ng

#include <sys/select.h>
#include <fcntl.h>
#include <stdio.h>
int main()
{
    int sock = open("/dev/urandom", O_RDONLY);
    int r;
    fd_set rfds;
    FD_ZERO(&rfds);
    FD_SET(sock, &rfds);
    r = select(1, &rfds, NULL, NULL, NULL);
    printf("end\n");
}

ok

#include <sys/select.h>
#include <fcntl.h>
#include <stdio.h>
int main()
{
    int sock = open("/dev/urandom", O_RDONLY);
    int r;
    fd_set rfds;
    FD_ZERO(&rfds);
    FD_SET(sock, &rfds);
    r = select(sock+1, &rfds, NULL, NULL, NULL);
    printf("end\n");
}