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.\"     From: @(#)inet.3	8.1 (Berkeley) 6/4/93
.\"
.Dd November 9, 2021
.Dt INET 3
.Os
.Sh NAME
.Nm inet_aton ,
.Nm inet_addr ,
.Nm inet_network ,
.Nm inet_ntoa ,
.Nm inet_ntoa_r ,
.Nm inet_ntop ,
.Nm inet_pton ,
.Nm inet_makeaddr ,
.Nm inet_lnaof ,
.Nm inet_netof
.Nd Internet address manipulation routines
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netinet/in.h
.In arpa/inet.h
.Ft int
.Fn inet_aton "const char *cp" "struct in_addr *pin"
.Ft in_addr_t
.Fn inet_addr "const char *cp"
.Ft in_addr_t
.Fn inet_network "const char *cp"
.Ft char *
.Fn inet_ntoa "struct in_addr in"
.Ft char *
.Fo inet_ntoa_r
.Fa "struct in_addr in"
.Fa "char *buf"
.Fa "socklen_t size"
.Fc
.Ft const char *
.Fo inet_ntop
.Fa "int af"
.Fa "const void * restrict src"
.Fa "char * restrict dst"
.Fa "socklen_t size"
.Fc
.Ft int
.Fn inet_pton "int af" "const char * restrict src" "void * restrict dst"
.Ft struct in_addr
.Fn inet_makeaddr "in_addr_t net" "in_addr_t lna"
.Ft in_addr_t
.Fn inet_lnaof "struct in_addr in"
.Ft in_addr_t
.Fn inet_netof "struct in_addr in"
.Sh DESCRIPTION
The routines
.Fn inet_aton ,
.Fn inet_addr
and
.Fn inet_network
interpret character strings representing
numbers expressed in the Internet standard
.Ql .\&
notation.
.Pp
The
.Fn inet_pton
function converts a presentation format address (that is, printable form
as held in a character string) to network format (usually a
.Ft struct in_addr
or some other internal binary representation, in network byte order).
It returns 1 if the address was valid for the specified address family, or
0 if the address was not parseable in the specified address family, or -1
if some system error occurred (in which case
.Va errno
will have been set).
This function is presently valid for
.Dv AF_INET
and
.Dv AF_INET6 .
.Pp
The
.Fn inet_aton
routine interprets the specified character string as an Internet address,
placing the address into the structure provided.
It returns 1 if the string was successfully interpreted,
or 0 if the string is invalid.
The
.Fn inet_addr
and
.Fn inet_network
functions return numbers suitable for use
as Internet addresses and Internet network
numbers, respectively.
.Pp
The function
.Fn inet_ntop
converts an address
.Fa *src
from network format
(usually a
.Ft struct in_addr
or some other binary form, in network byte order) to presentation format
(suitable for external display purposes).
The
.Fa size
argument specifies the size, in bytes, of the buffer
.Fa *dst .
.Dv INET_ADDRSTRLEN
and
.Dv INET6_ADDRSTRLEN
define the maximum size required to convert an address of the respective
type.
It returns NULL if a system error occurs (in which case,
.Va errno
will have been set), or it returns a pointer to the destination string.
This function is presently valid for
.Dv AF_INET
and
.Dv AF_INET6 .
.Pp
The routine
.Fn inet_ntoa
takes an Internet address and returns an
.Tn ASCII
string representing the address in
.Ql .\&
notation.
The routine
.Fn inet_ntoa_r
is the reentrant version of
.Fn inet_ntoa .
The deprecated routine
.Fn inet_makeaddr
takes an Internet network number and a local
host address on that network, and constructs an Internet address
from it.
It should only be assumed to work for historical class A/B/C networks.
The deprecated routines
.Fn inet_netof
and
.Fn inet_lnaof
break apart Internet host addresses, returning
the network number and local host address part,
respectively,
assuming the historical class A/B/C network masks.
.Pp
All Internet addresses are returned in network
order (bytes ordered from left to right).
All network numbers and local address parts are
returned as machine byte order integer values.
.Sh INTERNET ADDRESSES (IP VERSION 4)
The
.Fn inet_aton 
and
.Fn inet_addr
functions accept IPv4
values specified using the
.Ql .\&
notation in one
of the following forms:
.Bd -literal -offset indent
a.b.c.d
a.b.c
a.b
a
.Ed
.Pp
When four parts are specified, each is interpreted
as a byte of data and assigned, from left to right,
to the four bytes of an Internet address.
.Pp
When a three part address is specified, the last
part is interpreted as a 16-bit quantity and placed
in the least significant two bytes of the network address.
.Pp
When a two part address is supplied, the last part
is interpreted as a 24-bit quantity and placed in
the least significant three bytes of the network address.
.Pp
When only one part is given, the value is stored
directly in the network address without any byte
rearrangement.
.Pp
All numbers supplied as
.Dq parts
in a
.Ql .\&
notation
may be decimal, octal, or hexadecimal, as specified
in the C language (i.e., a leading 0x or 0X implies
hexadecimal; otherwise, a leading 0 implies octal;
otherwise, the number is interpreted as decimal).
.Pp
Note that
.Fn inet_pton
does not accept 1-, 2-, or 3-part dotted addresses; all four parts
must be specified and are interpreted only as decimal values.
This is a narrower input set than that accepted by
.Fn inet_aton .
.Sh DIAGNOSTICS
The constant
.Dv INADDR_NONE
is returned by
.Fn inet_addr
and
.Fn inet_network
for malformed requests.
.Sh ERRORS
The
.Fn inet_ntop
call fails if:
.Bl -tag -width Er
.It Bq Er ENOSPC
.Fa size
was not large enough to store the presentation form of the address.
.It Bq Er EAFNOSUPPORT
.Fa *src
was not an
.Dv AF_INET
or
.Dv AF_INET6
family address.
.El
.Sh SEE ALSO
.Xr byteorder 3 ,
.Xr getaddrinfo 3 ,
.Xr gethostbyname 3 ,
.Xr getnameinfo 3 ,
.Xr getnetent 3 ,
.Xr inet_net 3 ,
.Xr hosts 5 ,
.Xr networks 5
.Rs
.%R RFC
.%N 2373
.%D July 1998
.%T "IP Version 6 Addressing Architecture"
.Re
.Sh STANDARDS
The
.Fn inet_ntop
and
.Fn inet_pton
functions conform to
.St -xns5.2 .
.Sh HISTORY
These
functions appeared in
.Bx 4.2 .
.Sh BUGS
The value
.Dv INADDR_NONE
(0xffffffff) is a valid broadcast address, but
.Fn inet_addr
cannot return that value without indicating failure.
The newer
.Fn inet_aton
function does not share this problem.
The problem of host byte ordering versus network byte ordering is
confusing.
The string returned by
.Fn inet_ntoa
resides in a static memory area.
.Pp
The
.Fn inet_addr
function should return a
.Fa struct in_addr .