Confirm: safe and portable?
My guess is C struct elements are stored linearly, so this will be safe... but I'm not 100% certain.Code:int main( void ) { struct { int magic; void *data1; void *data2; ... void *datan; } args = { 0xdeadbeef }; myfunc( &args ); } myfunc( void *arg ) { int magic = ((struct { int magic; } *)arg)->magic; ... }
Doesn't offsetof() work?
thanks for pointing out that macro, but how can I use it without knowing the struct type?
The myfunc function does not know the exact kind of struct that is being passed (it can be one of many different structs). All it knows is that the first member is an integer magic code.
So I need a portable and safe way of getting the first member of a struct without knowing what lies in the rest of the struct.
Standard grantees that there'll be no padding in front of first struct member.
It should be portable and safe.
ResourceThe Standard makes some guarantees about the layout of structures and unions:
Members of a structure are allocated within the structure in the order of their appearance in the declaration and have ascending addresses.
There must not be any padding in front of the first member.
The address of a structure is the same as the address of its first member, provided that the appropriate cast is used. Given the previous declaration of struct wp_char, if item is of type struct wp_char, then (char *)item == &item.wp_cval.
Bit fields (see Section 6.4) don't actually have addresses, but are conceptually packed into units which obey the rules above.
excellent thanks Bayint! that's exactly the reassurance I was looking for.
I will cast the (void *) argument to the type of the first member.
