inet_net_ntop(3)



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

NAME
       inet_net_pton, inet_net_ntop - Internet network number conversion

SYNOPSIS
       #include <arpa/inet.h>

       int inet_net_pton(int af, const char *pres,
                         void *netp, size_t nsize);

       char *inet_net_ntop(int af, const void *netp, int bits,
                           char *pres, size_t psize);

       Link with -lresolv.

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

       inet_net_pton(), inet_net_ntop():
           Since glibc 2.20:
               _DEFAULT_SOURCE
           Before glibc 2.20:
               _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
       These  functions  convert  network  numbers between presentation (i.e.,
       printable) format and network (i.e., binary) format.

       For both functions, af specifies the address family for the conversion;
       the only supported value is AF_INET.

   inet_net_pton()
       The  inet_net_pton()  function  converts pres, a null-terminated string
       containing an Internet network number in presentation format to network
       format.   The result of the conversion, which is in network byte order,
       is placed in the buffer pointed to by net.  (The  netp  argument  typi-
       cally  points  to  an in_addr structure.)  The nsize argument specifies
       the number of bytes available in netp.

       On success, inet_net_pton() returns the number of bits in  the  network
       number field of the result placed in netp.  For a discussion of the in-
       put presentation format and the return value, see NOTES.

       Note: the buffer pointed to by netp should be zeroed out before calling
       inet_net_pton(),  since  the  call writes only as many bytes as are re-
       quired for the network number (or as are explicitly specified by pres),
       which  may  be  less than the number of bytes in a complete network ad-
       dress.

   inet_net_ntop()
       The inet_net_ntop() function converts the network number in the  buffer
       pointed  to  by  netp to presentation format; *netp is interpreted as a
       value in network byte order.  The bits argument specifies the number of
       bits in the network number in *netp.

       The  null-terminated presentation-format string is placed in the buffer
       pointed to by pres.  The psize argument specifies the number  of  bytes
       available  in  pres.  The presentation string is in CIDR format: a dot-
       ted-decimal number representing the  network  address,  followed  by  a
       slash, and the size of the network number in bits.

RETURN VALUE
       On  success,  inet_net_pton() returns the number of bits in the network
       number.  On error, it returns -1, and errno  is  set  to  indicate  the
       cause of the error.

       On  success,  inet_net_ntop() returns pres.  On error, it returns NULL,
       and errno is set to indicate the cause of the error.

ERRORS
       EAFNOSUPPORT
              af specified a value other than AF_INET.

       EMSGSIZE
              The size of the output buffer was insufficient.

       ENOENT (inet_net_pton()) pres was not in correct presentation format.

CONFORMING TO
       The inet_net_pton() and inet_net_ntop() functions are nonstandard,  but
       widely available.

NOTES
   Input presentation format for inet_net_pton()
       The network number may be specified either as a hexadecimal value or in
       dotted-decimal notation.

       Hexadecimal values are indicated by an initial "0x" or "0X".  The hexa-
       decimal digits populate the nibbles (half octets) of the network number
       from left to right in network byte order.

       In dotted-decimal notation, up to four octets are specified, as decimal
       numbers  separated  by  dots.  Thus, any of the following forms are ac-
       cepted:

           a.b.c.d
           a.b.c
           a.b
           a

       Each part is a number in the range 0 to 255 that populates one byte  of
       the resulting network number, going from left to right, in network-byte
       (big endian) order.  Where a part is omitted, the resulting byte in the
       network number is zero.

       For either hexadecimal or dotted-decimal format, the network number can
       optionally be followed by a slash and a number in the range  0  to  32,
       which specifies the size of the network number in bits.

   Return value of inet_net_pton()
       The  return  value of inet_net_pton() is the number of bits in the net-
       work number field.  If the input presentation string terminates with  a
       slash  and  an  explicit  size value, then that size becomes the return
       value of inet_net_pton().  Otherwise, the return value,  bits,  is  in-
       ferred as follows:

       *  If  the  most significant byte of the network number is greater than
          or equal to 240, then bits is 32.

       *  Otherwise, if the most significant byte of  the  network  number  is
          greater than or equal to 224, then bits is 4.

       *  Otherwise,  if  the  most  significant byte of the network number is
          greater than or equal to 192, then bits is 24.

