Provided by: manpages-posix-dev_2017a-2_all 
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 <netinet/in.h>
DESCRIPTION
The <netinet/in.h> header shall define the following types:
in_port_t Equivalent to the type uint16_t as described in <inttypes.h>.
in_addr_t Equivalent to the type uint32_t as described in <inttypes.h>.
The <netinet_in.h> header shall define the sa_family_t type as described in <sys/socket.h>.
The <netinet_in.h> header shall define the uint8_t and uint32_t types as described in <inttypes.h>.
Inclusion of the <netinet/in.h> header may also make visible all symbols from <inttypes.h> and
<sys/socket.h>.
The <netinet/in.h> header shall define the in_addr structure, which shall include at least the following
member:
in_addr_t s_addr
The <netinet/in.h> 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 <netinet/in.h> 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 <netinet/in.h> 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 <netinet/in.h> 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 <netinet/in.h>
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 <netinet/in.h> 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 <netinet/in.h>
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 <netinet/in.h> 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 <netinet/in.h> 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 <netinet/in.h> 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 <netinet/in.h> 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 <netinet/in.h> 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 <arpa/inet.h>.
Inclusion of the <netinet/in.h> header may also make visible all symbols from <arpa/inet.h>.
The <netinet/in.h> 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 <netinet/in.h> 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 <netinet/in.h> 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, <arpa_inet.h>, <inttypes.h>, <sys_socket.h>
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 .