What is the # for when formatting using %s

Question

I came across this example of an assertion and was wondering what the # is for:

#define ASSERT( x ) if ( !( x ) ) { \
    int *p = NULL; \
    DBGPRINTF("Assert failed: [%s]\r\n Halting.", #x); \
    *p=1; \
  } 

Answer 1

What you see is called stringification. It allows you to convert an argument of a macro into a string literal. You can read more about it here http://gcc.gnu.org/onlinedocs/cpp/Stringification.html.