On Mon, 6 May 1996, Kresimir Kukulj wrote:
> I would like a comment on this:
>
> #define CREATE(result, type, number) do {\
> if (!((result) = (type *) calloc ((number), sizeof(type))))\
> { perror("malloc failure"); abort(); } } while(0)
>
> Is < do {} while (0) > necessary?
>
>From comp.loang.c FAQ:
10.4: What's the best way to write a multi-statement macro?
A: The usual goal is to write a macro that can be invoked as if it
were a statement consisting of a single function call. This
means that the "caller" will be supplying the final semicolon,
so the macro body should not. The macro body cannot therefore
be a simple brace-enclosed compound statement, because syntax
errors would result if it were invoked (apparently as a single
statement, but with a resultant extra semicolon) as the if
branch of an if/else statement with an explicit else clause.
The traditional solution, therefore, is to use
#define MACRO(arg1, arg2) do { \
/* declarations */ \
stmt1; \
stmt2; \
/* ... */ \
} while(0) /* (no trailing ; ) */
When the caller appends a semicolon, this expansion becomes a
single statement regardless of context. (An optimizing compiler
will remove any "dead" tests or branches on the constant
condition 0, although lint may complain.)
If all of the statements in the intended macro are simple
expressions, with no declarations or loops, another technique is
to write a single, parenthesized expression using one or more
comma operators. (For an example, see the first DEBUG() macro
in question 10.26.) This technique also allows a value to be
"returned."
References: H&S Sec. 3.3.2 p. 45; CT&P Sec. 6.3 pp. 82-3.
:]
Gekke Eekhoorn of BUG.
PS: http://www.cis.ohio-state.edu/hypertext/faq/usenet/C-faq/faq/faq.html
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