getnetent / getnetbyaddr / getnetbyname / setnetent / endnetent (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 <netdb.h>

struct netent *getnetent(void);
struct netent *getnetbyname(const char *name);
struct netent *getnetbyaddr(uint32_t net, int type);
void setnetent(int stayopen);
void endnetent(void);

Unconditional in /usr/include/netdb.h: setnetent line 196, endnetent 202, getnetent 209, getnetbyaddr 216, getnetbyname 222. struct netent itself lives in the included bits/netdb.h (lines 26-32 on this host) with n_name, n_aliases, n_addrtype, n_net. The reentrant getnetent_r/getnetbyaddr_r twins (decls at netdb.h lines 235-249, __USE_MISC) are separate rows and not covered by this page’s IDs.

Semantics

The networks database – traditionally /etc/networks, nowadays the NSS networks service – maps network names to network numbers, which the manual calls “useful for routing programs such as route, but not useful for programs that simply communicate over the network” (manual, Networks Database). getnetent reads successive entries, opening the database when needed; getnetbyname and getnetbyaddr do keyed lookups (the latter taking the network number in host byte order and an address type, for which AF_INET is the value to pass). setnetent opens/rewinds, keeping the database open between calls when stayopen is nonzero; endnetent closes it. All five return pointers to a statically allocated struct netent, or NULL on error or end of data (getnetent(3)).

Gotchas and portability

• Byte-order asymmetry across the database family: n_net (and the getnetbyaddr argument) are host byte order, while servent.s_port is network order – a recurring interview trap.
• The manual warns the iterators “are not reentrant” and the result must be copied before the next call.
• musl 1.2.5: the whole family is absent – no struct netent in include/netdb.h, no sources under src/network/ (grep for netent returns nothing).
• Related: getaddrinfo page for the modern resolver path; sibling compact pages getprotobyname-getservbyport.

References

• glibc manual: socket.texi, node Networks Database
• man3: getnetent.3 (covers the family), networks(5)
• header: /usr/include/netdb.h
• symbols: nm -D --with-symbol-versions libc.so.6