Why do we need parentheses around block macro?

Question

In linux, container_of macro is enclosed in seemingly "extra" parentheses:

 #define container_of(ptr, type, member) ({ \
                const typeof( ((type *)0)->member ) *__mptr = (ptr); 
                (type *)( (char *)__mptr - offsetof(type,member) );})

Instead of it, can we just use

 #define container_of(ptr, type, member) { \
                const typeof( ((type *)0)->member ) *__mptr = (ptr); 
                (type *)( (char *)__mptr - offsetof(type,member) );}

?

Are the parentheses mandatory or are they just for precaution?

This should be tagged GCC rather than C....... – David Heffernan Nov 15 '11 at 20:04 — David Heffernan, Nov 15 '11 at 20:04

score 11 · Accepted Answer · edited May 23 '17 at 10:27

11

It's necessary. One of the "tricks" used is GCC's statement expressions that require this 'strange' ({ code }) syntax.

Code that uses this macro wouldn't compile without that in most use cases (and it's not valid C).

See also: Rationale behind the container_of macro in linux/list.h

And: container_of by Greg Kroah-Hartman.

edited May 23 '17 at 10:27

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answered Nov 15 '11 at 19:59

Mat

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score 9 · Answer 2 · edited May 23 '17 at 12:31

({...}) is a braced group, a code block that can be used inside an expression. The last statement determines its value.

Inside macros you use it to let macros behave the same way as functions. Mostly a static inline function would be preferable.

Why do we need parentheses around block macro?

2 Answers2

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