inet_ntoa / inet_lnaof / inet_makeaddr / inet_netof / inet_network (3) - glibc

Split from the chapter glibc: Sockets into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <arpa/inet.h>

char *inet_ntoa(struct in_addr in);                    /* line 53 */
in_addr_t inet_lnaof(struct in_addr in);               /* line 37 */
struct in_addr inet_makeaddr(in_addr_t net,
                             in_addr_t host);          /* lines 41-42 */
in_addr_t inet_netof(struct in_addr in);               /* line 45 */
in_addr_t inet_network(const char *cp);                /* line 49 */

All unconditional in /usr/include/arpa/inet.h (line numbers above; typedef uint32_t in_addr_t; wraps struct in_addr { in_addr_t s_addr; } from netinet/in.h).

Semantics

The pre-CIDR helpers. inet_network parses a numbers-and-dots string into a number in host byte order suitable as a network address, returning -1 for invalid input (inet.3); unlike inet_addr it does not return network order, and -1 is its failure value. inet_ntoa renders a network-order in_addr as dotted decimal in a statically allocated buffer, overwritten by each call – per-thread in glibc, but still shared across calls in one thread (manual, Host Address Functions). inet_netof extracts the network number and inet_lnaof the local part of a host address, both in host byte order, both applying the classful A/B/C masks from the leading bits; inet_makeaddr is their converse, rebuilding a network-order host address from network number plus local part (inet.3).

Gotchas and portability

• The classful trio “works only with traditional IPv4 class A, B and C network types. It doesn’t work with classless addresses and shouldn’t be used anymore” – the manual’s exact verdict, repeated on all three (socket.texi, Host Address Functions).
• inet_ntoa is MT-Safe only in the per-thread-buffer sense; its return is still invalidated by the next call in the same thread – inet_ntop with a caller buffer is the replacement.
• musl 1.2.5: all five present (src/network/inet_ntoa.c, inet_legacy.c; note inet_network there is just ntohl(inet_addr(p)), which keeps the same host-order contract for the common forms).

References

• glibc manual: socket.texi, nodes Host Address Functions, Abstract Host Addresses, Byte Order
• man3: inet.3 (covers the five), inet_addr.3 (inet_makeaddr link target)
• header: /usr/include/arpa/inet.h
• symbols: nm -D --with-symbol-versions libc.so.6