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    <p>Usually it's a mistake to test if the user is root. POSIX does not even require a root user, but leaves it to the implementation to determine how permissions work. Code such as:</p> <pre><code>if (i_am_root) do_privileged_op(); else print_error(); </code></pre> <p>will really annoy users with advanced privilege models where root is not necessary to perform the necessary privileged operations. I remember back in the early days of cd burning on Linux, I had to hack all over the <code>cdrecord</code> source to remove all the useless checks to see if it was running as root, when it worked just fine with permission to read <code>/dev/sga</code>.</p> <p>Instead, you should always <strong>attempt</strong> the privileged operation you need to perform, and check for <code>EPERM</code> or similar if it fails to notify the user that they have insufficient privileges (and perhaps should retry running as root).</p> <p>The one case where it's useful to check for root is checking if your program was invoked "suid-root". A reasonable test would be:</p> <pre><code>uid_t uid=getuid(), euid=geteuid(); if (uid&lt;0 || uid!=euid) { /* We might have elevated privileges beyond that of the user who invoked * the program, due to suid bit. Be very careful about trusting any data! */ } else { /* Anything goes. */ } </code></pre> <p>Note that I allowed for the possibility (far-fetched, but best to be paranoid) that either of the calls to get uid/euid could fail, and that in the failure case we should assume we're suid and a malicious user has somehow caused the syscalls to fail in an attempt to hide that we're suid.</p>
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    1. COAgreed, completely.
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    2. COThe Linux Standards Base does require a root user, however. Just for the record. Not that I disagree with the overall premise, of course. :)
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    3. COLSB has a lot of bugs, like requiring accidentally-left-visible internal glibc functions to be available to applications. :-)
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