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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |FILES |ATTRIBUTES |VERSIONS |STANDARDS |HISTORY |NOTES |EXAMPLES |SEE ALSO |COLOPHON | |
getaddrinfo(3) Library Functions Manualgetaddrinfo(3)getaddrinfo, freeaddrinfo, gai_strerror - network address and service translation
Standard C library (libc,-lc)
#include <sys/types.h>#include <sys/socket.h>#include <netdb.h>int getaddrinfo(const char *restrictnode,const char *restrictservice,const struct addrinfo *restricthints,struct addrinfo **restrictres);void freeaddrinfo(struct addrinfo *res);const char *gai_strerror(interrcode); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):getaddrinfo(),freeaddrinfo(),gai_strerror(): Since glibc 2.22: _POSIX_C_SOURCE >= 200112L glibc 2.21 and earlier: _POSIX_C_SOURCE
Givennode andservice, which identify an Internet host and a service,getaddrinfo() returns one or moreaddrinfo structures, each of which contains an Internet address that can be specified in a call tobind(2) orconnect(2). Thegetaddrinfo() function combines the functionality provided by thegethostbyname(3) andgetservbyname(3) functions into a single interface, but unlike the latter functions,getaddrinfo() is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependencies. Theaddrinfo structure used bygetaddrinfo() contains the following fields: struct addrinfo { int ai_flags; int ai_family; int ai_socktype; int ai_protocol; socklen_t ai_addrlen; struct sockaddr *ai_addr; char *ai_canonname; struct addrinfo *ai_next; }; Thehints argument points to anaddrinfo structure that specifies criteria for selecting the socket address structures returned in the list pointed to byres. Ifhints is not NULL it points to anaddrinfo structure whoseai_family,ai_socktype, andai_protocol specify criteria that limit the set of socket addresses returned bygetaddrinfo(), as follows:ai_family This field specifies the desired address family for the returned addresses. Valid values for this field includeAF_INETandAF_INET6. The valueAF_UNSPECindicates thatgetaddrinfo() should return socket addresses for any address family (either IPv4 or IPv6, for example) that can be used withnode andservice.ai_socktype This field specifies the preferred socket type, for exampleSOCK_STREAMorSOCK_DGRAM. Specifying 0 in this field indicates that socket addresses of any type can be returned bygetaddrinfo().ai_protocol This field specifies the protocol for the returned socket addresses. Specifying 0 in this field indicates that socket addresses with any protocol can be returned bygetaddrinfo().ai_flags This field specifies additional options, described below. Multiple flags are specified by bitwise OR-ing them together. All the other fields in the structure pointed to byhints must contain either 0 or a null pointer, as appropriate. Specifyinghints as NULL is equivalent to settingai_socktype andai_protocol to 0;ai_family toAF_UNSPEC; andai_flags to(AI_V4MAPPED | AI_ADDRCONFIG). (POSIX specifies different defaults forai_flags; see NOTES.)node specifies either a numerical network address (for IPv4, numbers-and-dots notation as supported byinet_aton(3); for IPv6, hexadecimal string format as supported byinet_pton(3)), or a network hostname, whose network addresses are looked up and resolved. Ifhints.ai_flags contains theAI_NUMERICHOSTflag, thennode must be a numerical network address. TheAI_NUMERICHOSTflag suppresses any potentially lengthy network host address lookups. If theAI_PASSIVEflag is specified inhints.ai_flags, andnode is NULL, then the returned socket addresses will be suitable forbind(2)ing a socket that willaccept(2) connections. The returned socket address will contain the "wildcard address" (INADDR_ANYfor IPv4 addresses,IN6ADDR_ANY_INITfor IPv6 address). The wildcard address is used by applications (typically servers) that intend to accept connections on any of the host's network