Variable length arg lists for macros
Richard Walker
walker at island.uu.net
Thu Sep 15 07:49:33 AEST 1988
In article <228 at itivax.UUCP> scs at itivax.UUCP (Steve C. Simmons) writes:
>In article <1036 at cbnews.ATT.COM> lvc at cbnews.ATT.COM (Lawrence V. Cipriani) writes:
>-In article <438 at ucsvc.unimelb.edu.au>, u5565522 at ucsvc.unimelb.edu.au (David Clunie) writes:
>-
>- #ifdef TRACE
>- #define trace(anything) anything
>- #else
>- #define trace(anything)
>- #endif
>-
> if ( x == m )
> trace( printf( "the consequences\n) ; )
> froob( some function ) ;
>
>This produced rather different programs depending on the definition of
>'trace'!
The problem illustrated above is caused by the bad usage of the macro. The
macro call should be formed as a statement to avoid the flow control problem.
In other words, the errors above are caused by the missing ';' at the end
of the trace macro call.
Especially, macros which expand into code with flow control
should be formed such that the semicolon cleanly terminates
the flow, e.g.:
#define COMPLEX_MACRO(foo) (if(SOME_TEST(foo)) {statements;}else)
Then the correct usage of COMPLEX_MACRO would be:
COMPLEX_MACRO(foo);
which also handles the case where COMPLEX_MACRO expands into nothing.
cat <flames_about_crufty_usage_of_code_in_macros >/dev/null
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