--- name: "freeaddrinfo" section: "3p" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/freeaddrinfo/3p" --- # freeaddrinfo(3p) get address information ## PROLOG This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. ## NAME freeaddrinfo, getaddrinfo — get address information ## SYNOPSIS ``` #include #include ``` void freeaddrinfo(struct addrinfo \**ai*); int getaddrinfo(const char \*restrict *nodename*,\ const char \*restrict *servname*,\ const struct addrinfo \*restrict *hints*,\ struct addrinfo \*\*restrict *res*); ## DESCRIPTION The *freeaddrinfo*() function shall free one or more **addrinfo** structures returned by *getaddrinfo*(), along with any additional storage associated with those structures. If the *ai\_next* field of the structure is not null, the entire list of structures shall be freed. The *freeaddrinfo*() function shall support the freeing of arbitrary sublists of an **addrinfo** list originally returned by *getaddrinfo*(). The *getaddrinfo*() function shall translate the name of a service location (for example, a host name) and/or a service name and shall return a set of socket addresses and associated information to be used in creating a socket with which to address the specified service. - **Note:** In many cases it is implemented by the Domain Name System, as documented in RFC 1034, RFC 1035, and RFC 1886. The *freeaddrinfo*() and *getaddrinfo*() functions shall be thread-safe. The *nodename* and *servname* arguments are either null pointers or pointers to null-terminated strings. One or both of these two arguments shall be supplied by the application as a non-null pointer. The format of a valid name depends on the address family or families. If a specific family is not given and the name could be interpreted as valid within multiple supported families, the implementation shall attempt to resolve the name in all supported families and, in absence of errors, one or more results shall be returned. If the *nodename* argument is not null, it can be a descriptive name or can be an address string. If the specified address family is AF\_INET, AF\_INET6, or AF\_UNSPEC, valid descriptive names include host names. If the specified address family is AF\_INET or AF\_UNSPEC, address strings using Internet standard dot notation as specified in *inet\_addr()* are valid. If the specified address family is AF\_INET6 or AF\_UNSPEC, standard IPv6 text forms described in *inet\_ntop()* are valid. If *nodename* is not null, the requested service location is named by *nodename*; otherwise, the requested service location is local to the caller. If *servname* is null, the call shall return network-level addresses for the specified *nodename.* If *servname* is not null, it is a null-terminated character string identifying the requested service. This can be either a descriptive name or a numeric representation suitable for use with the address family or families. If the specified address family is AF\_INET, AF\_INET6, or AF\_UNSPEC, the service can be specified as a string specifying a decimal port number. If the *hints* argument is not null, it refers to a structure containing input values that directs the operation by providing options and by limiting the returned information to a specific socket type, address family, and/or protocol, as described below. The application shall ensure that each of the *ai\_addrlen*, *ai\_addr*, *ai\_canonname*, and *ai\_next* members, as well as each of the non-standard additional members, if any, of this *hints* structure is initialized. If any of these members has a value other than the value that would result from default initialization, the behavior is implementation-defined. A value of AF\_UNSPEC for *ai\_family* means that the caller shall accept any address family. A value of zero for *ai\_socktype* means that the caller shall accept any socket type. A value of zero for *ai\_protocol* means that the caller shall accept any protocol. If *hints* is a null pointer, the behavior shall be as if it referred to a structure containing the value zero for the *ai\_flags*, *ai\_socktype*, and *ai\_protocol* fields, and AF\_UNSPEC for the *ai\_family* field. 