Catch moment when we have wifi connection - c

Question

I need to write program which follow such steps:
Start program (daemon)
Wait (sleep, block) until I have wifi connection up
Send/get some data from server
Wait until wifi connection goes down
goto 2
Problem with step 2. I dont know how to catch moment when there is established network connection. There is /proc/net/wireless entry, where information about available wireless connections appear, but trying to monitor it with inotify have no success. Network connection is established asynchronously.
Here is my test code with inotify (copied mostly from R.Loves book):
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/inotify.h>
#include <sys/select.h>
#define BUF_LEN 1024
int
main() {
int fd, wd, rc;
char buf[BUF_LEN];
ssize_t len, i = 0;
static fd_set read_fds;
fd = inotify_init();
if (fd == -1) {
perror("inotify_init");
exit(EXIT_FAILURE);
}
wd = inotify_add_watch(fd, "/proc/net/wireless", IN_ALL_EVENTS);
if (wd == -1) {
perror("inotify_add_watch");
exit(EXIT_FAILURE);
}
for (;;) {
FD_ZERO(&read_fds);
FD_SET(wd, &read_fds);
rc = select(wd + 1, &read_fds, NULL, NULL, NULL);
if (rc == -1)
perror("select");
len = read(fd, buf, BUF_LEN);
while (i < len) {
struct inotify_event *event = (struct inotify_event *) &buf[i];
printf("wd=%d mask=%d cookie=%d len=%d dir=%s\n",
event->wd, event->mask, event->cookie, event->len,
(event-> mask & IN_ISDIR) ? "yes" : "no");
if (event->len)
printf("name=%s\n", event->name);
i += sizeof(struct inotify_event) + event->len;
}
sleep(1);
}
return 0;
}
It only catches evernt when I do cat /proc/net/wireless
Question: How to catch moment, when I have network connection running (wifi), using only Linux features?
P.S. This is my first post here, hope everything is ok.

Answer 1

You can detect when a network connection (not just wifi) beomes link-ready through the netlink interface, rtnetlink.
This is not an easy interface to program against, so you might wish to invoke the process "ip monitor link" instead. If you see the interface have the LOWER_UP flag, that means it's ready to send/ receive (EDIT: You may also want to check the NO_CARRIER flag is absent; see Simon's comment).
However, there is also a problem that you may have a race condition with a daemon like NetworkManager, which will (if so configured) attempt to get an IP address after the link becomes available.