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3.4 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 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 (see Argument Prescan). Therefore,
by the time str gets to its argument, it has already been macro-expanded.
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