#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():
For both functions, af specifies the address family for the conversion; the only supported value is AF_INET.
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.
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.
On success, inet_net_ntop() returns pres. On error, it returns NULL, and errno is set to indicate the cause of the error.
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.
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.
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
/* 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); }