code.c

    int a;
        getsockopt(udp_fd, SOL_SOCKET, SO_RCVBUF, &a, &len);
        printf("started as                  %d, setting it as %d\n", a, tmp );
        if (setsockopt(udp_fd, SOL_SOCKET, SO_RCVBUF, &tmp, sizeof(tmp)) < 0) {
            log_net_error(h, AV_LOG_WARNING, "setsockopt(SO_RECVBUF)");
        }
        len = sizeof(tmp);
        if (getsockopt(udp_fd, SOL_SOCKET, SO_RCVBUF, &tmp, &len) < 0) {
            log_net_error(h, AV_LOG_WARNING, "getsockopt(SO_RCVBUF)");
        } else
            av_log(h, AV_LOG_DEBUG, "end receive buffer size reported is %d\n", tmp);

gistfile1.txt

...an input file: udp://227.0.0.1?buffer_size=2222.
started as                  212992, setting it as 2222
[udp @ 0x1a72ec0] end receive buffer size reported is 4444