I used the following method to get iphone mac address, but sometimes result is null. This problem can occur in what circumstances?

I used the following method to get iphone mac address, but sometimes result is null. This problem can occur in what circumstances?

+(NSString *) macAddress{
    int                    mib[6];
    size_t          开发者_Python百科      len;
    char                *buf;
    unsigned char        *ptr;
    struct if_msghdr    *ifm;
    struct sockaddr_dl    *sdl;

    mib[0] = CTL_NET;
    mib[1] = AF_ROUTE;
    mib[2] = 0;
    mib[3] = AF_LINK;
    mib[4] = NET_RT_IFLIST;

    if ((mib[5] = if_nametoindex("en0")) == 0) {
        printf("Error: if_nametoindex error/n");
        return NULL;
    }

    if (sysctl(mib, 6, NULL, &len, NULL, 0) < 0) {
        printf("Error: sysctl, take 1/n");
        return NULL;
    }

    if ((buf = malloc(len)) == NULL) {
        printf("Could not allocate memory. error!/n");
        return NULL;
    }

    if (sysctl(mib, 6, buf, &len, NULL, 0) < 0) {
        printf("Error: sysctl, take 2");
        return NULL;
    }

    ifm = (struct if_msghdr *)buf;
    sdl = (struct sockaddr_dl *)(ifm + 1);
    ptr = (unsigned char *)LLADDR(sdl);
    NSString *outstring = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x", *ptr, *(ptr+1), *(ptr+2), *(ptr+3), *(ptr+4), *(ptr+5)];
    free(buf);
    return [outstring uppercaseString];
}

bsd socket related functions won't activate iphone device's network capability *if it is on 3g and has not used network for several minutes.

I had problem with dns retrieval and connect() with c socket call.

If your sometimes is when your phone is on 3g and turned off(no network activity) for a while, then I would guess it's the same problem.