NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       inet_net_pton, inet_net_ntop - Internet network number conversion

SYNOPSIS         top

       #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         top

       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 typically 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
       input 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 required for the network number (or as are explicitly specified
       by pres), which may be less than the number of bytes in a complete
       network address.

   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 dotted-decimal number representing the network address,
       followed by a slash, and the size of the network number in bits.

RETURN VALUE         top

       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         top

       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         top

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

NOTES         top

   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
       hexadecimal 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 accepted:

           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
       network 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 inferred 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
       actually specified.

EXAMPLE         top

       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()
       allowing 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
       inference about its size (but any extra bytes that are explicitly
       specified will still be used by inet_net_pton(): to populate the
       result buffer):

           $ ./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         top

       inet(3), networks(5)

COLOPHON         top

       This page is part of release 4.10 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
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Linux                            2014-05-28                 INET_NET_PTON(3)