getsockopt return negative mss (on CLion)

Question

I got an issue, when i use the function getsockopt to get the MSS i got a negative number (or 0). But the function is successfull, the socket is correct value hSocketService is correct and working.

So i got:

getsockopt OK
Taille maximale d'un segment = -13312

Here is the code:

int tailleMsgRecu, nbreBytesRecus, finDetectee, tailleS, tailleO;


 /* 7. Recherche du MTU  -*-Mache pas-*-*/
    tailleO=sizeof(int);
    if (getsockopt(hSocketService, IPPROTO_TCP, TCP_MAXSEG, &tailleS, &tailleO) == -1)
    {
        printf("Erreur sur le getsockopt de la socket %d\n", errno);
        exit(1);
    }
    else
    {
        printf("getsockopt OK\n");
        printf("Taille maximale d'un segment = %d\n", tailleS);
    }

thanks

EDIT

I forgot to precise that I'm running the code on windows using cygwin with Clion.

I changed the code and now tailleS is of type socklen_t, and printing its value with printf("Taille maximale d'un segment = %u\n", tailleS); the result is Taille maximale d'un segment = 0.

So the issue is still the same, I'm not getting the right value even if getsockopt is working.

Answer 1

Your tailleS variable must be of type socklen_t which resolves to an unsigned value on most platforms - You do, however, use a signed integer. This garbles the value.

Printing that value should be done with a %u format specifier to tell printf that this variable is unsigned.

And a last remark: You seem to be aware that what you're retrieving is NOT the MTU, but rather the TCP segment size which is a different thing (close, but still different)