INET_NTOP(3P) POSIX Programmer's Manual INET_NTOP(3P)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.
inet_ntop, inet_pton — convert IPv4 and IPv6 addresses between binary
and text form
const char *inet_ntop(int af, const void *restrict src,
char *restrict dst, socklen_t size);
int inet_pton(int af, const char *restrict src, void *restrict dst);
The inet_ntop() function shall convert a numeric address into a text
string suitable for presentation. The af argument shall specify the
family of the address. This can be AF_INET or AF_INET6. The src argu‐
ment points to a buffer holding an IPv4 address if the af argument is
AF_INET, or an IPv6 address if the af argument is AF_INET6; the address
must be in network byte order. The dst argument points to a buffer
where the function stores the resulting text string; it shall not be
NULL. The size argument specifies the size of this buffer, which shall
be large enough to hold the text string (INET_ADDRSTRLEN characters for
IPv4, INET6_ADDRSTRLEN characters for IPv6).
The inet_pton() function shall convert an address in its standard text
presentation form into its numeric binary form. The af argument shall
specify the family of the address. The AF_INET and AF_INET6 address
families shall be supported. The src argument points to the string
being passed in. The dst argument points to a buffer into which the
function stores the numeric address; this shall be large enough to hold
the numeric address (32 bits for AF_INET, 128 bits for AF_INET6).
If the af argument of inet_pton() is AF_INET, the src string shall be
in the standard IPv4 dotted-decimal form:
where "ddd" is a one to three digit decimal number between 0 and 255
(see inet_addr()). The inet_pton() function does not accept other for‐
mats (such as the octal numbers, hexadecimal numbers, and fewer than
four numbers that inet_addr() accepts).
If the af argument of inet_pton() is AF_INET6, the src string shall be
in one of the following standard IPv6 text forms:
1. The preferred form is "x:x:x:x:x:x:x:x", where the 'x's are the
hexadecimal values of the eight 16-bit pieces of the address.
Leading zeros in individual fields can be omitted, but there shall
be at least one numeral in every field.
2. A string of contiguous zero fields in the preferred form can be
shown as "::". The "::" can only appear once in an address.
Unspecified addresses ("0:0:0:0:0:0:0:0") may be represented simply
3. A third form that is sometimes more convenient when dealing with a
mixed environment of IPv4 and IPv6 nodes is "x:x:x:x:x:x:d.d.d.d",
where the 'x's are the hexadecimal values of the six high-order
16-bit pieces of the address, and the 'd's are the decimal values
of the four low-order 8-bit pieces of the address (standard IPv4
Note: A more extensive description of the standard representations
of IPv6 addresses can be found in RFC 2373.
The inet_ntop() function shall return a pointer to the buffer contain‐
ing the text string if the conversion succeeds, and NULL otherwise, and
set errno to indicate the error.
The inet_pton() function shall return 1 if the conversion succeeds,
with the address pointed to by dst in network byte order. It shall
return 0 if the input is not a valid IPv4 dotted-decimal string or a
valid IPv6 address string, or −1 with errno set to [EAFNOSUPPORT] if
the af argument is unknown.
The inet_ntop() and inet_pton() functions shall fail if:
The af argument is invalid.
ENOSPC The size of the inet_ntop() result buffer is inadequate.
The following sections are informative.
The Base Definitions volume of POSIX.1‐2008, <arpa_inet.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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
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IEEE/The Open Group 2013 INET_NTOP(3P)