#define _GNU_SOURCE
#include <net/if.h>
#include <sys/socket.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include "__netlink.h"

struct ifnamemap {
	unsigned int index;
	unsigned char namelen;
	char name[IFNAMSIZ];
};

struct ifnameindexctx {
	unsigned int num;
	unsigned int str_bytes;
	struct ifnamemap *list;
};

static int __handle_link(void *pctx, struct nlmsghdr *h)
{
	struct ifnameindexctx *ctx = pctx;
	struct ifinfomsg *ifim = NLMSG_DATA(h);
	struct rtattr *rta;
	struct ifnamemap *e;

	for (rta = NLMSG_RTA(h, sizeof(*ifim)); NLMSG_RTAOK(rta, h); rta = RTA_NEXT(rta)) {
		if (rta->rta_type != IFLA_IFNAME) continue;
		if (RTA_DATALEN(rta) > IFNAMSIZ) return -ENOBUFS;

		ctx->num++;
		ctx->str_bytes += RTA_DATALEN(rta) + 1;
		e = realloc(ctx->list, sizeof(struct ifnamemap[ctx->num]));
		if (e == 0) return -ENOMEM;
		ctx->list = e;

		e = &ctx->list[ctx->num-1];
		e->index = ifim->ifi_index;
		e->namelen = RTA_DATALEN(rta);
		memcpy(e->name,  RTA_DATA(rta), IFNAMSIZ);
	}

	return 0;
}

struct if_nameindex *if_nameindex()
{
	struct ifnameindexctx _ctx, *ctx = &_ctx;
	struct if_nameindex *ifs = NULL;
	struct __netlink_handle *nh;
	int r, i;
	char *p;

	nh = __netlink_open(NETLINK_ROUTE);
	if (!nh) goto err;
	memset(ctx, 0, sizeof(*ctx));
	r = __netlink_enumerate(nh, RTM_GETLINK, __handle_link, ctx);
	__netlink_close(nh);
	if (r < 0) goto err;

	ifs = malloc(sizeof(struct if_nameindex[ctx->num+1]) + ctx->str_bytes);
	if (ifs == 0) goto err;

	p = (char*)ifs + sizeof(struct if_nameindex[ctx->num+1]);
	for (i = 0; i < ctx->num; i++) {
		ifs[i].if_index = ctx->list[i].index;
		ifs[i].if_name = p;
		memcpy(p, ctx->list[i].name, ctx->list[i].namelen);
		p += ctx->list[i].namelen;
		*p++ = 0;
	}
	ifs[i].if_index = 0;
	ifs[i].if_name = 0;
err:
	free(ctx->list);
	if (ifs == NULL) errno = ENOBUFS;
	return ifs;
}