--- name: "getaddrinfo" section: "3" platform: "macos" package: "Libinfo" version: "600" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/getaddrinfo/3" --- # getaddrinfo(3) socket address structure to host and service name ## NAME **getaddrinfo**, **freeaddrinfo** – socket address structure to host and service name ## SYNOPSIS **#include \** **#include \** **#include \** *int*\ **getaddrinfo**(*const char \*hostname*, *const char \*servname*, *const struct addrinfo \*hints*, *struct addrinfo \*\*res*); *void*\ **freeaddrinfo**(*struct addrinfo \*ai*); ## DESCRIPTION The **getaddrinfo**() function is used to get a list of IP addresses and port numbers for host *hostname* and service *servname*. It is a replacement for and provides more flexibility than the [gethostbyname(3)](/gethostbyname/3.md) and [getservbyname(3)](/getservbyname/3.md) functions. The *hostname* and *servname* arguments are either pointers to NUL-terminated strings or the null pointer. An acceptable value for *hostname* is either a valid host name or a numeric host address string consisting of a dotted decimal IPv4 address or an IPv6 address. The *servname* is either a decimal port number or a service name listed in [services(5)](/services/5.md). At least one of *hostname* and *servname* must be non-null. *hints* is an optional pointer to a struct addrinfo, as defined by ⟨*netdb.h*⟩: ``` struct addrinfo { int ai_flags; /* input flags */ int ai_family; /* protocol family for socket */ int ai_socktype; /* socket type */ int ai_protocol; /* protocol for socket */ socklen_t ai_addrlen; /* length of socket-address */ struct sockaddr *ai_addr; /* socket-address for socket */ char *ai_canonname; /* canonical name for service location */ struct addrinfo *ai_next; /* pointer to next in list */ }; ``` This structure can be used to provide hints concerning the type of socket that the caller supports or wishes to use. The caller can supply the following structure elements in *hints*: - *ai\_family* The protocol family that should be used. When *ai\_family* is set to PF\_UNSPEC, it means the caller will accept any protocol family supported by the operating system. - *ai\_socktype* Denotes the type of socket that is wanted: SOCK\_STREAM, SOCK\_DGRAM, or SOCK\_RAW. When *ai\_socktype* is zero the caller will accept any socket type. - *ai\_protocol* Indicates which transport protocol is desired, IPPROTO\_UDP or IPPROTO\_TCP. If *ai\_protocol* is zero the caller will accept any protocol. - *ai\_flags* The *ai\_flags* field to which the *hints* parameter points shall be set to zero or be the bitwise-inclusive OR of one or more of the values AI\_ADDRCONFIG, AI\_ALL, AI\_CANONNAME, AI\_NUMERICHOST, AI\_NUMERICSERV, AI\_PASSIVE, AI\_V4MAPPED, AI\_V4MAPPED\_CFG, and AI\_DEFAULT. - AI\_ADDRCONFIG If the AI\_ADDRCONFIG bit is set, IPv4 addresses shall be returned only if an IPv4 address is configured on the local system, and IPv6 addresses shall be returned only if an IPv6 address is configured on the local system. - AI\_ALL If the AI\_ALL bit is set with the AI\_V4MAPPED bit, then **getaddrinfo**() shall return all matching IPv6 and IPv4 addresses. The AI\_ALL bit without the AI\_V4MAPPED bit is ignored. - AI\_CANONNAME If the AI\_CANONNAME bit is set, a successful call to **getaddrinfo**() will return a NUL-terminated string containing the canonical name of the specified hostname in the *ai\_canonname* element of the first addrinfo structure returned. - AI\_NUMERICHOST If the AI\_NUMERICHOST bit is set, it indicates that *hostname* should be treated as a numeric string defining an IPv4 or IPv6 address and no name resolution should be attempted. - AI\_NUMERICSERV If the AI\_NUMERICSERV bit is set, then a non-null *servname* string supplied shall be a numeric port string. Otherwise, an EAI\_NONAME error shall be returned. This bit shall prevent any type of name resolution service (for example, NIS+) from being invoked. - AI\_PASSIVE If the AI\_PASSIVE bit is set it indicates that the returned socket address structure is intended for use in a call to [bind(2)](/bind/2.md). In this case, if the *hostname* argument is the 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, the returned socket address structure will be ready for use in a call to [connect(2)](/connect/2.md) for a connection-oriented protocol or [connect(2)](/connect/2.md), [sendto(2)](/sendto/2.md), or [sendmsg(2)](/sendmsg/2.md) if a connectionless protocol was chosen. The IP address portion of the socket address structure will be set to the loopback address if *hostname* is the null pointer and AI\_PASSIVE is not set. - AI\_V4MAPPED If the AI\_V4MAPPED flag is specified along with an *ai\_family* of PF\_INET6, then **getaddrinfo**() shall return IPv4-mapped IPv6 addresses on finding no matching IPv6 addresses ( *ai\_addrlen* shall be 16). The AI\_V4MAPPED flag shall be ignored unless *ai\_family* equals PF\_INET6. - AI\_V4MAPPED\_CFG The AI\_V4MAPPED\_CFG flag behaves exactly like the AI\_V4MAPPED flag if the kernel supports IPv4-mapped IPv6 addresses. Otherwise it is ignored. - AI\_DEFAULT AI\_DEFAULT is defined as ( AI\_V4MAPPED\_CFG \| AI\_ADDRCONFIG ). - AI\_UNUSABLE To override the automatic AI\_DEFAULT behavior that occurs when *ai\_flags* is zero pass AI\_UNUSABLE instead of zero. This suppresses the implicit setting of AI\_V4MAPPED\_CFG and AI\_ADDRCONFIG, thereby causing unusable addresses to be included in the results. If *ai\_flags* is zero, **getaddrinfo**() gives the AI\_DEFAULT behavior ( AI\_V4MAPPED\_CFG \| AI\_ADDRCONFIG ). To override this default behavior, pass any nonzero value for *ai\_flags*, by setting any desired flag values, or setting AI\_UNUSABLE if no other flags are desired. All other elements of the addrinfo structure passed via *hints* must be zero or the null pointer. If *hints* is the null pointer, **getaddrinfo**() behaves as if the caller provided a struct addrinfo with *ai\_family* set to PF\_UNSPEC and all other elements set to zero or NULL (which includes treating the *ai\_flags* field as effectively zero, giving the automatic default AI\_DEFAULT behavior). After a successful call to **getaddrinfo**(), *\*res* is a pointer to a linked list of one or more addrinfo structures. The list can be traversed by following the *ai\_next* pointer in each addrinfo structure until a null pointer is encountered. The three members *ai\_family,* *ai\_socktype,* and *ai\_protocol* in each returned addrinfo structure are suitable for a call to [socket(2)](/socket/2.md). For each addrinfo structure in the list, the *ai\_addr* member points to a filled-in socket address structure of length *ai\_addrlen*. This implementation of **getaddrinfo**() allows numeric IPv6 address notation with scope identifier, as documented in section 11 of RFC 4007. By appending the percent character and scope identifier to addresses, one can fill the sin6\_scope\_id field for addresses. This would make management of scoped addresses easier and allows cut-and-paste input of scoped addresses. At this moment the code supports only link-local addresses with the format. The scope identifier is hardcoded to the name of the hardware interface associated with the link( such as ne0). An example is “fe80::1%ne0”, which means“ fe80::1 on the link associated with the ne0 interface”. The current implementation assumes a one-to-one relationship between the interface and link, which is not necessarily true from the specification. All of the information returned by **getaddrinfo**() is dynamically allocated: the addrinfo structures themselves as well as the socket address structures and the canonical host name strings included in the addrinfo structures. Memory allocated for the dynamically allocated structures created by a successful call to **getaddrinfo**() is released by the **freeaddrinfo**() function. The *ai* pointer should be an addrinfo structure created by a call to **getaddrinfo**(). The current implementation supports synthesis of NAT64 mapped IPv6 addresses. If *hostname* is a numeric string defining an IPv4 address (for example, “192.0.2.1” ) and *ai\_family* is set to PF\_UNSPEC or PF\_INET6, **getaddrinfo**() will synthesize the appropriate IPv6 address(es) (for example, “64:ff9b::192.0.2.1” ) if the current interface supports IPv6, NAT64 and DNS64 and does not support IPv4. If the AI\_ADDRCONFIG flag is set, the IPv4 address will be suppressed on those interfaces. On non-qualifying interfaces, **getaddrinfo**() is guaranteed to return immediately without attempting any resolution, and will return the IPv4 address if *ai\_family* is PF\_UNSPEC or PF\_INET. NAT64 address synthesis can be disabled by setting the