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\_PASSIVE, AI\_CANONNAME, AI\_NUMERICHOST, AI\_NUMERICSERV, AI\_V4MAPPED, AI\_ALL, and AI\_ADDRCONFIG. If the AI\_PASSIVE flag is specified, the returned address information shall be suitable for use in binding a socket for accepting incoming connections for the specified service. In this case, if the *nodename* argument is null, then the IP address portion of the socket address structure shall be set to INADDR\_ANY for an IPv4 address or IN6ADDR\_ANY\_INIT for an IPv6 address. If the AI\_PASSIVE flag is not specified, the returned address information shall be suitable for a call to *connect*() (for a connection-mode protocol) or for a call to *connect*(), *sendto*(), or *sendmsg*() (for a connectionless protocol). In this case, if the *nodename* argument is null, then the IP address portion of the socket address structure shall be set to the loopback address. The AI\_PASSIVE flag shall be ignored if the *nodename* argument is not null. If the AI\_CANONNAME flag is specified and the *nodename* argument is not null, the function shall attempt to determine the canonical name corresponding to *nodename* (for example, if *nodename* is an alias or shorthand notation for a complete name). - **Note:** Since different implementations use different conceptual models, the terms \`\`canonical name'' and \`\`alias'' cannot be precisely defined for the general case. However, Domain Name System implementations are expected to interpret them as they are used in RFC 1034. A numeric host address string is not a \`\`name'', and thus does not have a \`\`canonical name'' form; no address to host name translation is performed. See below for handling of the case where a canonical name cannot be obtained. If the AI\_NUMERICHOST flag is specified, then a non-null *nodename* string supplied shall be a numeric host address string. Otherwise, an **\[EAI\_NONAME\]** error is returned. This flag shall prevent any type of name resolution service (for example, the DNS) from being invoked. If the AI\_NUMERICSERV flag is specified, then a non-null *servname* string supplied shall be a numeric port string. Otherwise, an **\[EAI\_NONAME\]** error shall be returned. This flag shall prevent any type of name resolution service (for example, NIS+) from being invoked. By default, with an *ai\_family* of AF\_INET6, *getaddrinfo*() shall return only IPv6 addresses. If the AI\_V4MAPPED flag is specified along with an *ai\_family* of AF\_INET6, then *getaddrinfo*() shall return IPv4-mapped IPv6 addresses on finding no matching IPv6 addresses. The AI\_V4MAPPED flag shall be ignored unless *ai\_family* equals AF\_INET6. If the AI\_ALL flag is used with the AI\_V4MAPPED flag, then *getaddrinfo*() shall return all matching IPv6 and IPv4 addresses. The AI\_ALL flag without the AI\_V4MAPPED flag shall be ignored. If the AI\_ADDRCONFIG flag is specified, 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. The *ai\_socktype* field to which argument *hints* points specifies the socket type for the service, as defined in *socket()*. If a specific socket type is not given (for example, a value of zero) and the service name could be interpreted as valid with multiple supported socket types, the implementation shall attempt to resolve the service name for all supported socket types and, in the absence of errors, all possible results shall be returned. A non-zero socket type value shall limit the returned information to values with the specified socket type. If the *ai\_family* field to which *hints* points has the value AF\_UNSPEC, addresses shall be returned for use with any address family that can be used with the specified *nodename* and/or *servname*. Otherwise, addresses shall be returned for use only with the specified address family. If *ai\_family* is not AF\_UNSPEC and *ai\_protocol* is not zero, then addresses shall be returned for use only with the specified address family and protocol; the value of *ai\_protocol* shall be interpreted as in a call to the *socket*() function with the corresponding values of *ai\_family* and *ai\_protocol*. ## RETURN VALUE A zero return value for *getaddrinfo*() indicates successful completion; a non-zero return value indicates failure. The possible values for the failures are listed in the ERRORS section. Upon successful return of *getaddrinfo*(), the location to which *res* points shall refer to a linked list of **addrinfo** structures, each of which shall specify a socket address and information for use in creating a socket with which to use that socket address. The list shall include at least one **addrinfo** structure. The *ai\_next* field of each structure contains a pointer to the next structure on the list, or a null pointer if it is the last structure on the list. Each structure on the list shall include values for use with a call to the *socket*() function, and a socket address for use with the *connect*() function or, if the AI\_PASSIVE flag was specified, for use with the *bind*() function. The fields *ai\_family*, *ai\_socktype*, and *ai\_protocol* shall be usable as the arguments to the *socket*() function to create a socket suitable for