fprintf`. If `x` were a macro, it would be expanded in the `if` statement, but not in the string.
The `do` and `while (0)` are a kludge to make it possible to write `WARN_IF (arg);`, which the resemblance of `WARN_IF` to a function would make C programmers want to do; see Swallowing the Semicolon.
Stringification in C involves more than putting double-quote characters around the fragment. The preprocessor backslash-escapes the quotes surrounding embedded string constants, and all backslashes within string and character constants, in order to get a valid C string constant with the proper contents. Thus, stringifying `p = "foo\n";` results in `"p = \"foo\\n\";"`. However, backslashes that are not inside string or character constants are not duplicated: `\n` by itself stringifies to `"\n"`.
All leading and trailing whitespace in text being stringified is ignored. Any sequence of whitespace in the middle of the text is converted to a single space in the stringified result. Comments are replaced by whitespace long before stringification happens, so they never appear in stringified text.
There is no way to convert a macro argument into a character constant.
If you want to stringify the result of expansion of a macro argument, you have to use two levels of macros.

    #define xstr(s) str(s)
    #define str(s) #s
    #define foo 4
    str (foo)
         expands to "foo"
    xstr (foo)
         expands to xstr (4)
         expands to str (4)
         expands to "4"

`s` is stringified when it is used in `str`, so it is not macro-expanded first. But `s` is an ordinary argument to `xstr`, so it is completely macro-expanded before `xstr` itself is expanded (see Argument Prescan). Therefore, by the time `str` gets to its argument, it has already been macro-expanded.