addresses. Ifnode is not NULL, then theAI_PASSIVEflag is ignored. If theAI_PASSIVEflag is not set inhints.ai_flags, then the returned socket addresses will be suitable for use withconnect(2),sendto(2), orsendmsg(2). Ifnode is NULL, then the network address will be set to the loopback interface address (INADDR_LOOPBACKfor IPv4 addresses,IN6ADDR_LOOPBACK_INITfor IPv6 address); this is used by applications that intend to communicate with peers running on the same host.service sets the port in each returned address structure. If this argument is a service name (seeservices(5)), it is translated to the corresponding port number. This argument can also be specified as a decimal number, which is simply converted to binary. Ifservice is NULL, then the port number of the returned socket addresses will be left uninitialized. IfAI_NUMERICSERVis specified inhints.ai_flags andservice is not NULL, thenservice must point to a string containing a numeric port number. This flag is used to inhibit the invocation of a name resolution service in cases where it is known not to be required. Eithernode orservice, but not both, may be NULL. Thegetaddrinfo() function allocates and initializes a linked list ofaddrinfo structures, one for each network address that matchesnode andservice, subject to any restrictions imposed byhints, and returns a pointer to the start of the list inres. The items in the linked list are linked by theai_next field. There are several reasons why the linked list may have more than oneaddrinfo structure, including: the network host is multihomed, accessible over multiple protocols (e.g., bothAF_INETandAF_INET6); or the same service is available from multiple socket types (oneSOCK_STREAMaddress and anotherSOCK_DGRAMaddress, for example). Normally, the application should try using the addresses in the order in which they are returned. The sorting function used withingetaddrinfo() is defined in RFC 3484; the order can be tweaked for a particular system by editing/etc/gai.conf (available since glibc 2.5). Ifhints.ai_flags includes theAI_CANONNAMEflag, then theai_canonname field of the first of theaddrinfo structures in the returned list is set to point to the official name of the host. The remaining fields of each returnedaddrinfo structure are initialized as follows: • Theai_family,ai_socktype, andai_protocol fields return the socket creation parameters (i.e., these fields have the same meaning as the corresponding arguments ofsocket(2)). For example,ai_family might returnAF_INETorAF_INET6;ai_socktype might returnSOCK_DGRAMorSOCK_STREAM; andai_protocol returns the protocol for the socket. • A pointer to the socket address is placed in theai_addr field, and the size of the socket address, in bytes, is placed in theai_addrlen field. Ifhints.ai_flags includes theAI_ADDRCONFIGflag, then IPv4 addresses are returned in the list pointed to byres only if the local system has at least one IPv4 address configured, and IPv6 addresses are returned only if the local system has at least one IPv6 address configured. The loopback address is not considered for this case as valid as a configured address. This flag is useful on, for example, IPv4-only systems, to ensure thatgetaddrinfo() does not return IPv6 socket addresses that would always fail inconnect(2) orbind(2). Ifhints.ai_flags specifies theAI_V4MAPPEDflag, andhints.ai_family was specified asAF_INET6, and no matching IPv6 addresses could be found, then return IPv4-mapped IPv6 addresses in the list pointed to byres. If bothAI_V4MAPPEDandAI_ALLare specified inhints.ai_flags, then return both IPv6 and IPv4-mapped IPv6 addresses in the list pointed to byres.AI_ALLis ignored ifAI_V4MAPPEDis not also specified. Thefreeaddrinfo() function frees the memory that was allocated for the dynamically allocated linked listres.Extensions to getaddrinfo() for Internationalized Domain Names Starting with glibc 2.3.4,getaddrinfo() has been extended