c - inotify_add_watch fails on /sys/class/net/eth0/operstate

Question

I have used inotify in Linux, to get the event raised when ever the network interface link changes. whenever interface link changes, /sys/class/net/eth40/operstate/ file gets modified. But in the below code snippet even though the file is getting modified, read function is still in blocked state.

#include <stdio.h>
#include <sys/inotify.h>
#include <stdlib.h>
#include <limits.h>
#include <signal.h>
#define FILE_TO_WATCH "/sys/class/net/eth40/operstate"
#define EVENT_SIZE (sizeof (struct inotify_event))
#define EVENT_BUFFER_LENGTH (1024 * EVENT_SIZE + NAME_MAX + 1)

void print_event(struct inotify_event *event) {

    int ret = 0;
    if (event->mask & IN_CREATE)
        printf("file created in directory
");
    if (event->mask & IN_DELETE)
        printf("file deleted in directory
");
    if (event->mask & IN_ACCESS)
        printf("file accessed
");
    if (event->mask & IN_CLOSE)
        printf("file closed after reading or writing 
");
    if (event->mask & IN_OPEN)
        printf("file opened
");

    if (event->len)
        printf("name: %s
", event->name);
}

int main(int argc, char** argv)
{
    int notify_fd;
    int watch_fd;
    long input_len;
    char *ptr;
    char buffer[EVENT_BUFFER_LENGTH];
    struct inotify_event *event;

    notify_fd = inotify_init();
    if (notify_fd < 0) {
        perror("cannot init inotify");
        exit(EXIT_FAILURE);
    }
    printf("done1
");
    watch_fd = inotify_add_watch(notify_fd,FILE_TO_WATCH,IN_ACCESS|IN_MODIFY);
    if (watch_fd < 0) {
        perror("cannot add file");
        exit(EXIT_FAILURE);
    }
    printf("done2
");
    while (1) {
        input_len = read(notify_fd, buffer, EVENT_BUFFER_LENGTH);
        if (input_len <= 0) {
            perror("error reading from inotify fd");
            exit(EXIT_FAILURE);
        }
        printf("done3
");
        ptr = buffer;
        while (ptr < buffer + input_len) {
            event = (struct inotify_event *) ptr;
            print_event(event);
            ptr += sizeof (struct inotify_event) +event->len;
        }
    }
}

Answer 1

/sys is not a regular file system, but a special in-memory file system called sysfs

To quote a kernel developer:

inotify does not and will not work on sysfs. Or procfs. Or devpts. Or any number of network filesystems. No matter how hard somebody might wish it to work, that's simply not feasible.

For network link events you can use rtnetlink, though stuff like this is hardly documented, here's an starting point example that will show you link (and a few other) events, you'll have to figure out which events/flags and similar that is relevant for your particular case.

#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <asm/types.h>
#include <asm/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <linux/if.h>
#include <linux/rtnetlink.h>

#define ENTRY(x) {x, #x}
struct {
    unsigned flag;
    const char *name;
} ifi_flag_map[] = {
    ENTRY(IFF_UP),
    ENTRY(IFF_BROADCAST),
    ENTRY(IFF_DEBUG),
    ENTRY(IFF_LOOPBACK),
    ENTRY(IFF_POINTOPOINT),
    ENTRY(IFF_NOTRAILERS),
    ENTRY(IFF_RUNNING),
    ENTRY(IFF_NOARP),
    ENTRY(IFF_PROMISC),
    ENTRY(IFF_ALLMULTI),
    ENTRY(IFF_MASTER),
    ENTRY(IFF_SLAVE),
    ENTRY(IFF_MULTICAST),
    ENTRY(IFF_PORTSEL),
    ENTRY(IFF_AUTOMEDIA),
    ENTRY(IFF_DYNAMIC),
    ENTRY(IFF_LOWER_UP),
    ENTRY(IFF_DORMANT),
    ENTRY(IFF_ECHO),
};
struct {
    unsigned type;
    const char *name;
} nlmrt_type_map[] = {
    ENTRY(RTM_NEWLINK ),
    ENTRY(RTM_DELLINK),
    ENTRY(RTM_GETLINK),
    ENTRY(RTM_SETLINK),
    ENTRY(RTM_NEWADDR ),
    ENTRY(RTM_DELADDR),
    ENTRY(RTM_GETADDR),
    ENTRY(RTM_NEWROUTE    ),
    ENTRY(RTM_DELROUTE),
    ENTRY(RTM_GETROUTE),
    ENTRY(RTM_NEWNEIGH    ),
    ENTRY(RTM_DELNEIGH),
    ENTRY(RTM_GETNEIGH),
    ENTRY(RTM_NEWRULE ),
    ENTRY(RTM_DELRULE),
    ENTRY(RTM_GETRULE),
    ENTRY(RTM_NEWQDISC    ),
    ENTRY(RTM_DELQDISC),
    ENTRY(RTM_GETQDISC),
    ENTRY(RTM_NEWTCLASS   ),
    ENTRY(RTM_DELTCLASS),
    ENTRY(RTM_GETTCLASS),
    ENTRY(RTM_NEWTFILTER  ),
    ENTRY(RTM_DELTFILTER),
    ENTRY(RTM_NEWACTION   ),
    ENTRY(RTM_DELACTION),
    ENTRY(RTM_GETACTION),
    ENTRY(RTM_NEWPREFIX   ),
    ENTRY(RTM_GETMULTICAST ),
    ENTRY(RTM_GETANYCAST  ),
    ENTRY(RTM_NEWNEIGHTBL ),
    ENTRY(RTM_GETNEIGHTBL ),
    ENTRY(RTM_SETNEIGHTBL),
    ENTRY(RTM_NEWNDUSEROPT ),
    ENTRY(RTM_NEWADDRLABEL ),
    ENTRY(RTM_DELADDRLABEL),
    ENTRY(RTM_GETADDRLABEL),
    ENTRY(RTM_GETDCB ),
    ENTRY(RTM_SETDCB),
    ENTRY(RTM_NEWNETCONF ),
    ENTRY(RTM_GETNETCONF ),
    ENTRY(RTM_NEWMDB ),
    ENTRY(RTM_DELMDB ),
    ENTRY(RTM_GETMDB ),
};

