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.

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 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.

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);
}