setsockopt fails for IPPROTO_TCP IP_TOS in C

Question

My code fails. I am running as root (same behavior as normal user)

First I want to set the TOS and then get the value.

int tos_local = 0x28;
if (setsockopt(sockfd, IPPROTO_TCP, IP_TOS,  &tos_local, sizeof(tos_local))) {
    error("error at socket option");
} else {
    int tos=0;
    int toslen=0;

    if (getsockopt(sockfd, IPPROTO_TCP, IP_TOS,  &tos, &toslen) < 0) {
            error("error to get option");
    }else {
            printf ("changing tos opt = %d\n",tos);
    }
}

the printf prints

changing tos opt = 0

I would expect to print 0x28 (40).

What is the problem?

The correct answer:

    if (setsockopt(sockfd, **IPPROTO_IP**, IP_TOS,  &tos_local, sizeof(tos_local))) {

    int tos=0;
    int toslen=sizeof(tos); //that line here

    if (getsockopt(sockfd, IPPROTO_IP, IP_TOS,  &tos, &toslen) < 0) {

Answer 1

IP_TOS has level IPPROTO_IP, not IPPROTO_TCP.

See the documentation.

This affects both setting and getting the option.

Also, what Seth said about initializing the length parameter, which affects only getsockopt.

Answer 2

When calling getsockopt, you pass in the size of the memory pointed to by &tos. In other words initialize toslen to sizeof(tos).