use with the returned address. The fields *ai\_addr* and *ai\_addrlen* are usable as the arguments to the *connect*() or *bind*() functions with such a socket, according to the AI\_PASSIVE flag. If *nodename* is not null, and if requested by the AI\_CANONNAME flag, the *ai\_canonname* field of the first returned **addrinfo** structure shall point to a null-terminated string containing the canonical name corresponding to the input *nodename*; if the canonical name is not available, then *ai\_canonname* shall refer to the *nodename* argument or a string with the same contents. The contents of the *ai\_flags* field of the returned structures are undefined. All fields in socket address structures returned by *getaddrinfo*() that are not filled in through an explicit argument (for example, *sin6\_flowinfo*) shall be set to zero. - **Note:** This makes it easier to compare socket address structures. ## ERRORS The *getaddrinfo*() function shall fail and return the corresponding error value if: - \[EAI\_AGAIN\] The name could not be resolved at this time. Future attempts may succeed. - \[EAI\_BADFLAGS\] The *flags* parameter had an invalid value. - \[EAI\_FAIL\] A non-recoverable error occurred when attempting to resolve the name. - \[EAI\_FAMILY\] The address family was not recognized. - \[EAI\_MEMORY\] There was a memory allocation failure when trying to allocate storage for the return value. - \[EAI\_NONAME\] The name does not resolve for the supplied parameters. Neither *nodename* nor *servname* were supplied. At least one of these shall be supplied. - \[EAI\_SERVICE\] The service passed was not recognized for the specified socket type. - \[EAI\_SOCKTYPE\] The intended socket type was not recognized. - \[EAI\_SYSTEM\] A system error occurred; the error code can be found in *errno*. *The following sections are informative.* ## EXAMPLES The following (incomplete) program demonstrates the use of *getaddrinfo*() to obtain the socket address structure(s) for the service named in the program's command-line argument. The program then loops through each of the address structures attempting to create and bind a socket to the address, until it performs a successful *bind*(). ``` #include #include #include #include #include #include ``` int main(int argc, char \*argv\[\]) {\ struct addrinfo \*result, \*rp;\ int sfd, s; if (argc != 2) {\ fprintf(stderr, "Usage: %s port\\n", argv\[0\]);\ exit(EXIT\_FAILURE);\ } struct addrinfo hints = {0};\ hints.ai\_family = AF\_UNSPEC;\ hints.ai\_socktype = SOCK\_DGRAM;\ hints.ai\_flags = AI\_PASSIVE;\ hints.ai\_protocol = 0; 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 a successful bind().\ If [socket(2)](/socket/2.md) (or [bind(2)](/bind/2.md)) fails, 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);\ } if (rp == NULL) { /\* No address succeeded \*/\ fprintf(stderr, "Could not bind\\n");\ exit(EXIT\_FAILURE);\ } freeaddrinfo(result); /\* No longer needed \*/ /\* ... use socket bound to sfd ... \*/ } ## APPLICATION USAGE If the caller handles only TCP and not UDP, for example, then the *ai\_protocol* member of the *hints* structure should be set to IPPROTO\_TCP 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 AF\_INET when *getaddrinfo*() is called. Although it is common practice to initialize the *hints* structure using: ``` struct addrinfo hints; memset(&hints, 0, sizeof hints); ``` this method is not portable according to this standard, because the structure can contain pointer or floating-point members that are not required to have an all-bits-zero representation after default initialization. Portable methods make use of default initialization; for example: ``` struct addrinfo hints = { 0 }; ``` or: ``` static struct addrinfo hints_init; struct addrinfo hints = hints_init; ``` A future version of this standard may require that a pointer object with an all-bits-zero representation is a null pointer, and that **addrinfo** does not have any floating-point members if a floating-point object with an all-bits-zero representation does not have the value 0.0. The term \`\`canonical name'' is misleading; it is taken from the Domain Name System (RFC 2181). It should be noted that the canonical name is a result of alias processing, and not necessarily a unique attribute of a host, address, or set of addresses. See RFC 2181 for more discussion of this in the Domain Name System context. ## RATIONALE None. ## FUTURE DIRECTIONS None. ## SEE ALSO *connect()*, *endservent()*, *gai\_strerror()*, *getnameinfo()*, *socket()* The Base Definitions volume of POSIX.1‐2017, ***\***, ***\*** ## COPYRIGHT Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting\_bugs.html .