Question

I am writing an application that requires root user privileges to execute. If executed by a non root user, it exits and terminates with a perror message such as:

    pthread_getschedparam: Operation not permitted

I would like to make the application more user friendly. As part of its early initialization I would like it to check if it is being executed by root or not. And if not root, it would present a message indicating that it can only be run by root, and then terminate.

Answer 1

getuid or geteuid would be the obvious choices.

getuid checks the credentials of the actual user.

The added e in geteuid stands for effective. It checks the effective credentials.

Just for example, if you use sudo to run a program as root (superuser), your actual credentials are still your own account, but your effective credentials are those of the root account (or a member of the wheel group, etc.)

For example, consider code like this:

#include <unistd.h>
#include <iostream>

int main() { 
    auto me = getuid();
    auto myprivs = geteuid();


    if (me == myprivs)
        std::cout << "Running as self\n";
    else
        std::cout << "Running as somebody else\n";
}

If you run this normally, getuid() and geteuid() will return the same value, so it’ll say “running as self”. If you do sudo ./a.out instead, getuid() will still return your user ID, but geteuid() will return the credentials for root or wheel, so it’ll say “Running as somebody else”.