26.5.4 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 #, preprocessing 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. But you can write a series of string constants and stringified arguments. After preprocessing replaces the stringified arguments with string constants, the consecutive string constants will be concatenated into one long string constant (see String Constants).

Here is an example that uses stringification and concatenation of string constants:

#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);. The resemblance of WARN_IF to a function makes that a natural way to write it. See Swallowing the Semicolon.

Stringification in C involves more than putting double-quote characters around the fragment. It also 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.

To stringify the result of expansion of a macro argument, you have to use two levels of macros, like this:

#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 text, that text already been macro-expanded.