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getifaddrs.3: New page documenting getifaddrs(3) and freeifaddrs(3)
Many edits and changes of Petr's initial draft by mtk Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Copyright (c) 2008 Petr Baudis <pasky@suse.cz>
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.\" and copyright (c) 2009, Linux Foundation, written by Michael Kerrisk
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.\" <mtk.manpages@gmail.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\"
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.\" 2008-12-08 Petr Baudis <pasky@suse.cz>
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.\" Rewrite the BSD manpage in the Linux man pages style and account
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.\" for glibc specificities, provide an example.
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.\" 2009-01-14 mtk, many edits and changes, rewrote example program.
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.\"
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.TH GETIFADDRS 3 2009-01-14 "GNU" "Linux Programmer's Manual"
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.\" FIXME Make a link for freeifaddrs.3
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.\" FIXME Check other changes in Petr's mail
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.SH NAME
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getifaddrs, freeifaddrs \- get interface addresses
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.SH SYNOPSIS
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.nf
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.B #include <sys/types.h>
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.B #include <ifaddrs.h>
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.sp
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.BI "int getifaddrs(struct ifaddrs **" "ifap" );
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.sp
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.BI "void freeifaddrs(struct ifaddrs *" "ifa" );
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.fi
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.SH DESCRIPTION
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The
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.BR getifaddrs ()
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function creates a linked list of structures describing
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the network interfaces of the local system,
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and stores the address of the first item of the list in
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.IR *ifap .
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The list consists of
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.I ifaddrs
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structures, defined as follows:
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.sp
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.in +4n
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.nf
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struct ifaddrs {
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struct ifaddrs *ifa_next; /* Next item in list */
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char *ifa_name; /* Name of interface */
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unsigned int ifa_flags; /* Flags from SIOCGIFFLAGS */
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struct sockaddr *ifa_addr; /* Address of interface */
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struct sockaddr *ifa_netmask; /* Netmask of interface */
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union {
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struct sockaddr *ifu_broadaddr;
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/* Broadcast address of interface */
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struct sockaddr *ifu_dstaddr;
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/* Point-to-point destination address */
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} ifa_ifu;
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#define ifa_broadaddr ifa_ifu.ifu_broadaddr
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#define ifa_dstaddr ifa_ifu.ifu_dstaddr
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void *ifa_data; /* Address-specific data */
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};
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.fi
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.in
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.PP
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The
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.I ifa_next
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field contains a pointer to the next structure on the list,
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or NULL if this is the last item of the list.
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.PP
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The
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.I ifa_name
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points to the null-terminated interface name.
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.\" The constant
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.\" .B IF NAMESIZE
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.\" indicates the maximum length of this field.
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.PP
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The
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.I ifa_flags
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field contains the interface flags, as returned by the
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.B SIOCGIFFLAGS
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.BR ioctl (2)
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operation (see
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.BR netdevice (7)
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for a list of these flags).
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.PP
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The
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.I ifa_addr
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field points to a structure containing the interface address.
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(The
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.I sa_family
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sub-field should be consulted to determine the format of the
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address structure.)
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.PP
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The
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.I ifa_netmask
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field points to a structure containing the netmask associated with
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.IR ifa_addr ,
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if applicable for the address family.
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.PP
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Depending on whether the bit
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.B IFF_BROADCAST
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or
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.B IFF_POINTOPOINT
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is set in
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.I ifa_flags
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(only one can be set at a time),
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either
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.I ifa_broadaddr
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will contain the broadcast address associated with
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.I ifa_addr
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(if applicable for the address family) or
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.I ifa_dstaddr
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will contain the destination address of the point-to-point interface.
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.PP
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The
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.I ifa_data
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field points to a buffer containing address-family-specific data;
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this field may be NULL if there is no such data for this interface.
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.PP
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The data returned by
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.BR getifaddrs ()
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is dynamically allocated and should be freed using
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.BR freeifaddrs ()
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when no longer needed.
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.SH RETURN VALUES
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On success,
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.BR getifaddrs ()
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returns zero;
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on error, -1 is returned, and
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.I errno
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is set appropriately.
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.SH ERRORS
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.BR getifaddrs ()
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may fail and set
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.I errno
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for any of the errors specified for
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.BR socket (2),
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.BR bind (2),
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.\" FIXME Petr, I added getsockname() and recvmsg(); do you agree?