to selectively allow the incoming and outgoing hostnames to be transparently converted to and from the Internationalized Domain Name (IDN) format (see RFC 3490,Internationalizing Domain Namesin Applications (IDNA)). Four new flags are defined:AI_IDNIf this flag is specified, then the node name given innode is converted to IDN format if necessary. The source encoding is that of the current locale. If the input name contains non-ASCII characters, then the IDN encoding is used. Those parts of the node name (delimited by dots) that contain non-ASCII characters are encoded using ASCII Compatible Encoding (ACE) before being passed to the name resolution functions.AI_CANONIDN After a successful name lookup, and if theAI_CANONNAME flag was specified,getaddrinfo() will return the canonical name of the node corresponding to theaddrinfo structure value passed back. The return value is an exact copy of the value returned by the name resolution function. If the name is encoded using ACE, then it will contain thexn-- prefix for one or more components of the name. To convert these components into a readable form theAI_CANONIDNflag can be passed in addition toAI_CANONNAME. The resulting string is encoded using the current locale's encoding.AI_IDN_ALLOW_UNASSIGNEDAI_IDN_USE_STD3_ASCII_RULES 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.getaddrinfo() returns 0 if it succeeds, or one of the following nonzero error codes:EAI_ADDRFAMILY The specified network host does not have any network addresses in the requested address family.EAI_AGAIN The name server returned a temporary failure indication. Try again later.EAI_BADFLAGShints.ai_flags contains invalid flags; or,hints.ai_flags includedAI_CANONNAMEandnode was NULL.EAI_FAIL The name server returned a permanent failure indication.EAI_FAMILY The requested address family is not supported.EAI_MEMORY Out of memory.EAI_NODATA The specified network host exists, but does not have any network addresses defined.EAI_NONAME Thenode orservice is not known; or bothnode andservice are NULL; orAI_NUMERICSERVwas specified inhints.ai_flags andservice was not a numeric port-number string.EAI_SERVICE The requested service is not available for the requested socket type. It may be available through another socket type. For example, this error could occur ifservice was "shell" (a service available only on stream sockets), and eitherhints.ai_protocol wasIPPROTO_UDP, orhints.ai_socktype wasSOCK_DGRAM; or the error could occur ifservice was not NULL, andhints.ai_socktype wasSOCK_RAW (a socket type that does not support the concept of services).EAI_SOCKTYPE The requested socket type is not supported. This could occur, for example, ifhints.ai_socktype andhints.ai_protocol are inconsistent (e.g.,SOCK_DGRAMandIPPROTO_TCP, respectively).EAI_SYSTEM Other system error;errno is set to indicate the error. Thegai_strerror() function translates these error codes to a human readable string, suitable for error reporting.
/etc/gai.conf
For an explanation of the terms used in this section, seeattributes(7). ┌───────────────────────────┬───────────────┬────────────────────┐ │Interface│Attribute│Value│ ├───────────────────────────┼───────────────┼────────────────────┤ │getaddrinfo() │ Thread safety │ MT-Safe env locale │ ├───────────────────────────┼───────────────┼────────────────────┤ │freeaddrinfo(), │ Thread safety │ MT-Safe │ │gai_strerror() │ │ │ └───────────────────────────┴───────────────┴────────────────────┘
According to POSIX.1, specifyinghints as NULL should causeai_flags to be assumed as 0. The GNU C library instead assumes a value of(AI_V4MAPPED | AI_ADDRCONFIG)for this case, since this value is considered an improvement on the specification.
POSIX.1-2008.getaddrinfo() RFC 2553.
POSIX.1-2001.AI_ADDRCONFIGAI_ALLAI_V4MAPPED glibc 2.3.3.AI_NUMERICSERV glibc 2.3.4.
getaddrinfo() supports theaddress%scope-id notation for specifying the IPv6 scope-ID.
