It is strange if the condition in c: if (((void) (d), 0)) {...}

Question

I read the Xen source code and saw something like this:

#define is_domain_direct_mapped(d) ((void)(d), 0)

is_domain_direct_mapped is then used in the if statement as follows (d is a pointer to a structure variable):

if( is_domain_direct_mapped(d) ) {...}

So, after the compiler replaces is_domain_direct_mapped with its definition, we have something like:

if( ((void)(d), 0) ) {...}

The above if statement is very strange to me. This is the first time I see such an expression. How should this work?

+4

Bao bui 25 sept. '17 at 2:31

2 answers

.

( d) . (0) .

+2

Ry︁ · Accepted Answer · 2017-09-25T14:34:37+0000

It should always produce 0, but also evaluate d, in case of something with side effects like p++.

, 0: ARM.

#define is_domain_direct_mapped(d) ((d) == hardware_domain && dom0_11_mapping)

, d .

/memory.c, : [1]:

#define is_domain_is_direct_mapped(d) ((void)(d), 0)

, .

( ), ,

#define is_domain_direct_mapped(d) ((void)(d), 0)