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.BR getsockname (2),
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.BR recvmsg (2),
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.BR sendto (2),
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.BR malloc (3),
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or
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.BR realloc (3).
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.SH VERSIONS
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The
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.BR getifaddrs ()
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function first appeared in glibc 2.3, but before glibc 2.3.3,
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the implementation only supported IPv4 addresses;
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IPv6 support was added in glibc 2.3.3.
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.SH CONFORMING TO
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Not in POSIX.1-2001.
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This function first appeared in BSDi and is
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present on the BSD systems, but with slightly different
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semantics documented\(emreturning one entry per interface,
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not per address.
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This means
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.I ifa_addr
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and other fields can actually be NULL if the interface has no address,
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and no link-level address is returned if the interface has an IP address
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assigned.
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Also, the way of choosing either
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.I ifa_broadaddr
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or
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.I ifa_dstaddr
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differs on various systems.
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.\" , but the BSD-derived documentation generally
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.\" appears to be confused and obsolete on this point.
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.\" i.e., commonly it still says one of them will be NULL, even if
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.\" the ifa_ifu union is already present
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.SH NOTES
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The addresses returned on Linux will usually be the IPv4 and IPv6 addresses
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assigned to the interface, but also one
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.B AF_PACKET
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address per interface containing lower-level details about the interface
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and its physical layer.
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In this case, the
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.I ifa_data
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field may contain a pointer to a
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.IR "struct net_device_stats" ,
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defined in
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.IR <linux/netdevice.h> ,
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which contains various interface attributes and statistics.
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.SH EXAMPLE
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The program below demonstrates the use of
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.BR getifaddrs (),
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.BR freeifaddrs (),
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and
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.BR inet_ntop (3).
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Here is what we see when running this program on one system:
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.in +4n
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.nf
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$ \fB./a.out\fP
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lo address family: 17 (AF_PACKET)
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eth0 address family: 17 (AF_PACKET)
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lo address family: 2 (AF_INET)
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address: <127.0.0.1>
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eth0 address family: 2 (AF_INET)
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address: <10.1.1.4>
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lo address family: 10 (AF_INET6)
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address: <::1>
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eth0 address family: 10 (AF_INET6)
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address: <fe80::2d0:59ff:feda:eb51%eth0>
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.fi
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.in
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.SS Program source
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\&
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.nf
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#include <arpa/inet.h>
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#include <sys/socket.h>
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#include <netdb.h>
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#include <ifaddrs.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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int
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main(int argc, char *argv[])
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{
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struct ifaddrs *ifaddr;
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struct net_device_stats *ndsp;
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int family, s;
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char host[NI_MAXHOST];
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if (getifaddrs(&ifaddr) == \-1) {
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perror("getifaddrs");
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exit(EXIT_FAILURE);
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}
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while (ifaddr != NULL) { /* Walk through linked list */
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family = ifaddr\->ifa_addr\->sa_family;
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/* Display interface name and family (including symbolic
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form of the latter for the common families) */
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printf("%\-6s address family: %d%s\\n",
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ifaddr\->ifa_name, family,
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(family == AF_PACKET) ? " (AF_PACKET)" :
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(family == AF_INET) ? " (AF_INET)" :
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(family == AF_INET6) ? " (AF_INET6)" : "");
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/* For an AF_INET* interface address, display the address */
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if (family == AF_INET || family == AF_INET6) {
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s = getnameinfo(ifaddr\->ifa_addr,
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(family == AF_INET) ? sizeof(struct sockaddr_in) :
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sizeof(struct sockaddr_in6),
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host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
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if (s != 0) {
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printf("getnameinfo() failed: %s\\n", gai_strerror(s));
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exit(EXIT_FAILURE);
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}
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printf("\\taddress: <%s>\\n", host);
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}
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ifaddr = ifaddr\->ifa_next;
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}
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freeifaddrs(ifaddr);
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exit(EXIT_SUCCESS);
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}
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.fi
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.SH SEE ALSO
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.BR bind (2),
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.BR getsockname (2),
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.BR socket (2),
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.BR packet (7),
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.BR ifconfig (8)
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