--- name: "netinet_in.h" section: "0p" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/netinet_in.h" --- # netinet\_in.h(0p) Internet address family ## 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 netinet/in.h — Internet address family ## SYNOPSIS ``` #include ``` ## DESCRIPTION The *\* header shall define the following types: - **in\_port\_t** Equivalent to the type **uint16\_t** as described in *\*. - **in\_addr\_t** Equivalent to the type **uint32\_t** as described in *\*. The *\* header shall define the **sa\_family\_t** type as described in *\*. The *\* header shall define the **uint8\_t** and **uint32\_t** types as described in *\*. Inclusion of the *\* header may also make visible all symbols from *\* and *\*. The *\* header shall define the **in\_addr** structure, which shall include at least the following member: ``` in_addr_t s_addr ``` The *\* header shall define the **sockaddr\_in** structure, which shall include at least the following members: ``` sa_family_t sin_family AF_INET. in_port_t sin_port Port number. struct in_addr sin_addr IP address. ``` The *sin\_port* and *sin\_addr* members shall be in network byte order. The **sockaddr\_in** structure is used to store addresses for the Internet address family. Pointers to this type shall be cast by applications to **struct sockaddr \*** for use with socket functions. The *\* header shall define the **in6\_addr** structure, which shall include at least the following member: ``` uint8_t s6_addr[16] ``` This array is used to contain a 128-bit IPv6 address, stored in network byte order. The *\* header shall define the **sockaddr\_in6** structure, which shall include at least the following members: ``` sa_family_t sin6_family AF_INET6. in_port_t sin6_port Port number. uint32_t sin6_flowinfo IPv6 traffic class and flow information. struct in6_addr sin6_addr IPv6 address. uint32_t sin6_scope_id Set of interfaces for a scope. ``` The *sin6\_port* and *sin6\_addr* members shall be in network byte order. Prior to calling a function in this standard which reads values from a **sockaddr\_in6** structure (for example, *bind*() or *connect*()), the application shall ensure that all members of the structure, including any additional non-standard members, if any, are initialized. If the **sockaddr\_in6** structure has a non-standard member, and that member has a value other than the value that would result from default initialization, the behavior of any function in this standard that reads values from the **sockaddr\_in6** structure is implementation-defined. All functions in this standard that return data in a **sockaddr\_in6** structure (for example, *getaddrinfo*() or *accept*()) shall initialize the structure in a way that meets the above requirements, and shall ensure that each non-standard member, if any, has a value that produces the same behavior as default initialization would in all functions in this standard which read values from a **sockaddr\_in6** structure. The *sin6\_scope\_id* field is a 32-bit integer that identifies a set of interfaces as appropriate for the scope of the address carried in the *sin6\_addr* field. For a link scope *sin6\_addr*, the application shall ensure that *sin6\_scope\_id* is a link index. For a site scope *sin6\_addr*, the application shall ensure that *sin6\_scope\_id* is a site index. The mapping of *sin6\_scope\_id* to an interface or set of interfaces is implementation-defined. The *\* header shall declare the following external variable: ``` const struct in6_addr in6addr_any ``` This variable is initialized by the system to contain the wildcard IPv6 address. The *\* header also defines the IN6ADDR\_ANY\_INIT macro. This macro must be constant at compile time and can be used to initialize a variable of type **struct in6\_addr** to the IPv6 wildcard address. The *\* header shall declare the following external variable: ``` const struct in6_addr in6addr_loopback ``` This variable is initialized by the system to contain the loopback IPv6 address. The *\* header also defines the IN6ADDR\_LOOPBACK\_INIT macro. This macro must be constant at compile time and can be used to initialize a variable of type **struct in6\_addr** to the IPv6 loopback address. The *\* header shall define the **ipv6\_mreq** structure, which shall include at least the following members: ``` struct in6_addr ipv6mr_multiaddr IPv6 multicast address. unsigned ipv6mr_interface Interface index. ``` The *\* header shall define the following symbolic constants for use as values of the *level* argument of *getsockopt*() and *setsockopt*(): - IPPROTO\_IP Internet protocol. - IPPROTO\_IPV6 Internet Protocol Version 6. - IPPROTO\_ICMP Control message protocol. - IPPROTO\_RAW Raw IP Packets Protocol. - IPPROTO\_TCP Transmission control protocol. - IPPROTO\_UDP User datagram protocol. The *\* header shall define the following symbolic constant for use as a local address in the structure passed to *bind*(): - INADDR\_ANY IPv4 wildcard address. The *\* header shall define the following symbolic constant for use as a destination address in the structures passed to *connect*(), *sendmsg*(), and *sendto*(): - INADDR\_BROADCAST IPv4 broadcast address. The *\* header shall define the following symbolic constant, with the value specified, to help applications declare buffers of the proper size to store IPv4 addresses in string form: - INET\_ADDRSTRLEN 16\. Length of the string form for IP. The *htonl*(), *htons*(), *ntohl*(), and *ntohs*() functions shall be available as described in *\*. Inclusion of the *\* header may also make visible all symbols from *\*. The *\* header shall define the following symbolic constant, with the value specified, to help applications declare buffers of the proper size to store IPv6 addresses in string form: - INET6\_ADDRSTRLEN 46\. Length of the string form for IPv6.\ The *\* header shall define the following symbolic constants, with distinct integer values, for use in the *option\_name* argument in the *getsockopt*() or *setsockopt*() functions at protocol level IPPROTO\_IPV6: - IPV6\_JOIN\_GROUP Join a multicast group. - IPV6\_LEAVE\_GROUP Quit a multicast group. - IPV6\_MULTICAST\_HOPS Multicast hop limit. - IPV6\_MULTICAST\_IF Interface to use for outgoing multicast packets. - IPV6\_MULTICAST\_LOOP Multicast packets are delivered back to the local application. - IPV6\_UNICAST\_HOPS Unicast hop limit. - IPV6\_V6ONLY Restrict AF\_INET6 socket to IPv6 communications only. The *\* header shall define the following macros that test for special IPv6 addresses. Each macro is of type **int** and takes a single argument of type **const struct in6\_addr \***: - IN6\_IS\_ADDR\_UNSPECIFIED Unspecified address. - IN6\_IS\_ADDR\_LOOPBACK Loopback address. - IN6\_IS\_ADDR\_MULTICAST Multicast address. - IN6\_IS\_ADDR\_LINKLOCAL Unicast link-local address. - IN6\_IS\_ADDR\_SITELOCAL Unicast site-local address. - IN6\_IS\_ADDR\_V4MAPPED IPv4 mapped address. - IN6\_IS\_ADDR\_V4COMPAT IPv4-compatible address. - IN6\_IS\_ADDR\_MC\_NODELOCAL Multicast node-local address. - IN6\_IS\_ADDR\_MC\_LINKLOCAL Multicast link-local address. - IN6\_IS\_ADDR\_MC\_SITELOCAL Multicast site-local address. - IN6\_IS\_ADDR\_MC\_ORGLOCAL Multicast organization-local address. - IN6\_IS\_ADDR\_MC\_GLOBAL Multicast global address. *The following sections are informative.* ## APPLICATION USAGE Although applications are required to initialize all members (including any non-standard ones) of a **sockaddr\_in6** structure, the same is not required for the **sockaddr\_in** structure, since historically many applications only initialized the standard members. Despite this, applications are encouraged to initialize **sockaddr\_in** structures in a manner similar to the required initialization of **sockaddr\_in6** structures. Although it is common practice to initialize a **sockaddr\_in6** structure using: ``` struct sockaddr_in6 sa; memset(&sa, 0, sizeof sa); ``` 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 sockaddr_in6 sa = { 0 }; ``` or: ``` static struct sockaddr_in6 sa_init; struct sockaddr_in6 sa = sa_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 **sockaddr\_in6** 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. ## RATIONALE The INADDR\_ANY and INADDR\_BROADCAST values are byte-order-neutral and thus their byte order is not specified. Many implementations have additional constants as extensions, such as INADDR\_LOOPBACK, that are not byte-order-neutral. Traditionally, these constants are in host byte order, requiring the use of *htonl*() when using them in a **sockaddr\_in** structure. ## FUTURE DIRECTIONS None. ## SEE ALSO *Section 4.10*, *Host and Network Byte Orders*, ***\***, ***\***, ***\*** The System Interfaces volume of POSIX.1‐2017, *connect()*, *getsockopt()*, *htonl()*, *sendmsg()*, *sendto()*, *setsockopt()* ## 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 .