cpp.info: Stringification

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Stringification

   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);
==> 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 *Note 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)
==> "foo"
xstr (foo)
==> xstr (4)
==> str (4)
==> "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 (*note
Argument Prescan::).  Therefore, by the time `str' gets to its
argument, it has already been macro-expanded.