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INET(3) Linux Programmer's Manual INET(3)

NAME

     inet_aton,    inet_addr,    inet_network,   inet_ntoa,   inet_makeaddr,
     inet_lnaof, inet_netof - Internet address manipulation routines

SYNOPSIS

     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <arpa/inet.h>

     int inet_aton(const char *cp, struct in_addr *inp);

     in_addr_t inet_addr(const char *cp);

     in_addr_t inet_network(const char *cp);

     char *inet_ntoa(struct in_addr in);

     struct in_addr inet_makeaddr(in_addr_t net, in_addr_t host);

     in_addr_t inet_lnaof(struct in_addr in);

     in_addr_t inet_netof(struct in_addr in);

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     inet_aton(), inet_ntoa():
         Since glibc 2.19:
             _DEFAULT_SOURCE
         In glibc up to and including 2.19:
             _BSD_SOURCE || _BSD_SOURCE

DESCRIPTION

     inet_aton() converts the Internet host address cp from  the  IPv4  num-
     bers-and-dots  notation  into  binary  form (in network byte order) and
     stores it in the structure that inp  points  to.   inet_aton()  returns
     nonzero  if the address is valid, zero if not.  The address supplied in
     cp can have one of the following forms:

     a.b.c.d   Each of the four  numeric  parts  specifies  a  byte  of  the
               address;  the  bytes  are  assigned in left-to-right order to
               produce the binary address.

     a.b.c     Parts a and b specify the  first  two  bytes  of  the  binary
               address.   Part  c  is  interpreted  as  a  16-bit value that
               defines the rightmost two bytes of the binary address.   This
               notation  is  suitable for specifying (outmoded) Class B net-
               work addresses.

     a.b       Part a specifies the first byte of the binary address.   Part
               b is interpreted as a 24-bit value that defines the rightmost
               three bytes of the binary address.  This notation is suitable
               for specifying (outmoded) Class A network addresses.

     a         The  value  a is interpreted as a 32-bit value that is stored
               directly into the binary address without any byte  rearrange-
               ment.

     In  all  of  the  above  forms, components of the dotted address can be
     specified in decimal, octal (with a leading 0), or hexadecimal, with  a
     leading  0X).   Addresses in any of these forms are collectively termed
     IPV4 numbers-and-dots notation.  The form that uses exactly four  deci-
     mal  numbers  is  referred to as IPv4 dotted-decimal notation (or some-
     times: IPv4 dotted-quad notation).

     inet_aton() returns 1 if the supplied string  was  successfully  inter-
     preted, or 0 if the string is invalid (errno is not set on error).

     The  inet_addr()  function  converts  the Internet host address cp from
     IPv4 numbers-and-dots notation into binary data in network byte  order.
     If  the input is invalid, INADDR_NONE (usually -1) is returned.  Use of
     this  function  is  problematic  because  -1   is   a   valid   address
     (255.255.255.255).    Avoid   its   use   in   favor   of  inet_aton(),
     inet_pton(3), or getaddrinfo(3), which provide a cleaner way  to  indi-
     cate error return.

     The  inet_network() function converts cp, a string in IPv4 numbers-and-
     dots notation, into a number in host byte order suitable for use as  an
     Internet  network  address.   On  success,  the  converted  address  is
     returned.  If the input is invalid, -1 is returned.

     The inet_ntoa() function converts the Internet host address  in,  given
     in  network  byte  order,  to a string in IPv4 dotted-decimal notation.
     The string is returned in a statically allocated buffer,  which  subse-
     quent calls will overwrite.

     The inet_lnaof() function returns the local network address part of the
     Internet address in.  The returned value is in host byte order.

     The inet_netof() function returns the network number part of the Inter-
     net address in.  The returned value is in host byte order.

     The  inet_makeaddr()  function  is  the  converse  of  inet_netof() and
     inet_lnaof().  It returns an Internet  host  address  in  network  byte
     order,  created  by  combining  the  network  number net with the local
     address host, both in host byte order.

     The  structure  in_addr  as  used  in   inet_ntoa(),   inet_makeaddr(),
     inet_lnaof() and inet_netof() is defined in <netinet/in.h> as:

         typedef uint32_t in_addr_t;

         struct in_addr {
             in_addr_t s_addr; };

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     +-------------------------------+---------------+----------------+
     |Interface                      | Attribute     | Value          |
     +-------------------------------+---------------+----------------+
     |inet_aton(), inet_addr(),      | Thread safety | MT-Safe locale |
     |inet_network(), inet_ntoa()    |               |                |
     +-------------------------------+---------------+----------------+
     |inet_makeaddr(), inet_lnaof(), | Thread safety | MT-Safe        |
     |inet_netof()                   |               |                |
     +-------------------------------+---------------+----------------+

CONFORMING TO

     inet_addr(), inet_ntoa(): POSIX.1-2001, POSIX.1-2008, 4.3BSD.

     inet_aton()  is not specified in POSIX.1, but is available on most sys-
     tems.

NOTES

     On x86 architectures, the host byte order  is  Least  Significant  Byte
     first  (little  endian), whereas the network byte order, as used on the
     Internet, is Most Significant Byte first (big endian).

     inet_lnaof(), inet_netof(), and inet_makeaddr()  are  legacy  functions
     that assume they are dealing with classful network addresses.  Classful
     networking divides IPv4 network addresses into host and network  compo-
     nents at byte boundaries, as follows:

     Class A   This  address  type  is  indicated by the value 0 in the most
               significant bit of the (network byte ordered)  address.   The
               network  address  is  contained in the most significant byte,
               and the host address occupies the remaining three bytes.

     Class B   This address type is indicated by the binary value 10 in  the
               most  significant  two  bits  of  the  address.   The network
               address is contained in the two most significant  bytes,  and
               the host address occupies the remaining two bytes.

     Class C   This address type is indicated by the binary value 110 in the
               most significant three bits  of  the  address.   The  network
               address is contained in the three most significant bytes, and
               the host address occupies the remaining byte.

     Classful network addresses are now obsolete, having been superseded  by
     Classless  Inter-Domain  Routing  (CIDR),  which divides addresses into
     network and host components at arbitrary bit (rather than byte)  bound-
     aries.

EXAMPLE

     An  example  of  the use of inet_aton() and inet_ntoa() is shown below.
     Here are some example runs:

         $ ./a.out 226.000.000.037      # Last byte is in octal 226.0.0.31 $
         ./a.out 0x7f.1               # First byte is in hex 127.0.0.1

 Program source
      #define _BSD_SOURCE #include <arpa/inet.h> #include <stdio.h> #include
     <stdlib.h>

     int main(int argc, char *argv[]) {
         struct in_addr addr;

         if (argc != 2) {
             fprintf(stderr, "%s <dotted-address>\n", argv[0]);
             exit(EXIT_FAILURE);
         }

         if (inet_aton(argv[1], &addr) == 0) {
             fprintf(stderr, "Invalid address\n");
             exit(EXIT_FAILURE);
         }

         printf("%s\n", inet_ntoa(addr));
         exit(EXIT_SUCCESS); }