AI\_NUMERICHOST flag. To best support NAT64 networks, it is recommended to resolve all IP address literals with *ai\_family* set to PF\_UNSPEC and *ai\_flags* set to AI\_DEFAULT. Note that NAT64 address synthesis is always disabled for IPv4 addresses in the following ranges: 0.0.0.0/8, 127.0.0.0/8, 169.254.0.0/16, 192.0.0.0/29, 192.88.99.0/24, 224.0.0.0/4, and 255.255.255.255/32. Additionally, NAT64 address synthesis is disabled when the network uses the well-known prefix (64:ff9b::/96) for IPv4 addresses in the following ranges: 10.0.0.0/8, 100.64.0.0/10, 172.16.0.0/12, and 192.168.0.0/16. Historically, passing a host's own hostname to **getaddrinfo**() has been a popular technique for determining that host's IP address(es), but this is fragile, and doesn't work reliably in all cases. The appropriate way for software to discover the IP address(es) of the host it is running on is to use [getifaddrs(3)](/getifaddrs/3.md). The **getaddrinfo**() implementations on all versions of OS X and iOS are now, and always have been, thread-safe. Previous versions of this man page incorrectly reported that **getaddrinfo**() was not thread-safe. ## RETURN VALUES **getaddrinfo**() returns zero on success or one of the error codes listed in [gai\_strerror(3)](/gai_strerror/3.md) if an error occurs. ## EXAMPLES The following code tries to connect to “www.kame.net” service “http” via a stream socket. It loops through all the addresses available, regardless of address family. If the destination resolves to an IPv4 address, it will use an PF\_INET socket. Similarly, if it resolves to IPv6, an PF\_INET6 socket is used. Observe that there is no hardcoded reference to a particular address family. The code works even if **getaddrinfo**() returns addresses that are not IPv4/v6. ``` struct addrinfo hints, *res, *res0; int error; int s; const char *cause = NULL; memset(&hints, 0, sizeof(hints)); hints.ai_family = PF_UNSPEC; hints.ai_socktype = SOCK_STREAM; error = getaddrinfo("www.kame.net", "http", &hints, &res0); if (error) { errx(1, "%s", gai_strerror(error)); /*NOTREACHED*/ } s = -1; for (res = res0; res; res = res->ai_next) { s = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (s < 0) { cause = "socket"; continue; } if (connect(s, res->ai_addr, res->ai_addrlen) < 0) { cause = "connect"; close(s); s = -1; continue; } break; /* okay we got one */ } if (s < 0) { err(1, "%s", cause); /*NOTREACHED*/ } freeaddrinfo(res0); ``` The following example tries to open a wildcard listening socket onto service “http”, for all the address families available. ``` struct addrinfo hints, *res, *res0; int error; int s[MAXSOCK]; int nsock; const char *cause = NULL; memset(&hints, 0, sizeof(hints)); hints.ai_family = PF_UNSPEC; hints.ai_socktype = SOCK_STREAM; hints.ai_flags = AI_PASSIVE; error = getaddrinfo(NULL, "http", &hints, &res0); if (error) { errx(1, "%s", gai_strerror(error)); /*NOTREACHED*/ } nsock = 0; for (res = res0; res && nsock < MAXSOCK; res = res->ai_next) { s[nsock] = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (s[nsock] < 0) { cause = "socket"; continue; } if (bind(s[nsock], res->ai_addr, res->ai_addrlen) < 0) { cause = "bind"; close(s[nsock]); continue; } (void) listen(s[nsock], 5); nsock++; } if (nsock == 0) { err(1, "%s", cause); /*NOTREACHED*/ } freeaddrinfo(res0); ``` ## SEE ALSO [bind(2)](/bind/2.md), [connect(2)](/connect/2.md), [send(2)](/send/2.md), [socket(2)](/socket/2.md), [gai\_strerror(3)](/gai_strerror/3.md), [gethostbyname(3)](/gethostbyname/3.md), [getnameinfo(3)](/getnameinfo/3.md), [getservbyname(3)](/getservbyname/3.md), [resolver(3)](/resolver/3.md), [hosts(5)](/hosts/5.md), [resolv.conf(5)](/resolv.conf/5.md), [services(5)](/services/5.md), [hostname(7)](/hostname/7.md), [named(8)](/named/8.md) R. Gilligan, S. Thomson, J. Bound, J. McCann, and W. Stevens, *Basic Socket Interface Extensions for IPv6*, RFC 3493, February 2003. S. Deering, B. Haberman, T. Jinmei, E. Nordmark, and B. Zill, *IPv6 Scoped Address Architecture*, RFC 4007, March 2005. Craig Metz, “Protocol Independence Using the Sockets API”, *Proceedings of the freenix track: 2000 USENIX annual technical conference*, June 2000. ## STANDARDS The **getaddrinfo**() function is defined by the IEEE Std 1003.1-2004 (“POSIX.1”) specification and documented in RFC 3493, “Basic Socket Interface Extensions for IPv6”.