setuid和getuid似乎不起作用

Question

I'm writing an Qt application accessing Linux raw socket, so I need root privilege to run the program. So I did this:

In my program, I have this at the beginning:

if (getuid() != 0)
{
    fprintf(stderr, "You must be root to run this program. UID = %i\n", getuid());
    exit(-1);
}

Then, I did "chmod 6777 myProgram" as root.

But, when I try to run it as a normal user, it says: "You must be root to run this program. UID = 1002", where 1002 is the user ID I'm currently using.

Can any one post a clue?

Thanks

Answer 1

You're mixing up getuid() and geteuid() . From the man page of getuid() :

The getuid() function returns the real user ID of the calling process. The geteuid() function returns the effective user ID of the calling process.

The real user ID is that of the user who has invoked the program. As the effective user ID gives the process additional permissions during execution of set-user-ID mode processes, getuid() is used to determine the real-user-id of the calling process.

Linux's man page is even more succinct (that previous one was from Mac OS X):

When a normal program is executed, the effective and real user ID of the process are set to the ID of the user executing the file. When a set ID program is executed the real user ID is set to the calling user and the effective user ID corresponds to the set ID bit on the file being executed.

For setuid programs, the file needs to be owned by the userid that you want to setuid to, which is in most cases root.