       *  Otherwise, if the most significant byte of  the  network  number  is
          greater than or equal to 128, then bits is 16.

       *  Otherwise, bits is 8.

       If  the  resulting  bits  value from the above steps is greater than or
       equal to 8, but the number of octets specified in  the  network  number
       exceed  bits/8,  then bits is set to 8 times the number of octets actu-
       ally specified.

EXAMPLES
       The  program  below  demonstrates  the  use  of   inet_net_pton()   and
       inet_net_ntop().   It  uses inet_net_pton() to convert the presentation
       format network address provided in its first command-line  argument  to
       binary  form,  displays the return value from inet_net_pton().  It then
       uses inet_net_ntop() to convert the binary form  back  to  presentation
       format, and displays the resulting string.

       In order to demonstrate that inet_net_pton() may not write to all bytes
       of its netp argument, the program allows an  optional  second  command-
       line   argument,   a  number  used  to  initialize  the  buffer  before
       inet_net_pton() is called.  As its final line of  output,  the  program
       displays all of the bytes of the buffer returned by inet_net_pton() al-
       lowing  the  user  to  see  which  bytes  have  not  been  touched   by
       inet_net_pton().

       An  example run, showing that inet_net_pton() infers the number of bits
       in the network number:

           $ ./a.out 193.168
           inet_net_pton() returned: 24
           inet_net_ntop() yielded:  193.168.0/24
           Raw address:              c1a80000

       Demonstrate that inet_net_pton() does not zero out unused bytes in  its
       result buffer:

           $ ./a.out 193.168 0xffffffff
           inet_net_pton() returned: 24
           inet_net_ntop() yielded:  193.168.0/24
           Raw address:              c1a800ff

       Demonstrate  that  inet_net_pton()  will widen the inferred size of the
       network number, if the supplied number of  bytes  in  the  presentation
       string exceeds the inferred value:

           $ ./a.out 193.168.1.128
           inet_net_pton() returned: 32
           inet_net_ntop() yielded:  193.168.1.128/32
           Raw address:              c1a80180

       Explicitly  specifying the size of the network number overrides any in-
       ference about its size (but any extra bytes that are explicitly  speci-
       fied will still be used by inet_net_pton(): to populate the result buf-
       fer):

           $ ./a.out 193.168.1.128/24
           inet_net_pton() returned: 24
           inet_net_ntop() yielded:  193.168.1/24
           Raw address:              c1a80180

   Program source
       /* Link with "-lresolv" */

       #include <arpa/inet.h>
       #include <stdio.h>
       #include <stdlib.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       int
       main(int argc, char *argv[])
       {
           char buf[100];
           struct in_addr addr;
           int bits;

           if (argc < 2) {
               fprintf(stderr,
                       "Usage: %s presentation-form [addr-init-value]\n",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           /* If argv[2] is supplied (a numeric value), use it to initialize
              the output buffer given to inet_net_pton(), so that we can see
              that inet_net_pton() initializes only those bytes needed for
              the network number. If argv[2] is not supplied, then initialize
              the buffer to zero (as is recommended practice). */

           addr.s_addr = (argc > 2) ? strtod(argv[2], NULL) : 0;

           /* Convert presentation network number in argv[1] to binary */

           bits = inet_net_pton(AF_INET, argv[1], &addr, sizeof(addr));
           if (bits == -1)
               errExit("inet_net_ntop");

           printf("inet_net_pton() returned: %d\n", bits);

           /* Convert binary format back to presentation, using 'bits'
              returned by inet_net_pton() */

           if (inet_net_ntop(AF_INET, &addr, bits, buf, sizeof(buf)) == NULL)
               errExit("inet_net_ntop");

           printf("inet_net_ntop() yielded:  %s\n", buf);

           /* Display 'addr' in raw form (in network byte order), so we can
              see bytes not displayed by inet_net_ntop(); some of those bytes
              may not have been touched by inet_net_ntop(), and so will still
              have any initial value that was specified in argv[2]. */

           printf("Raw address:              %x\n", htonl(addr.s_addr));

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       inet(3), networks(5)

COLOPHON
       This page is part of release 5.07 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-06-09                  INET_NET_PTON(3)

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