GNU Info

(cpp-295.info)Swallow Semicolon

---

Next: Side Effects Prev: Macro Parentheses Up: Macro Pitfalls

---

Enter node , (file) or (file)node

Swallowing the Semicolon
........................

   Often it is desirable to define a macro that expands into a compound
statement.  Consider, for example, the following macro, that advances a
pointer (the argument `p' says where to find it) across whitespace
characters:

     #define SKIP_SPACES(p, limit)  \
     { register char *lim = (limit); \
       while (p != lim) {            \
         if (*p++ != ' ') {          \
           p--; break; }}}

Here Backslash-Newline is used to split the macro definition, which must
be a single line, so that it resembles the way such C code would be
laid out if not part of a macro definition.

   A call to this macro might be `SKIP_SPACES (p, lim)'.  Strictly
speaking, the call expands to a compound statement, which is a complete
statement with no need for a semicolon to end it.  But it looks like a
function call.  So it minimizes confusion if you can use it like a
function call, writing a semicolon afterward, as in `SKIP_SPACES (p,
lim);'

   But this can cause trouble before `else' statements, because the
semicolon is actually a null statement.  Suppose you write

     if (*p != 0)
       SKIP_SPACES (p, lim);
     else ...

The presence of two statements--the compound statement and a null
statement--in between the `if' condition and the `else' makes invalid C
code.

   The definition of the macro `SKIP_SPACES' can be altered to solve
this problem, using a `do ... while' statement.  Here is how:

     #define SKIP_SPACES(p, limit)     \
     do { register char *lim = (limit); \
          while (p != lim) {            \
            if (*p++ != ' ') {          \
              p--; break; }}}           \
     while (0)

   Now `SKIP_SPACES (p, lim);' expands into

     do {...} while (0);

which is one statement.