6.4.9 Comments
1 Except within a character constant, a string literal, or a comment, the characters /*
  introduce a comment. The contents of such a comment are examined only to identify
  multibyte characters and to find the characters */ that terminate it.69)
2 Except within a character constant, a string literal, or a comment, the characters //
  introduce a comment that includes all multibyte characters up to, but not including, the
  next new-line character. The contents of such a comment are examined only to identify
  multibyte characters and to find the terminating new-line character.
  EXAMPLE
3
                                            
         "a//b"                          // four-character string literal
         #include "//e"                  // undefined behavior
         // */                           // comment, not syntax error
         f = g/**//h;                    // equivalent to f = g / h;
         //\                             // part of a two-line comment
         i();
         /\
         / j();                          // part of a two-line comment
         #define glue(x,y) x##y
         glue(/,/) k();                  // syntax error, not comment
         /*//*/ l();                     // equivalent to l();
         m = n//**/o
            + p;                         // equivalent to m = n + p;