The following programs demonstrate the use ofgetaddrinfo(),gai_strerror(),freeaddrinfo(), andgetnameinfo(3). The programs are an echo server and client for UDP datagrams.Server program #include <netdb.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/socket.h> #include <sys/types.h> #include <unistd.h> #define BUF_SIZE 500 int main(int argc, char *argv[]) { int sfd, s; char buf[BUF_SIZE]; ssize_t nread; socklen_t peer_addrlen; struct addrinfo hints; struct addrinfo *result, *rp; struct sockaddr_storage peer_addr; if (argc != 2) { fprintf(stderr, "Usage: %s port\n", argv[0]); exit(EXIT_FAILURE); } memset(&hints, 0, sizeof(hints)); hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */ hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */ hints.ai_flags = AI_PASSIVE; /* For wildcard IP address */ hints.ai_protocol = 0; /* Any protocol */ hints.ai_canonname = NULL; hints.ai_addr = NULL; hints.ai_next = NULL; s = getaddrinfo(NULL, argv[1], &hints, &result); if (s != 0) { fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s)); exit(EXIT_FAILURE); } /* getaddrinfo() returns a list of address structures. Try each address until we successfully bind(2). If socket(2) (or bind(2)) fails, we (close the socket and) try the next address. */ for (rp = result; rp != NULL; rp = rp->ai_next) { sfd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol); if (sfd == -1) continue; if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0) break; /* Success */ close(sfd); } freeaddrinfo(result); /* No longer needed */ if (rp == NULL) { /* No address succeeded */ fprintf(stderr, "Could not bind\n"); exit(EXIT_FAILURE); } /* Read datagrams and echo them back to sender. */ for (;;) { char host[NI_MAXHOST], service[NI_MAXSERV]; peer_addrlen = sizeof(peer_addr); nread = recvfrom(sfd, buf, BUF_SIZE, 0, (struct sockaddr *) &peer_addr, &peer_addrlen); if (nread == -1) continue; /* Ignore failed request */ s = getnameinfo((struct sockaddr *) &peer_addr, peer_addrlen, host, NI_MAXHOST, service, NI_MAXSERV, NI_NUMERICSERV); if (s == 0) printf("Received %zd bytes from %s:%s\n", nread, host, service); else fprintf(stderr, "getnameinfo: %s\n", gai_strerror(s)); if (sendto(sfd, buf, nread, 0, (struct sockaddr *) &peer_addr, peer_addrlen) != nread) { fprintf(stderr, "Error sending response\n"); } } }Client program #include <netdb.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/socket.h> #include <sys/types.h> #include <unistd.h> #define BUF_SIZE 500 int main(int argc, char *argv[]) { int sfd, s; char buf[BUF_SIZE]; size_t size; ssize_t nread; struct addrinfo hints; struct addrinfo *result, *rp; if (argc < 3) { fprintf(stderr, "Usage: %s host port msg...\n", argv[0]); exit(EXIT_FAILURE); } /* Obtain address(es) matching host/port. */ memset(&hints, 0, sizeof(hints)); hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */ hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */ hints.ai_flags = 0; hints.ai_protocol = 0; /* Any protocol */ s = getaddrinfo(argv[1], argv[2], &hints, &result); if (s != 0) { fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s)); exit(EXIT_FAILURE); } /* getaddrinfo() returns a list of address structures. Try each address until we successfully connect(2). If socket(2) (or connect(2)) fails, we (close the socket and) try the next address. */ for (rp = result; rp != NULL; rp = rp->ai_next) { sfd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol); if (sfd == -1) continue; if (connect(sfd, rp->ai_addr, rp->ai_addrlen) != -1) break; /* Success */ close(sfd); } freeaddrinfo(result); /* No longer needed */ if (rp == NULL) { /* No address succeeded */ fprintf(stderr, "Could not connect\n"); exit(EXIT_FAILURE); } /* Send remaining command-line arguments as separate datagrams, and read responses from server. */ for (size_t j = 3; j < argc; j++) { size = strlen(argv[j]) + 1; /* +1 for terminating null byte */ if (size > BUF_SIZE) { fprintf(stderr, "Ignoring long message in argument %zu\n", j); continue; } if (write(sfd, argv[j], size) != size) { fprintf(stderr, "partial/failed write\n"); exit(EXIT_FAILURE); } nread = read(sfd, buf, BUF_SIZE); if (nread == -1) { perror("read"); exit(EXIT_FAILURE); } printf("Received %zd bytes: %s\n", nread, buf); } exit(EXIT_SUCCESS); }getaddrinfo_a(3),gethostbyname(3),getnameinfo(3),inet(3),gai.conf(5),hostname(7),ip(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17getaddrinfo(3)Pages that refer to this page:getent(1), pmdanetcheck(1), bind(2), connect(2), recv(2), send(2), socket(2), getaddrinfo_a(3), gethostbyname(3), getipnodebyname(3), getnameinfo(3), inet(3), inet_pton(3), NULL(3const), resolver(3), sockaddr(3type), gai.conf(5), resolv.conf(5), hostname(7), agetty(8), systemd-machined.service(8), systemd-resolved.service(8)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |