getnameinfo - address-to-name translation in protocol-independent manner
#include <sys/socket.h> #include <netdb.h> int getnameinfo(const struct sockaddr *addr, socklen_t addrlen, char *host, socklen_t hostlen, char *serv, socklen_t servlen, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getnameinfo(): Since glibc 2.22: _POSIX_C_SOURCE >= 200112L Glibc 2.21 and earlier: _POSIX_C_SOURCE
The getnameinfo() function is the inverse of getaddrinfo(3): it converts a socket address to a corresponding host and service, in a protocol-independent manner. It combines the functionality of gethostbyaddr(3) and getservbyport(3), but unlike those functions, getnameinfo() is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependencies.
addr argument is a pointer to a generic socket address
structure (of type
addrlen that holds the input IP address and port number.
serv are pointers to
caller-allocated buffers (of size
respectively) into which getnameinfo() places
null-terminated strings containing the host and service names
The caller can specify that no hostname (or no service name) is
required by providing a NULL
or a zero
servlen) argument. However, at
least one of hostname or service name must be requested.
flags argument modifies the behavior of
getnameinfo() as follows:
If set, then an error is returned if the hostname cannot be determined.
If set, then the service is datagram (UDP) based rather than stream (TCP) based. This is required for the few ports (512–514) that have different services for UDP and TCP.
If set, return only the hostname part of the fully qualified domain name for local hosts.
If set, then the numeric form of the hostname is returned. (When not set, this will still happen in case the node's name cannot be determined.)
If set, then the numeric form of the service address is returned. (When not set, this will still happen in case the service's name cannot be determined.)
Starting with glibc 2.3.4, getnameinfo() has been
extended to selectively allow hostnames to be transparently converted to
and from the Internationalized Domain Name (IDN) format (see RFC 3490,
Internationalizing Domain Names in Applications (IDNA)). Three
new flags are defined:
If this flag is used, then the name found in the lookup process is converted from IDN format to the locale's encoding if necessary. ASCII-only names are not affected by the conversion, which makes this flag usable in existing programs and environments.
Setting these flags will enable the IDNA_ALLOW_UNASSIGNED (allow unassigned Unicode code points) and IDNA_USE_STD3_ASCII_RULES (check output to make sure it is a STD3 conforming hostname) flags respectively to be used in the IDNA handling.
On success, 0 is returned, and node and service names, if requested, are filled with null-terminated strings, possibly truncated to fit the specified buffer lengths. On error, one of the following nonzero error codes is returned:
The name could not be resolved at this time. Try again later.
flags argument has an invalid value.
A nonrecoverable error occurred.
The address family was not recognized, or the address length was invalid for the specified family.
Out of memory.
The name does not resolve for the supplied arguments. NI_NAMEREQD is set and the host's name cannot be located, or neither hostname nor service name were requested.
The buffer pointed to by
serv was too
A system error occurred. The error code can be found in
The gai_strerror(3) function translates these error codes to a human readable string, suitable for error reporting.
The following code tries to get the numeric hostname and service name, for a given socket address. Note that there is no hardcoded reference to a particular address family.
struct sockaddr *addr; /* input */ socklen_t addrlen; /* input */ char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV]; if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf), sbuf, sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0) printf("host=%s, serv=%s\n", hbuf, sbuf);
The following version checks if the socket address has a reverse address mapping.
struct sockaddr *addr; /* input */ socklen_t addrlen; /* input */ char hbuf[NI_MAXHOST]; if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf), NULL, 0, NI_NAMEREQD)) printf("could not resolve hostname"); else printf("host=%s\n", hbuf);
POSIX.1-2001, POSIX.1-2008, RFC 2553.
In order to assist the programmer in choosing reasonable sizes for
the supplied buffers,
<netdb.h> defines the constants
#define NI_MAXHOST 1025 #define NI_MAXSERV 32
Since glibc 2.8, these definitions are exposed only if suitable feature test macros are defined, namely: _GNU_SOURCE, _DEFAULT_SOURCE (since glibc 2.19), or (in glibc versions up to and including 2.19) _BSD_SOURCE or _SVID_SOURCE.
The former is the constant MAXDNAME in recent
versions of BIND's
<arpa/nameser.h> header file. The
latter is a guess based on the services listed in the current Assigned
Before glibc version 2.2, the
arguments were typed as
R. Gilligan, S. Thomson, J. Bound and W. Stevens, Basic Socket Interface Extensions for IPv6, RFC 2553, March 1999.
Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped Addresses, internet draft, work in progress ftp://ftp.ietf.org/internet-drafts/draft-ietf-ipngwg-scopedaddr-format-02.txt">.
Protocol Independence Using the Sockets API,
Proceedings of the freenix track: 2000 USENIX annual technical
conference, June 2000
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