Similar to getifaddrs on old glibc version - c

Question

There is very useful funcion call getifaddrs which retrieves all machin network addresses. The problem is that I'm using old glibc version which doesn't have this function. Is there any replacement for it? I was looking and found getipnodebyname but it is unuseful when address isn't mapped in /etc/hosts file.

Answer 1

To add to the previous answer, here is an example for the SIOCGIFCONF-approach. You have to do something like this:
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <net/if.h>
#include <arpa/inet.h>
int fd;
int get_iface_list(struct ifconf *ifconf)
{
int rval;
if((rval = ioctl(fd, SIOCGIFCONF , (char*) ifconf )) < 0 )
perror("ioctl(SIOGIFCONF)");
return rval;
}
int main()
{
static struct ifreq ifreqs[100];
static struct ifconf ifc;
char *ptr;
fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fd < 0)
return 1;
ifc.ifc_buf = (char*) (ifreqs);
ifc.ifc_len = sizeof(ifreqs);
if(get_iface_list(&ifc) < 0) return -1;
/* Go through the list of interfaces */
for (ptr = ifc.ifc_buf; ptr < ifc.ifc_buf + ifc.ifc_len;)
{
struct ifreq *ifr = (struct ifreq*)ptr;
int len = (sizeof(struct sockaddr) > ifr->ifr_addr.sa_len) ?
sizeof(struct sockaddr) : ifr->ifr_addr.sa_len;
ptr += sizeof(ifr->ifr_name) + len;
/* Do what you need with the ifr-structure.
* ifr->ifr_addr contains either sockaddr_dl,
* sockaddr_in or sockaddr_in6 depending on
* what addresses and L2 protocols the interface
* has associated in it.
*/
}
close(fd);
return 0;
}
There are some gotchas, of course. According to Unix Network Programming chapter 17.6 ioctl(fd, SIOCGIFCONF, array) may not return an error on some platforms if the array pointed in the argument is too small. The data will then be concatenated. Only way to work around this is to call ioctl() in a loop until you get same result length twice while increasing the size of the array. Of course, since this is 2012, I'm not sure how relevant this is anymore.
Size of ifreqs array is purely a guess in this case. Keep in mind though that the array will contain one struct ifreq for every L2 and L3 address associated with a interface. For example, assuming you have also IPv6 addresses, for lo-interface you'd get three entries: ethernet, IPv4 and IPv6. Therefore reserve enough space or apply the kludge.
To get broadcast addresses and other additional information, you will need to additional ioctl() calls in the loop. All possible options depends on what your OS provides, of course.
For more information I'd recommend reading Unix Network Programming by W. Richard Stevens. It is the most comprehesive book about this subject.

Answer 2

The traditional way to do the equivalent was with the SIOCGIFCONF operation to ioctl. Any socket can be used for the operation. It's not as easy as a single function call though.