Node:Swallowing the Semicolon, Next:Duplication of Side Effects, Previous:Operator Precedence Problems, Up:Macro Pitfalls

#### 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)  \
         { 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 logical line, so that it resembles the way such 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. However, since it looks like a function call, it minimizes confusion if you can use it like a function call, writing a semicolon afterward, as in `SKIP_SPACES (p, lim);`

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 { 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. The loop executes exactly once; most compilers generate no extra code for it.