Hello All,
Does anybody understand why RFCs propose creating struct sockaddr_storage with
code like
/*
* Desired design of maximum size and alignment
*/
#define _SS_MAXSIZE 128 /* Implementation specific max size */
#define _SS_ALIGNSIZE (sizeof (int64_t))
/* Implementation specific desired alignment */
/*
* Definitions used for sockaddr_storage structure paddings design.
*/
#define _SS_PAD1SIZE (_SS_ALIGNSIZE - sizeof (sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE - (sizeof (sa_family_t)+
_SS_PAD1SIZE + _SS_ALIGNSIZE))
struct sockaddr_storage {
sa_family_t ss_family; /* address family */
/* Following fields are implementation specific */
char _ss_pad1[_SS_PAD1SIZE];
/* 6 byte pad, this is to make implementation */
/* specific pad up to alignment field that */
/* follows explicit in the data structure */
int64_t _ss_align; /* field to force desired structure */
/* storage alignment */
char _ss_pad2[_SS_PAD2SIZE];
/* 112 byte pad to achieve desired size, */
/* _SS_MAXSIZE value minus size of ss_family */
/* __ss_pad1, __ss_align fields is 112 */
};
instead of
struct sockaddr_storage
{
union {
sa_family_t stor_family;
struct sockaddr_in s4;
struct sockaddr_in s6;
}sock_store;
#define ss_family sock_store.stor_family;
}
Is there a legitimate concern that this would not actually work as desired on some platforms due
to alignment issues when declared using a union? Doesn't ANSI C guarantee that all members
of the union are at offset zero? I seem to recall reading that someplace. And with this definition
wouldn't it guarantee alignment on any platform without platform-specific padding like the RFC
version?
Regards,
MT
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