macro unsafe?

[spiderling66]

hi guys.

i hava no idea why the macro is unsafe, i just want the number(x) is always positive. could anyone tell me why, please?

#define number(x) x < 0 ? -x : x

 

[Salem]

Try various things like

y = number( x & 1 );
y = number( x++ );
y = number( printf("hello") );

Macros are very tricky to write which are safe against operator precedence and multiple evaluation, especially when there are side effects involved.

Fix operator precedence by putting everything in ()

#define number(x) ((x) < 0 ? -(x) : (x))

--

 

[spiderling66]

thanks Salem.

 

[chipperMDW]

#define number(x) x < 0 ? -x : x

result = 6 + number(4 == 5) * 3;

is equivalent to

result = 6 + 4 < 0 ? -4 : 4 * 3;

]#define number(x) ((x) < 0 ? -(x) : (x))

result = 6 + number(4 == 5) * 3;

is equivalent to

result = 6 + ((4) < 0 ? -(4) : (4)) * 3;

Those have different meanings. The second one is more predictable from looking at the unexpanded macro call.