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 {}?
views:
1241answers:
3
A:
My guess would be something to do with portability across platforms and compilers.
OJ
2009-05-29 00:08:09
+31
A:
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.
SPWorley
2009-05-29 00:09:22
But the OP's question stands. Why not just { (ptr)->next ... } instead of do { (ptr)->next ... } while (0);?
Josh K
2009-05-29 00:26:20
arno explained that. It would expand to "{ (ptr)->next ... };" thus, a statement followed by a second statement. If syntax however is "if ( expression ) statement else statement" . The else would not be associated with any if, since you would have written "if ( expression ) statement statement" (one "{ ... }" and one ";" statement).
Johannes Schaub - litb
2009-05-29 00:37:12
As Amo said, it's a clever trick that allows a macro to *be* a C statement that must end with a semicolon. It makes the macro act exactly like a function call, so far as statement construction and termination (with ';') is concerned.
Eddie
2009-05-29 01:53:29
Note, however, that in this case it's all completely unnecessary, as the body of the macro could be written much more cleanly as: `(ptr)->next=(ptr)->prev=(ptr)`.
Jerry Coffin
2009-12-02 04:19:52
+14
A:
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
rodion
2009-05-29 00:11:47
This doesn't explain the do .. while(0) part of the macro, just the use of {} braces.
SPWorley
2009-05-29 18:05:14