List all network interfaces with netlink

if.c

#include <stdio.h>
#include <unistd.h>
#include <memory.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/types.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>

#define BUF_SIZE 8192

int fd;
int b;
int msg_len;
int attr_len;
char buf[BUF_SIZE];
struct sockaddr_nl kernel;
struct sockaddr_nl local;
struct msghdr msg;
struct iovec iov;
struct nlmsghdr *nlmsg_ptr;
struct ifinfomsg *ifi_ptr;
struct rtattr *attr_ptr;

struct nl_req_s
{
	struct nlmsghdr hdr;
	struct rtgenmsg gen;
};
typedef struct nl_req_s nl_req_t;

nl_req_t req;

int main()
{
	memset(&kernel, 0, sizeof(kernel));
	memset(&local, 0, sizeof(local));
	memset(&msg, 0, sizeof(msg));
	memset(&req, 0, sizeof(req));

	kernel.nl_family = AF_NETLINK;

	local.nl_family = AF_NETLINK;
	local.nl_pid = getpid();
	local.nl_groups = 0;

	req.hdr.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtgenmsg));
	req.hdr.nlmsg_type = RTM_GETLINK;
	req.hdr.nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP;
	req.hdr.nlmsg_pid = getpid();
	req.hdr.nlmsg_seq = 1;
	req.gen.rtgen_family = AF_PACKET;

	iov.iov_base = &req;
	iov.iov_len = req.hdr.nlmsg_len;

	msg.msg_iov = &iov;
	msg.msg_iovlen = 1;
	msg.msg_name = &kernel;
	msg.msg_namelen = sizeof(kernel);

	fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
	if (fd < 0) return -1;
	b = bind(fd, (struct sockaddr *)&local, sizeof(local));
	if (b < 0) return -2;
	
	sendmsg(fd, (struct msghdr *)&msg, 0);

	memset(&iov, 0, sizeof(iov));
	iov.iov_base = buf;
	iov.iov_len = BUF_SIZE;

	msg_len = recvmsg(fd, &msg, 0);

	nlmsg_ptr = (struct nlmsghdr *)buf;
	while(NLMSG_OK(nlmsg_ptr, msg_len))
	{
		if(nlmsg_ptr->nlmsg_type != RTM_NEWLINK) continue;
		
		ifi_ptr = NLMSG_DATA(nlmsg_ptr);
		attr_ptr = IFLA_RTA(ifi_ptr);
		attr_len = nlmsg_ptr->nlmsg_len - NLMSG_LENGTH(sizeof(*ifi_ptr));
		
		while(RTA_OK(attr_ptr, attr_len))
		{
			if(attr_ptr->rta_type == IFLA_IFNAME) printf("%s\n", (char *)RTA_DATA(attr_ptr)); 
			attr_ptr = RTA_NEXT(attr_ptr, attr_len);
		}
		nlmsg_ptr = NLMSG_NEXT(nlmsg_ptr, msg_len);
	}
}

Not bad. Very clear code. But there is an error. You are receiving only 1 netlink message. So not all the interfaces are present.
you need another cycle over recvmsg()

while(1){
msg_len = recvmsg();
if(msg_len < 0) break;
nlmsg_ptr = (struct nlmsghdr *)iov.iov_base;
if(nlmsg_ptr->nlmsg_type == NLMSG_DONE) break;
....
}

Something like this.