Compile Time Assertion..

The most seen assertion are during runtime, but this one is at compile time, to give the error more quickly..

I do not quite understand why need this kind of check.. The following is code from OVS..

#define BUILD_ASSERT(EXPR) \
　　typedef char AssertOnCompileFailed[(EXPR)?:-]

#define BUILD_ASSERT_DECL(EXPR) BUILD_ASSERT(EXPR)
typedef struct _NlMsgErr {
　　INT error;
　　NL_MSG_HDR nlMsg;
} NL_MSG_ERR, *PNL_MSG_ERR;
BUILD_ASSERT_DECL(sizeof(NL_MSG_ERR)==);

I think maybe the typedef of struct is constructed at compile time, so assert at compile time is not a bad idea..