char *
inet_ntoa(struct in_addr in);
inet_ntoa() has two deficiencies, one important
and one trivial: it doesn't support IPv6 and it returns a
pointer to a statically allocated buffer, so it's not thread
safe (I'll let you figure out which is which). Luckily, there's another API: addr2ascii():
char *
addr2ascii(int af, const void *addrp, int len, char *buf);
If you pass buf=0,
addr2ascii() will return a pointer to a static
buffer like inet_ntoa(). However, if you pass it
an allocated buffer it will return the result in
buf. Unfortunately, if you actually try to
use addr2ascii() in threaded code you will
quickly discover something unpleasant, at least on FreeBSD: you occasionally
get the result "[inet_ntoa error]" or some fraction thereof.
The answer is hidden in the
EXAMPLES section of the man page:
In actuality, this cannot be done because addr2ascii() and ascii2addr() are implemented in terms of the inet(3) functions, rather than the other way around.
More specifically, on FreeBSD, it looks like this:
case AF_INET:
if (len != sizeof(struct in_addr)) {
errno = ENAMETOOLONG;
return 0;
}
strcpy(buf, inet_ntoa(*(const struct in_addr *)addrp));
break;
In other words, even though addr2ascii() doesn't
explicitly use a static buffer, since it depends on
inet_ntoa() it's still not thread safe.
In order to get thread safety, you need to use yet another
API:
const char *
inet_ntop(int af, const void *restrict src, char *restrict dst,
socklen_t size);
Outstanding!
UPDATE: Clarified that this is a problem on FreeBSD. I don't know if it's an
issue on all other platforms. Linux, for instance, doesn't have addr2ascii()
UPDATE2: Trivial vs. important.
