inet_net_pton()¶

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 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()¶

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()¶

• 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.

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);
}