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>GETADDRINFO(3) System Library Functions Manual GETADDRINFO(3)
NAME
getaddrinfo, freeaddrinfo, gai_strerror - nodename-to-address translation
in protocol-independent manner
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int
getaddrinfo(const char *nodename, const char *servname,
const struct addrinfo *hints, struct addrinfo **res);
void
freeaddrinfo(struct addrinfo *ai);
char *
gai_strerror(int ecode);
DESCRIPTION
The getaddrinfo() function is defined for protocol-independent nodename-
to-address translation. It performs the functionality of
gethostbyname(3) and getservbyname(3), but in a more sophisticated man-
ner.
The addrinfo structure is defined as a result of including the <netdb.h>
header:
struct addrinfo { *
int ai_flags; /* AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST */
int ai_family; /* PF_xxx */
int ai_socktype; /* SOCK_xxx */
int ai_protocol; /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
size_t ai_addrlen; /* length of ai_addr */
char *ai_canonname; /* canonical name for nodename */
struct sockaddr *ai_addr; /* binary address */
struct addrinfo *ai_next; /* next structure in linked list */
};
The nodename and servname arguments are pointers to NUL-terminated
strings or NULL. One or both of these two arguments must be a non-null
pointer. In the normal client scenario, both the nodename and servname
are specified. In the normal server scenario, only the servname is spec-
ified. A non-null nodename string can be either a node name or a numeric
host address string (i.e., a dotted-decimal IPv4 address or an IPv6 hex
address). A non-null servname string can be either a service name or a
decimal port number.
The caller can optionally pass an addrinfo structure, pointed to by the
third argument, to provide hints concerning the type of socket that the
caller supports. In this hints structure all members other than
ai_flags, ai_family, ai_socktype, and ai_protocol must be zero or a null
pointer. A value of PF_UNSPEC for ai_family means the caller will accept
any protocol family. A value of 0 for ai_socktype means the caller will
accept any socket type. A value of 0 for ai_protocol means the caller
will accept any protocol. For example, if the caller handles only TCP
and not UDP, then the ai_socktype member of the hints structure should be
set to SOCK_STREAM when getaddrinfo() is called. If the caller handles
only IPv4 and not IPv6, then the ai_family member of the hints structure
should be set to PF_INET when getaddrinfo() is called. If the third
argument to getaddrinfo() is a null pointer, this is the same as if the
caller had filled in an addrinfo structure initialized to zero with
ai_family set to PF_UNSPEC.
Upon successful return a pointer to a linked list of one or more addrinfo
structures is returned through the final argument. The caller can pro-
cess each addrinfo structure in this list by following the ai_next
pointer, until a null pointer is encountered. In each returned addrinfo
structure the three members ai_family, ai_socktype, and ai_protocol are
the corresponding arguments for a call to the socket() function. In each
addrinfo structure the ai_addr member points to a filled-in socket
address structure whose length is specified by the ai_addrlen member.
If the AI_PASSIVE bit is set in the ai_flags member of the hints struc-
ture, then the caller plans to use the returned socket address structure
in a call to bind(). In this case, if the nodename argument is a null
pointer, then the IP address portion of the socket address structure will
be set to INADDR_ANY for an IPv4 address or IN6ADDR_ANY_INIT for an IPv6
address.
If the AI_PASSIVE bit is not set in the ai_flags member of the hints
structure, then the returned socket address structure will be ready for a
call to connect() (for a connection-oriented protocol) or either
connect(), sendto(), or sendmsg() (for a connectionless protocol). In
this case, if the nodename argument is a null pointer, then the IP
address portion of the socket address structure will be set to the loop-
back address.
If the AI_CANONNAME bit is set in the ai_flags member of the hints struc-
ture, then upon successful return the ai_canonname member of the first
addrinfo structure in the linked list will point to a NUL-terminated
string containing the canonical name of the specified nodename.
If the AI_NUMERICHOST bit is set in the ai_flags member of the hints
structure, then a non-null nodename string must be a numeric host address
string. Otherwise an error of EAI_NONAME is returned. This flag pre-
vents any type of name resolution service (e.g., the DNS) from being
called.
The arguments to getaddrinfo() must sufficiently be consistent and unam-
biguous. Here are pitfall cases you may encounter:
o getaddrinfo() will raise an error if members of the hints structure
are not consistent. For example, for internet address families,
getaddrinfo() will raise an error if you specify SOCK_STREAM to
ai_socktype while you specify IPPROTO_UDP to ai_protocol.
o If you specify a servname which is defined only for certain
ai_socktype, getaddrinfo() will raise an error because the arguments
are not consistent. For example, getaddrinfo() will raise an error
if you ask for ``tftp'' service on SOCK_STREAM.
o For internet address families, if you specify servname while you set
ai_socktype to SOCK_RAW, getaddrinfo() will raise an error, because
service names are not defined for the internet SOCK_RAW space.
o If you specify a numeric servname, while leaving ai_socktype and
ai_protocol unspecified, getaddrinfo() will raise an error. This is
because the numeric servname does not identify any socket type, and
getaddrinfo() is not allowed to glob the argument in such case.
All of the information returned by getaddrinfo() is dynamically allo-
cated: the addrinfo structures, the socket address structures, and canon-
ical node name strings pointed to by the addrinfo structures. To return
this information to the system the function freeaddrinfo() is called.
The addrinfo structure pointed to by the ai argument is freed, along with
any dynamic storage pointed to by the structure. This operation is
repeated until a NULL ai_next pointer is encountered.
To aid applications in printing error messages based on the EAI_xxx codes
returned by getaddrinfo(), gai_strerror() is defined. The argument is
one of the EAI_xxx values defined earlier and the return value points to
a string describing the error. If the argument is not one of the EAI_xxx
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