What's the use of do while(0) when we define a macro? [duplicate]
Possible Duplicates:
Do-While and if-else statements in C/C++ macros
do { … } while (0) — what is it good for?
I'm reading the linux kernel and I found many macros like this:
#define INIT_LIST_HEAD(ptr) do { \
(ptr)->next = (ptr); (ptr)->prev = (ptr); \
} while (0)
Why do they use this rather than define it simply in a {}?
Solution 1:
You can follow it with a semicolon and make it look and act more like a function. It also works with if/else clauses properly then.
Without the while(0), your code above would not work with
if (doit)
INIT_LIST_HEAD(x);
else
displayError(x);
since the semicolon after the macro would "eat" the else clause, and the above wouldn't even compile.
Solution 2:
It allows you to group several statements into one macro.
Assume you did something like:
if (foo)
INIT_LIST_HEAD(bar);
If the macro was defined without the encapsulating do { ... } while (0);, the above code would expand to
if (foo)
(bar)->next = (bar);
(bar)->prev = (bar);
This is clearly not what was intended, as only the first statement will be executed if foo holds. The second statement would be executed regardless of whether foo holds.
Edit: Further explanation at http://c-faq.com/cpp/multistmt.html and http://developer.apple.com/documentation/DeveloperTools/gcc-4.0.1/cpp/Swallowing-the-Semicolon.html#Swallowing-the-Semicolon