void print_type(unsigned type)
{
    size_t i;

    for (i = 0; i < sizeof nlmrt_type_map/sizeof nlmrt_type_map[0]; i++) {
        if (type == nlmrt_type_map[i].type) {
            printf("Msg Type: %s
", nlmrt_type_map[i].name);
            return;
        }
    }

    printf("Msg Type: unknown(%d)
", type);
}
void print_flags(unsigned flags, unsigned change)
{
    size_t i;

    printf("flags: ");

    for (i = 0; i < sizeof ifi_flag_map/sizeof ifi_flag_map[0]; i++) {
        if (flags & ifi_flag_map[i].flag) {
            if (change & ifi_flag_map[i].flag) {
                printf("%s(C) ", ifi_flag_map[i].name);
            } else {
                printf("%s ", ifi_flag_map[i].name);
            }
        }
    }
    puts("");
}
oid read_msg(int fd)
{
    int len;
    char buf[4096];
    struct iovec iov = { buf, sizeof(buf) };
    struct sockaddr_nl sa;
    struct msghdr msg = { (void *)&sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
    struct nlmsghdr *nh;

    len = recvmsg(fd, &msg, 0);
    if(len == -1) {
        perror("recvmsg");
        return;
    }

    for (nh = (struct nlmsghdr *) buf; NLMSG_OK (nh, len);
         nh = NLMSG_NEXT (nh, len)) {
         struct ifinfomsg *ifimsg;
        /* The end of multipart message. */
         printf("netlink message: len = %u, type = %u, flags = 0x%X, seq = %u, pid = %u
",
            nh->nlmsg_len,
            nh->nlmsg_type,
            nh->nlmsg_flags,
            nh->nlmsg_seq,
            nh->nlmsg_pid);

        if (nh->nlmsg_type == NLMSG_DONE)
            return;

       if (nh->nlmsg_type == NLMSG_ERROR) {
            continue;
       }

       ifimsg = NLMSG_DATA(nh);
       printf("ifi_family = %u, ifi_type = %u, ifi_index = %u, ifi_flags = 0x%X, ifi_change = 0x%X
",
               ifimsg->ifi_family ,
               ifimsg->ifi_type ,
               ifimsg->ifi_index ,
               ifimsg->ifi_flags ,
               ifimsg->ifi_change);
       print_type(nh->nlmsg_type);
       print_flags(ifimsg->ifi_flags, ifimsg->ifi_change);
    }
}
int main(int argc, char *argv[])
{
    struct sockaddr_nl sa;
    int fd;

    memset(&sa, 0, sizeof(sa));
    sa.nl_family = AF_NETLINK;
    sa.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR;

    fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if(fd == -1) {
        perror("socket");
        return 1;
    }

    if(bind(fd, (struct sockaddr *) &sa, sizeof(sa)) == -1) {
        perror("bind");
        return 1;
    }
    for(;;) {
        read_msg(fd);
    }

    return 0;
}